• To review the ideas of computer science, programming, and problem-solving.
  • To understand abstraction and the role it plays in the problem-solving process.
  • To understand and implement the notion of an abstract data type.
  • To review the Python programming language.

Getting Started

The way we think about programming has undergone many changes in the years since the first electronic computers required patch cables and switches to convey instructions from human to machine. As is the case with many aspects of society, changes in computing technology provide computer scientists with a growing number of tools and platforms on which to practice their craft. Advances such as faster processors, high-speed networks, and large memory capacities have created a spiral of complexity through which computer scientists must navigate. Throughout all of this rapid evolution, a number of basic principles have remained constant. The science of computing is concerned with using computers to solve problems.

You have no doubt spent considerable time learning the basics of problem-solving and hopefully feel confident in your ability to take a problem statement and develop a solution. You have also learned that writing computer programs is often hard. The complexity of large problems and the corresponding complexity of the solutions can tend to overshadow the fundamental ideas related to the problem-solving process.

This chapter emphasizes two important areas for the rest of the text. First, it reviews the framework within which computer science and the study of algorithms and data structures must fit, in particular, the reasons why we need to study these topics and how understanding these topics helps us to become better problem solvers. Second, we review the Python programming language. Although we cannot provide a detailed, exhaustive reference, we will give examples and explanations for the basic constructs and ideas that will occur throughout the remaining chapters.

What Is Computer Science?

Computer science is often difficult to define. This is probably due to the unfortunate use of the word “computer” in the name. As you are perhaps aware, computer science is not simply the study of computers. Although computers play an important supporting role as a tool in the discipline, they are just that–tools.

Computer science is the study of problems, problem-solving, and the solutions that come out of the problem-solving process. Given a problem, a computer scientist’s goal is to develop an algorithm, a step-by-step list of instructions for solving any instance of the problem that might arise. Algorithms are finite processes that if followed will solve the problem. Algorithms are solutions.

Computer science can be thought of as the study of algorithms. However, we must be careful to include the fact that some problems may not have a solution. Although proving this statement is beyond the scope of this text, the fact that some problems cannot be solved is important for those who study computer science. We can fully define computer science, then, by including both types of problems and stating that computer science is the study of solutions to problems as well as the study of problems with no solutions.

It is also very common to include the word computable when describing problems and solutions. We say that a problem is computable if an algorithm exists for solving it. An alternative definition for computer science, then, is to say that computer science is the study of problems that are and that are not computable, the study of the existence and the nonexistence of algorithms. In any case, you will note that the word “computer” did not come up at all. Solutions are considered independent from the machine.

Computer science, as it pertains to the problem-solving process itself, is also the study of abstraction. Abstraction allows us to view the problem and solution in such a way as to separate the so-called logical and physical perspectives. The basic idea is familiar to us in a common example.

Consider the automobile that you may have driven to school or work today. As a driver, a user of the car, you have certain interactions that take place in order to utilize the car for its intended purpose. You get in, insert the key, start the car, shift, brake, accelerate, and steer in order to drive. From an abstraction point of view, we can say that you are seeing the logical perspective of the automobile. You are using the functions provided by the car designers for the purpose of transporting you from one location to another. These functions are sometimes also referred to as the interface.

On the other hand, the mechanic who must repair your automobile takes a very different point of view. She not only knows how to drive but must know all of the details necessary to carry out all the functions that we take for granted. She needs to understand how the engine works, how the transmission shifts gears, how temperature is controlled, and so on. This is known as the physical perspective, the details that take place “under the hood.”

The same thing happens when we use computers. Most people use computers to write documents, send and receive email, surf the web, play music, store images, and play games without any knowledge of the details that take place to allow those types of applications to work. They view computers from a logical or user perspective. Computer scientists, programmers, technology support staff, and system administrators take a very different view of the computer. They must know the details of how operating systems work, how network protocols are configured, and how to code various scripts that control function. They must be able to control the low-level details that a user simply assumes.

The common point for both of these examples is that the user of the abstraction, sometimes also called the client, does not need to know the details as long as the user is aware of the way the interface works. This interface is the way we as users communicate with the underlying complexities of the implementation. As another example of abstraction, consider the Python math module. Once we import the module, we can perform computations such as

>>> import math
>>> math.sqrt(16)

This is an example of procedural abstraction. We do not necessarily know how the square root is being calculated, but we know what the function is called and how to use it. If we perform the import correctly, we can assume that the function will provide us with the correct results. We know that someone implemented a solution to the square root problem but we only need to know how to use it. This is sometimes referred to as a “black box” view of a process. We simply describe the interface: the name of the function, what is needed (the parameters), and what will be returned. The details are hidden inside (see Figure 1).


Figure 1: Procedural Abstraction

What Is Programming?

Programming is the process of taking an algorithm and encoding it into a notation, a programming language, so that it can be executed by a computer. Although many programming languages and many different types of computers exist, the important first step is the need to have the solution. Without an algorithm there can be no program.

Computer science is not the study of programming. Programming, however, is an important part of what a computer scientist does. Programming is often the way that we create a representation for our solutions. Therefore, this language representation and the process of creating it becomes a fundamental part of the discipline.

Algorithms describe the solution to a problem in terms of the data needed to represent the problem instance and the set of steps necessary to produce the intended result. Programming languages must provide a notational way to represent both the process and the data. To this end, languages provide control constructs and data types.

Control constructs allow algorithmic steps to be represented in a convenient yet unambiguous way. At a minimum, algorithms require constructs that perform sequential processing, selection for decision-making, and iteration for repetitive control. As long as the language provides these basic statements, it can be used for algorithm representation.

All data items in the computer are represented as strings of binary digits. In order to give these strings meaning, we need to have data types. Data types provide an interpretation for this binary data so that we can think about the data in terms that make sense with respect to the problem being solved. These low-level, built-in data types (sometimes called the primitive data types) provide the building blocks for algorithm development.

For example, most programming languages provide a data type for integers. Strings of binary digits in the computer’s memory can be interpreted as integers and given the typical meanings that we commonly associate with integers (e.g. 23, 654, and -19). In addition, a data type also provides a description of the operations that the data items can participate in. With integers, operations such as addition, subtraction, and multiplication are common. We have come to expect that numeric types of data can participate in these arithmetic operations.

The difficulty that often arises for us is the fact that problems and their solutions are very complex. These simple, language-provided constructs and data types, although certainly sufficient to represent complex solutions, are typically at a disadvantage as we work through the problem-solving process. We need ways to control this complexity and assist with the creation of solutions.

Why Study Data Structures and Abstract Data Types?

To manage the complexity of problems and the problem-solving process, computer scientists use abstractions to allow them to focus on the “big picture” without getting lost in the details. By creating models of the problem domain, we are able to utilize a better and more efficient problem-solving process. These models allow us to describe the data that our algorithms will manipulate in a much more consistent way with respect to the problem itself.

Earlier, we referred to procedural abstraction as a process that hides the details of a particular function to allow the user or client to view it at a very high level. We now turn our attention to a similar idea, that of data abstraction. An abstract data type, sometimes abbreviated ADT, is a logical description of how we view the data and the operations that are allowed without regard to how they will be implemented. This means that we are concerned only with what the data is representing and not with how it will eventually be constructed. By providing this level of abstraction, we are creating an encapsulation around the data. The idea is that by encapsulating the details of the implementation, we are hiding them from the user’s view. This is called information hiding.

Figure 2 shows a picture of what an abstract data type is and how it operates. The user interacts with the interface, using the operations that have been specified by the abstract data type. The abstract data type is the shell that the user interacts with. The implementation is hidden one level deeper. The user is not concerned with the details of the implementation.


Figure 2: Abstract Data Type

The implementation of an abstract data type, often referred to as a data structure, will require that we provide a physical view of the data using some collection of programming constructs and primitive data types. As we discussed earlier, the separation of these two perspectives will allow us to define the complex data models for our problems without giving any indication as to the details of how the model will actually be built. This provides an implementation-independent view of the data. Since there will usually be many different ways to implement an abstract data type, this implementation independence allows the programmer to switch the details of the implementation without changing the way the user of the data interacts with it. The user can remain focused on the problem-solving process.

Why Study Algorithms?

Computer scientists learn by experience. We learn by seeing others solve problems and by solving problems by ourselves. Being exposed to different problem-solving techniques and seeing how different algorithms are designed helps us to take on the next challenging problem that we are given. By considering a number of different algorithms, we can begin to develop pattern recognition so that the next time a similar problem arises, we are better able to solve it.

Algorithms are often quite different from one another. Consider the example of sqrt seen earlier. It is entirely possible that there are many different ways to implement the details to compute the square root function. One algorithm may use many fewer resources than another. One algorithm might take 10 times as long to return the result as the other. We would like to have some way to compare these two solutions. Even though they both work, one is perhaps “better” than the other. We might suggest that one is more efficient or that one simply works faster or uses less memory. As we study algorithms, we can learn analysis techniques that allow us to compare and contrast solutions based solely on their own characteristics, not the characteristics of the program or computer used to implement them.

In the worst case scenario, we may have a problem that is intractable, meaning that there is no algorithm that can solve the problem in a realistic amount of time. It is important to be able to distinguish between those problems that have solutions, those that do not, and those where solutions exist but require too much time or other resources to work reasonably.

There will often be trade-offs that we will need to identify and decide upon. As computer scientists, in addition to our ability to solve problems, we will also need to know and understand solution evaluation techniques. In the end, there are often many ways to solve a problem. Finding a solution and then deciding whether it is a good one are tasks that we will do over and over again.

Review of Basic Python

In this section, we will review the programming language Python and also provide some more detailed examples of the ideas from the previous section. If you are new to Python or find that you need more information about any of the topics presented, we recommend that you consult a resource such as the Python Language Reference or a Python Tutorial. Our goal here is to reacquaint you with the language and also reinforce some of the concepts that will be central to later chapters.

Python is a modern, easy-to-learn, object-oriented programming language. It has a powerful set of built-in data types and easy-to-use control constructs. Since Python is an interpreted language, it is most easily reviewed by simply looking at and describing interactive sessions. You should recall that the interpreter displays the familiar >>> prompt and then evaluates the Python construct that you provide. For example,

>>> print("Algorithms and Data Structures")
Algorithms and Data Structures

shows the prompt, the print function, the result, and the next prompt.

Getting Started with Data

We stated above that Python supports the object-oriented programming paradigm. This means that Python considers data to be the focal point of the problem-solving process. In Python, as well as in any other object-oriented programming language, we define a class to be a description of what the data look like (the state) and what the data can do (the behavior). Classes are analogous to abstract data types because a user of a class only sees the state and behavior of a data item. Data items are called objects in the object-oriented paradigm. An object is an instance of a class.

Built-in Atomic Data Types

We will begin our review by considering the atomic data types. Python has two main built-in numeric classes that implement the integer and floating point data types. These Python classes are called int and float. The standard arithmetic operations, +, -, *, /, and ** (exponentiation), can be used with parentheses forcing the order of operations away from normal operator precedence. Other very useful operations are the remainder (modulo) operator, %, and integer division, //. Note that when two integers are divided, the result is a floating point. The integer division operator returns the integer portion of the quotient by truncating any fractional part.

Basic Arithmetic Operators (intro_1)

The boolean data type, implemented as the Python bool class, will be quite useful for representing truth values. The possible state values for a boolean object are True and False with the standard boolean operators, and, or, and not.

>>> True
>>> False
>>> False or True
>>> not (False or True)
>>> True and True

Boolean data objects are also used as results for comparison operators such as equality (==) and greater than (\(>\)). In addition, relational operators and logical operators can be combined together to form complex logical questions. Table 1 shows the relational and logical operators with examples shown in the session that follows.

Table 1: Relational and Logical Operators
Operation Name Operator Explanation
less than \(<\) Less than operator
greater than \(>\) Greater than operator
less than or equal \(<=\) Less than or equal to operator
greater than or equal \(>=\) Greater than or equal to operator
equal \(==\) Equality operator
not equal \(!=\) Not equal operator
logical and \(and\) Both operands True for result to be True
logical or \(or\) One or the other operand is True for the result to be True
logical not \(not\) Negates the truth value, False becomes True, True becomes False

Basic Relational and Logical Operators (intro_2)

Identifiers are used in programming languages as names. In Python, identifiers start with a letter or an underscore (_), are case sensitive, and can be of any length. Remember that it is always a good idea to use names that convey meaning so that your program code is easier to read and understand.

A Python variable is created when a name is used for the first time on the left-hand side of an assignment statement. Assignment statements provide a way to associate a name with a value. The variable will hold a reference to a piece of data and not the data itself. Consider the following session:

>>> theSum = 0
>>> theSum
>>> theSum = theSum + 1
>>> theSum
>>> theSum = True
>>> theSum

The assignment statement theSum = 0 creates a variable called theSum and lets it hold the reference to the data object 0 (see Figure 3). In general, the right-hand side of the assignment statement is evaluated and a reference to the resulting data object is “assigned” to the name on the left-hand side. At this point in our example, the type of the variable is integer as that is the type of the data currently being referred to by thesum. If the type of the data changes (see Figure 4), as shown above with the boolean value True, so does the type of the variable (thesum is now of the type boolean). The assignment statement changes the reference being held by the variable. This is a dynamic characteristic of Python. The same variable can refer to many different types of data.


Figure 3: Variables Hold References to Data Objects


Figure 4: Assignment Changes the Reference

Built-in Collection Data Types

In addition to the numeric and boolean classes, Python has a number of very powerful built-in collection classes. Lists, strings, and tuples are ordered collections that are very similar in general structure but have specific differences that must be understood for them to be used properly. Sets and dictionaries are unordered collections.

A list is an ordered collection of zero or more references to Python data objects. Lists are written as comma-delimited values enclosed in square brackets. The empty list is simply [ ]. Lists are heterogeneous, meaning that the data objects need not all be from the same class and the collection can be assigned to a variable as below. The following fragment shows a variety of Python data objects in a list.

>>> [1,3,True,6.5]
[1, 3, True, 6.5]
>>> myList = [1,3,True,6.5]
>>> myList
[1, 3, True, 6.5]

Note that when Python evaluates a list, the list itself is returned. However, in order to remember the list for later processing, its reference needs to be assigned to a variable.

Since lists are considered to be sequentially ordered, they support a number of operations that can be applied to any Python sequence. Table 2 reviews these operations and the following session gives examples of their use.

Table 2: Operations on Any Sequence in Python
Operation Name Operator Explanation
indexing [ ] Access an element of a sequence
concatenation + Combine sequences together
repetition * Concatenate a repeated number of times
membership in Ask whether an item is in a sequence
length len Ask the number of items in the sequence
slicing [ : ] Extract a part of a sequence

Note that the indices for lists (sequences) start counting with 0. The slice operation, myList[1:3], returns a list of items starting with the item indexed by 1 up to but not including the item indexed by 3.

Sometimes, you will want to initialize a list. This can quickly be accomplished by using repetition. For example,

>>> myList = [0] * 6
>>> myList
[0, 0, 0, 0, 0, 0]

One very important aside relating to the repetition operator is that the result is a repetition of references to the data objects in the sequence. This can best be seen by considering the following session:

Repetition of References (intro_3)

The variable A holds a collection of three references to the original list called myList. Note that a change to one element of myList shows up in all three occurrences in A.

Lists support a number of methods that will be used to build data structures. Table 3 provides a summary. Examples of their use follow.

Table 3: Methods Provided by Lists in Python
Method Name Use Explanation
append alist.append(item) Adds a new item to the end of a list
insert alist.insert(i,item) Inserts an item at the ith position in a list
pop alist.pop() Removes and returns the last item in a list
pop alist.pop(i) Removes and returns the ith item in a list
sort alist.sort() Modifies a list to be sorted
reverse alist.reverse() Modifies a list to be in reverse order
del del alist[i] Deletes the item in the ith position
index alist.index(item) Returns the index of the first occurrence of item
count alist.count(item) Returns the number of occurrences of item
remove alist.remove(item) Removes the first occurrence of item

Examples of List Methods (intro_5)

You can see that some of the methods, such as pop, return a value and also modify the list. Others, such as reverse, simply modify the list with no return value. pop will default to the end of the list but can also remove and return a specific item. The index range starting from 0 is again used for these methods. You should also notice the familiar “dot” notation for asking an object to invoke a method. myList.append(False) can be read as “ask the object myList to perform its append method and send it the value False.” Even simple data objects such as integers can invoke methods in this way.

>>> (54).__add__(21)

In this fragment we are asking the integer object 54 to execute its add method (called __add__ in Python) and passing it 21 as the value to add. The result is the sum, 75. Of course, we usually write this as 54+21. We will say much more about these methods later in this section.

One common Python function that is often discussed in conjunction with lists is the range function. range produces a range object that represents a sequence of values. By using the list function, it is possible to see the value of the range object as a list. This is illustrated below.

>>> range(10)
range(0, 10)
>>> list(range(10))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(5,10)
range(5, 10)
>>> list(range(5,10))
[5, 6, 7, 8, 9]
>>> list(range(5,10,2))
[5, 7, 9]
>>> list(range(10,1,-1))
[10, 9, 8, 7, 6, 5, 4, 3, 2]

The range object represents a sequence of integers. By default, it will start with 0. If you provide more parameters, it will start and end at particular points and can even skip items. In our first example, range(10), the sequence starts with 0 and goes up to but does not include 10. In our second example, range(5,10) starts at 5 and goes up to but not including 10. range(5,10,2) performs similarly but skips by twos (again, 10 is not included).

Strings are sequential collections of zero or more letters, numbers and other symbols. We call these letters, numbers and other symbols characters. Literal string values are differentiated from identifiers by using quotation marks (either single or double).

>>> "David"
>>> myName = "David"
>>> myName[3]
>>> myName*2
>>> len(myName)

Since strings are sequences, all of the sequence operations described above work as you would expect. In addition, strings have a number of methods, some of which are shown in Table 4. For example,

>>> myName
>>> myName.upper()
'  David   '
>>> myName.find('v')
>>> myName.split('v')
['Da', 'id']

Of these, split will be very useful for processing data. split will take a string and return a list of strings using the split character as a division point. In the example, v is the division point. If no division is specified, the split method looks for whitespace characters such as tab, newline and space.

Table 4: Methods Provided by Strings in Python
Method Name Use Explanation
center Returns a string centered in a field of size w
count astring.count(item) Returns the number of occurrences of item in the string
ljust astring.ljust(w) Returns a string left-justified in a field of size w
lower astring.lower() Returns a string in all lowercase
rjust astring.rjust(w) Returns a string right-justified in a field of size w
find astring.find(item) Returns the index of the first occurrence of item
split astring.split(schar) Splits a string into substrings at schar

A major difference between lists and strings is that lists can be modified while strings cannot. This is referred to as mutability. Lists are mutable; strings are immutable. For example, you can change an item in a list by using indexing and assignment. With a string that change is not allowed.

>>> myList
[1, 3, True, 6.5]
>>> myList[0]=2**10
>>> myList
[1024, 3, True, 6.5]
>>> myName
>>> myName[0]='X'

Traceback (most recent call last):
  File "<pyshell#84>", line 1, in -toplevel-
TypeError: object doesn't support item assignment

Tuples are very similar to lists in that they are heterogeneous sequences of data. The difference is that a tuple is immutable, like a string. A tuple cannot be changed. Tuples are written as comma-delimited values enclosed in parentheses. As sequences, they can use any operation described above. For example,

>>> myTuple = (2,True,4.96)
>>> myTuple
(2, True, 4.96)
>>> len(myTuple)
>>> myTuple[0]
>>> myTuple * 3
(2, True, 4.96, 2, True, 4.96, 2, True, 4.96)
>>> myTuple[0:2]
(2, True)

However, if you try to change an item in a tuple, you will get an error. Note that the error message provides location and reason for the problem.

>>> myTuple[1]=False

Traceback (most recent call last):
  File "<pyshell#137>", line 1, in -toplevel-
TypeError: object doesn't support item assignment

A set is an unordered collection of zero or more immutable Python data objects. Sets do not allow duplicates and are written as comma-delimited values enclosed in curly braces. The empty set is represented by set(). Sets are heterogeneous, and the collection can be assigned to a variable as below.

>>> {3,6,"cat",4.5,False}
{False, 4.5, 3, 6, 'cat'}
>>> mySet = {3,6,"cat",4.5,False}
>>> mySet
{False, 4.5, 3, 6, 'cat'}

Even though sets are not considered to be sequential, they do support a few of the familiar operations presented earlier. Table 5 reviews these operations and the following session gives examples of their use.

Table 5: Operations on a Set in Python
Operation Name Operator Explanation
membership in Set membership
length len Returns the cardinality of the set
| aset | otherset Returns a new set with all elements from both sets
& aset & otherset Returns a new set with only those elements common to both sets
- aset - otherset Returns a new set with all items from the first set not in second
<= aset <= otherset Asks whether all elements of the first set are in the second
>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>> len(mySet)
>>> False in mySet
>>> "dog" in mySet

Sets support a number of methods that should be familiar to those who have worked with them in a mathematics setting. Table 6 provides a summary. Examples of their use follow. Note that union, intersection, issubset, and difference all have operators that can be used as well.

Table 6: Methods Provided by Sets in Python
Method Name Use Explanation
union aset.union(otherset) Returns a new set with all elements from both sets
intersection aset.intersection(otherset) Returns a new set with only those elements common to both sets
difference aset.difference(otherset) Returns a new set with all items from first set not in second
issubset aset.issubset(otherset) Asks whether all elements of one set are in the other
add aset.add(item) Adds item to the set
remove aset.remove(item) Removes item from the set
pop aset.pop() Removes an arbitrary element from the set
clear aset.clear() Removes all elements from the set
>>> mySet
{False, 4.5, 3, 6, 'cat'}
>>> yourSet = {99,3,100}
>>> mySet.union(yourSet)
{False, 4.5, 3, 100, 6, 'cat', 99}
>>> mySet | yourSet
{False, 4.5, 3, 100, 6, 'cat', 99}
>>> mySet.intersection(yourSet)
>>> mySet & yourSet
>>> mySet.difference(yourSet)
{False, 4.5, 6, 'cat'}
>>> mySet - yourSet
{False, 4.5, 6, 'cat'}
>>> {3,100}.issubset(yourSet)
>>> {3,100}<=yourSet
>>> mySet.add("house")
>>> mySet
{False, 4.5, 3, 6, 'house', 'cat'}
>>> mySet.remove(4.5)
>>> mySet
{False, 3, 6, 'house', 'cat'}
>>> mySet.pop()
>>> mySet
{3, 6, 'house', 'cat'}
>>> mySet.clear()
>>> mySet

Our final Python collection is an unordered structure called a dictionary. Dictionaries are collections of associated pairs of items where each pair consists of a key and a value. This key-value pair is typically written as key:value. Dictionaries are written as comma-delimited key:value pairs enclosed in curly braces. For example,

>>> capitals = {'Iowa':'DesMoines','Wisconsin':'Madison'}
>>> capitals
{'Wisconsin': 'Madison', 'Iowa': 'DesMoines'}

We can manipulate a dictionary by accessing a value via its key or by adding another key-value pair. The syntax for access looks much like a sequence access except that instead of using the index of the item we use the key value. To add a new value is similar.

Using a Dictionary (intro_7)

It is important to note that the dictionary is maintained in no particular order with respect to the keys. The first pair added ('Utah': 'SaltLakeCity') was placed first in the dictionary and the second pair added ('California': 'Sacramento') was placed last. The placement of a key is dependent on the idea of “hashing,” which will be explained in more detail in Chapter 4. We also show the length function performing the same role as with previous collections.

Dictionaries have both methods and operators. Table 7 and Table 8 describe them, and the session shows them in action. The keys, values, and items methods all return objects that contain the values of interest. You can use the list function to convert them to lists. You will also see that there are two variations on the get method. If the key is not present in the dictionary, get will return None. However, a second, optional parameter can specify a return value instead.

Table 7: Operators Provided by Dictionaries in Python
Operator Use Explanation
[] myDict[k] Returns the value associated with k, otherwise its an error
in key in adict Returns True if key is in the dictionary, False otherwise
del del adict[key] Removes the entry from the dictionary
>>> phoneext={'david':1410,'brad':1137}
>>> phoneext
{'brad': 1137, 'david': 1410}
>>> phoneext.keys()
dict_keys(['brad', 'david'])
>>> list(phoneext.keys())
['brad', 'david']
>>> phoneext.values()
dict_values([1137, 1410])
>>> list(phoneext.values())
[1137, 1410]
>>> phoneext.items()
dict_items([('brad', 1137), ('david', 1410)])
>>> list(phoneext.items())
[('brad', 1137), ('david', 1410)]
>>> phoneext.get("kent")
>>> phoneext.get("kent","NO ENTRY")
Table 8: Methods Provided by Dictionaries in Python
Method Name Use Explanation
keys adict.keys() Returns the keys of the dictionary in a dict_keys object
values adict.values() Returns the values of the dictionary in a dict_values object
items adict.items() Returns the key-value pairs in a dict_items object
get adict.get(k) Returns the value associated with k, None otherwise
get adict.get(k,alt) Returns the value associated with k, alt otherwise


This workspace is provided for your convenience. You can use this activecode window to try out anything you like.


Input and Output

We often have a need to interact with users, either to get data or to provide some sort of result. Most programs today use a dialog box as a way of asking the user to provide some type of input. While Python does have a way to create dialog boxes, there is a much simpler function that we can use. Python provides us with a function that allows us to ask a user to enter some data and returns a reference to the data in the form of a string. The function is called input.

Python’s input function takes a single parameter that is a string. This string is often called the prompt because it contains some helpful text prompting the user to enter something. For example, you might call input as follows:

aName = input('Please enter your name: ')

Now whatever the user types after the prompt will be stored in the aName variable. Using the input function, we can easily write instructions that will prompt the user to enter data and then incorporate that data into further processing. For example, in the following two statements, the first asks the user for their name and the second prints the result of some simple processing based on the string that is provided.

The input Function Returns a String (strstuff)

It is important to note that the value returned from the input function will be a string representing the exact characters that were entered after the prompt. If you want this string interpreted as another type, you must provide the type conversion explicitly. In the statements below, the string that is entered by the user is converted to a float so that it can be used in further arithmetic processing.

sradius = input("Please enter the radius of the circle ")
radius = float(sradius)
diameter = 2 * radius

String Formatting

We have already seen that the print function provides a very simple way to output values from a Python program. print takes zero or more parameters and displays them using a single blank as the default separator. It is possible to change the separator character by setting the sep argument. In addition, each print ends with a newline character by default. This behavior can be changed by setting the end argument. These variations are shown in the following session:

>>> print("Hello")
>>> print("Hello","World")
Hello World
>>> print("Hello","World", sep="***")
>>> print("Hello","World", end="***")
Hello World***>>>

It is often useful to have more control over the look of your output. Fortunately, Python provides us with an alternative called formatted strings. A formatted string is a template in which words or spaces that will remain constant are combined with placeholders for variables that will be inserted into the string. For example, the statement

print(aName, "is", age, "years old.")

contains the words is and years old, but the name and the age will change depending on the variable values at the time of execution. Using a formatted string, we write the previous statement as

print("%s is %d years old." % (aName, age))

This simple example illustrates a new string expression. The % operator is a string operator called the format operator. The left side of the expression holds the template or format string, and the right side holds a collection of values that will be substituted into the format string. Note that the number of values in the collection on the right side corresponds with the number of % characters in the format string. Values are taken—in order, left to right—from the collection and inserted into the format string.

Let’s look at both sides of this formatting expression in more detail. The format string may contain one or more conversion specifications. A conversion character tells the format operator what type of value is going to be inserted into that position in the string. In the example above, the %s specifies a string, while the %d specifies an integer. Other possible type specifications include i, u, f, e, g, c, or %. Table 9 summarizes all of the various type specifications.

Table 9: String Formatting Conversion Characters
Character Output Format
d, i Integer
u Unsigned integer
f Floating point as m.ddddd
e Floating point as m.ddddde+/-xx
E Floating point as m.dddddE+/-xx
g Use %e for exponents less than \(-4\) or greater than \(+5\), otherwise use %f
c Single character
s String, or any Python data object that can be converted to a string by using the str function.
% Insert a literal % character

In addition to the format character, you can also include a format modifier between the % and the format character. Format modifiers may be used to left-justify or right-justifiy the value with a specified field width. Modifiers can also be used to specify the field width along with a number of digits after the decimal point. Table 10 explains these format modifiers

Table 10: Additional formatting options
Modifier Example Description
number %20d Put the value in a field width of 20
- %-20d Put the value in a field 20 characters wide, left-justified
+ %+20d Put the value in a field 20 characters wide, right-justified
0 %020d Put the value in a field 20 characters wide, fill in with leading zeros.
. %20.2f Put the value in a field 20 characters wide with 2 characters to the right of the decimal point.
(name) %(name)d Get the value from the supplied dictionary using name as the key.

The right side of the format operator is a collection of values that will be inserted into the format string. The collection will be either a tuple or a dictionary. If the collection is a tuple, the values are inserted in order of position. That is, the first element in the tuple corresponds to the first format character in the format string. If the collection is a dictionary, the values are inserted according to their keys. In this case all format characters must use the (name) modifier to specify the name of the key.

>>> price = 24
>>> item = "banana"
>>> print("The %s costs %d cents"%(item,price))
The banana costs 24 cents
>>> print("The %+10s costs %5.2f cents"%(item,price))
The     banana costs 24.00 cents
>>> print("The %+10s costs %10.2f cents"%(item,price))
The     banana costs      24.00 cents
>>> itemdict = {"item":"banana","cost":24}
>>> print("The %(item)s costs %(cost)7.1f cents"%itemdict)
The banana costs    24.0 cents

In addition to format strings that use format characters and format modifiers, Python strings also include a format method that can be used in conjunction with a new Formatter class to implement complex string formatting. More about these features can be found in the Python library reference manual.

Control Structures

As we noted earlier, algorithms require two important control structures: iteration and selection. Both of these are supported by Python in various forms. The programmer can choose the statement that is most useful for the given circumstance.

For iteration, Python provides a standard while statement and a very powerful for statement. The while statement repeats a body of code as long as a condition is true. For example,

>>> counter = 1
>>> while counter <= 5:
...     print("Hello, world")
...     counter = counter + 1

Hello, world
Hello, world
Hello, world
Hello, world
Hello, world

prints out the phrase “Hello, world” five times. The condition on the while statement is evaluated at the start of each repetition. If the condition is True, the body of the statement will execute. It is easy to see the structure of a Python while statement due to the mandatory indentation pattern that the language enforces.

The while statement is a very general purpose iterative structure that we will use in a number of different algorithms. In many cases, a compound condition will control the iteration. A fragment such as

while counter <= 10 and not done:

would cause the body of the statement to be executed only in the case where both parts of the condition are satisfied. The value of the variable counter would need to be less than or equal to 10 and the value of the variable done would need to be False (not False is True) so that True and True results in True.

Even though this type of construct is very useful in a wide variety of situations, another iterative structure, the for statement, can be used in conjunction with many of the Python collections. The for statement can be used to iterate over the members of a collection, so long as the collection is a sequence. So, for example,

>>> for item in [1,3,6,2,5]:
...    print(item)

assigns the variable item to be each successive value in the list [1,3,6,2,5]. The body of the iteration is then executed. This works for any collection that is a sequence (lists, tuples, and strings).

A common use of the for statement is to implement definite iteration over a range of values. The statement

>>> for item in range(5):
...    print(item**2)

will perform the print function five times. The range function will return a range object representing the sequence 0,1,2,3,4 and each value will be assigned to the variable item. This value is then squared and printed.

The other very useful version of this iteration structure is used to process each character of a string. The following code fragment iterates over a list of strings and for each string processes each character by appending it to a list. The result is a list of all the letters in all of the words.

Processing Each Character in a List of Strings (intro_8)

Selection statements allow programmers to ask questions and then, based on the result, perform different actions. Most programming languages provide two versions of this useful construct: the ifelse and the if. A simple example of a binary selection uses the ifelse statement.

if n<0:
   print("Sorry, value is negative")

In this example, the object referred to by n is checked to see if it is less than zero. If it is, a message is printed stating that it is negative. If it is not, the statement performs the else clause and computes the square root.

Selection constructs, as with any control construct, can be nested so that the result of one question helps decide whether to ask the next. For example, assume that score is a variable holding a reference to a score for a computer science test.

if score >= 90:
   if score >=80:
      if score >= 70:
         if score >= 60:

This fragment will classify a value called score by printing the letter grade earned. If the score is greater than or equal to 90, the statement will print A. If it is not (else), the next question is asked. If the score is greater than or equal to 80 then it must be between 80 and 89 since the answer to the first question was false. In this case print B is printed. You can see that the Python indentation pattern helps to make sense of the association between if and else without requiring any additional syntactic elements.

An alternative syntax for this type of nested selection uses the elif keyword. The else and the next if are combined so as to eliminate the need for additional nesting levels. Note that the final else is still necessary to provide the default case if all other conditions fail.

if score >= 90:
elif score >=80:
elif score >= 70:
elif score >= 60:

Python also has a single way selection construct, the if statement. With this statement, if the condition is true, an action is performed. In the case where the condition is false, processing simply continues on to the next statement after the if. For example, the following fragment will first check to see if the value of a variable n is negative. If it is, then it is modified by the absolute value function. Regardless, the next action is to compute the square root.

if n<0:
   n = abs(n)

Self Check

Test your understanding of what we have covered so far by trying the following exercise. Modify the code from Activecode 8 so that the final list only contains a single copy of each letter.


Returning to lists, there is an alternative method for creating a list that uses iteration and selection constructs. The is known as a list comprehension. A list comprehension allows you to easily create a list based on some processing or selection criteria. For example, if we would like to create a list of the first 10 perfect squares, we could use a for statement:

>>> sqlist=[]
>>> for x in range(1,11):

>>> sqlist
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Using a list comprehension, we can do this in one step as

>>> sqlist=[x*x for x in range(1,11)]
>>> sqlist
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

The variable x takes on the values 1 through 10 as specified by the for construct. The value of x*x is then computed and added to the list that is being constructed. The general syntax for a list comprehension also allows a selection criteria to be added so that only certain items get added. For example,

>>> sqlist=[x*x for x in range(1,11) if x%2 != 0]
>>> sqlist
[1, 9, 25, 49, 81]

This list comprehension constructed a list that only contained the squares of the odd numbers in the range from 1 to 10. Any sequence that supports iteration can be used within a list comprehension to construct a new list.

>>>[ch.upper() for ch in 'comprehension' if ch not in 'aeiou']
['C', 'M', 'P', 'R', 'H', 'N', 'S', 'N']

Self Check

Test your understanding of list comprehensions by redoing Activecode 8 using list comprehensions. For an extra challence, see if you can figure out how to remove the duplicates.


Exception Handling

There are two types of errors that typically occur when writing programs. The first, known as a syntax error, simply means that the programmer has made a mistake in the structure of a statement or expression. For example, it is incorrect to write a for statement and forget the colon.

>>> for i in range(10)
SyntaxError: invalid syntax (<pyshell#61>, line 1)

In this case, the Python interpreter has found that it cannot complete the processing of this instruction since it does not conform to the rules of the language. Syntax errors are usually more frequent when you are first learning a language.

The other type of error, known as a logic error, denotes a situation where the program executes but gives the wrong result. This can be due to an error in the underlying algorithm or an error in your translation of that algorithm. In some cases, logic errors lead to very bad situations such as trying to divide by zero or trying to access an item in a list where the index of the item is outside the bounds of the list. In this case, the logic error leads to a runtime error that causes the program to terminate. These types of runtime errors are typically called exceptions.

Most of the time, beginning programmers simply think of exceptions as fatal runtime errors that cause the end of execution. However, most programming languages provide a way to deal with these errors that will allow the programmer to have some type of intervention if they so choose. In addition, programmers can create their own exceptions if they detect a situation in the program execution that warrants it.

When an exception occurs, we say that it has been “raised.” You can “handle” the exception that has been raised by using a try statement. For example, consider the following session that asks the user for an integer and then calls the square root function from the math library. If the user enters a value that is greater than or equal to 0, the print will show the square root. However, if the user enters a negative value, the square root function will report a ValueError exception.

>>> anumber = int(input("Please enter an integer "))
Please enter an integer -23
>>> print(math.sqrt(anumber))
Traceback (most recent call last):
  File "<pyshell#102>", line 1, in <module>
ValueError: math domain error

We can handle this exception by calling the print function from within a try block. A corresponding except block “catches” the exception and prints a message back to the user in the event that an exception occurs. For example:

>>> try:
       print("Bad Value for square root")
       print("Using absolute value instead")

Bad Value for square root
Using absolute value instead

will catch the fact that an exception is raised by sqrt and will instead print the messages back to the user and use the absolute value to be sure that we are taking the square root of a non-negative number. This means that the program will not terminate but instead will continue on to the next statements.

It is also possible for a programmer to cause a runtime exception by using the raise statement. For example, instead of calling the square root function with a negative number, we could have checked the value first and then raised our own exception. The code fragment below shows the result of creating a new RuntimeError exception. Note that the program would still terminate but now the exception that caused the termination is something explicitly created by the programmer.

>>> if anumber < 0:
...    raise RuntimeError("You can't use a negative number")
... else:
...    print(math.sqrt(anumber))
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
RuntimeError: You can't use a negative number

There are many kinds of exceptions that can be raised in addition to the RuntimeError shown above. See the Python reference manual for a list of all the available exception types and for how to create your own.

Defining Functions

The earlier example of procedural abstraction called upon a Python function called sqrt from the math module to compute the square root. In general, we can hide the details of any computation by defining a function. A function definition requires a name, a group of parameters, and a body. It may also explicitly return a value. For example, the simple function defined below returns the square of the value you pass into it.

>>> def square(n):
...    return n**2
>>> square(3)
>>> square(square(3))

The syntax for this function definition includes the name, square, and a parenthesized list of formal parameters. For this function, n is the only formal parameter, which suggests that square needs only one piece of data to do its work. The details, hidden “inside the box,” simply compute the result of n**2 and return it. We can invoke or call the square function by asking the Python environment to evaluate it, passing an actual parameter value, in this case, 3. Note that the call to square returns an integer that can in turn be passed to another invocation.

We could implement our own square root function by using a well-known technique called “Newton’s Method.” Newton’s Method for approximating square roots performs an iterative computation that converges on the correct value. The equation \(newguess = \frac {1}{2} * (oldguess + \frac {n}{oldguess})\) takes a value \(n\) and repeatedly guesses the square root by making each \(newguess\) the \(oldguess\) in the subsequent iteration. The initial guess used here is \(\frac {n}{2}\). Listing 1 shows a function definition that accepts a value \(n\) and returns the square root of \(n\) after making 20 guesses. Again, the details of Newton’s Method are hidden inside the function definition and the user does not have to know anything about the implementation to use the function for its intended purpose. Listing 1 also shows the use of the # character as a comment marker. Any characters that follow the # on a line are ignored.

Listing 1

def squareroot(n):
    root = n/2    #initial guess will be 1/2 of n
    for k in range(20):
        root = (1/2)*(root + (n / root))

    return root

Self Check

Here’s a self check that really covers everything so far. You may have heard of the infinite monkey theorem? The theorem states that a monkey hitting keys at random on a typewriter keyboard for an infinite amount of time will almost surely type a given text, such as the complete works of William Shakespeare. Well, suppose we replace a monkey with a Python function. How long do you think it would take for a Python function to generate just one sentence of Shakespeare? The sentence we’ll shoot for is: “methinks it is like a weasel”

You’re not going to want to run this one in the browser, so fire up your favorite Python IDE. The way we’ll simulate this is to write a function that generates a string that is 27 characters long by choosing random letters from the 26 letters in the alphabet plus the space. We’ll write another function that will score each generated string by comparing the randomly generated string to the goal.

A third function will repeatedly call generate and score, then if 100% of the letters are correct we are done. If the letters are not correct then we will generate a whole new string.To make it easier to follow your program’s progress this third function should print out the best string generated so far and its score every 1000 tries.

Self Check Challenge

See if you can improve upon the program in the self check by keeping letters that are correct and only modifying one character in the best string so far. This is a type of algorithm in the class of ‘hill climbing’ algorithms, that is we only keep the result if it is better than the previous one.

Object-Oriented Programming in Python: Defining Classes

We stated earlier that Python is an object-oriented programming language. So far, we have used a number of built-in classes to show examples of data and control structures. One of the most powerful features in an object-oriented programming language is the ability to allow a programmer (problem solver) to create new classes that model data that is needed to solve the problem.

Remember that we use abstract data types to provide the logical description of what a data object looks like (its state) and what it can do (its methods). By building a class that implements an abstract data type, a programmer can take advantage of the abstraction process and at the same time provide the details necessary to actually use the abstraction in a program. Whenever we want to implement an abstract data type, we will do so with a new class.

A Fraction Class

A very common example to show the details of implementing a user-defined class is to construct a class to implement the abstract data type Fraction. We have already seen that Python provides a number of numeric classes for our use. There are times, however, that it would be most appropriate to be able to create data objects that “look like” fractions.

A fraction such as \(\frac {3}{5}\) consists of two parts. The top value, known as the numerator, can be any integer. The bottom value, called the denominator, can be any integer greater than 0 (negative fractions have a negative numerator). Although it is possible to create a floating point approximation for any fraction, in this case we would like to represent the fraction as an exact value.

The operations for the Fraction type will allow a Fraction data object to behave like any other numeric value. We need to be able to add, subtract, multiply, and divide fractions. We also want to be able to show fractions using the standard “slash” form, for example 3/5. In addition, all fraction methods should return results in their lowest terms so that no matter what computation is performed, we always end up with the most common form.

In Python, we define a new class by providing a name and a set of method definitions that are syntactically similar to function definitions. For this example,

class Fraction:

   #the methods go here

provides the framework for us to define the methods. The first method that all classes should provide is the constructor. The constructor defines the way in which data objects are created. To create a Fraction object, we will need to provide two pieces of data, the numerator and the denominator. In Python, the constructor method is always called __init__ (two underscores before and after init) and is shown in Listing 2.

Listing 2

class Fraction:

    def __init__(self,top,bottom):

        self.num = top
        self.den = bottom

Notice that the formal parameter list contains three items (self, top, bottom). self is a special parameter that will always be used as a reference back to the object itself. It must always be the first formal parameter; however, it will never be given an actual parameter value upon invocation. As described earlier, fractions require two pieces of state data, the numerator and the denominator. The notation self.num in the constructor defines the fraction object to have an internal data object called num as part of its state. Likewise, self.den creates the denominator. The values of the two formal parameters are initially assigned to the state, allowing the new fraction object to know its starting value.

To create an instance of the Fraction class, we must invoke the constructor. This happens by using the name of the class and passing actual values for the necessary state (note that we never directly invoke __init__). For example,

myfraction = Fraction(3,5)

creates an object called myfraction representing the fraction \(\frac {3}{5}\) (three-fifths). Figure 5 shows this object as it is now implemented.


Figure 5: An Instance of the Fraction Class

The next thing we need to do is implement the behavior that the abstract data type requires. To begin, consider what happens when we try to print a Fraction object.

>>> myf = Fraction(3,5)
>>> print(myf)
<__main__.Fraction instance at 0x409b1acc>

The fraction object, myf, does not know how to respond to this request to print. The print function requires that the object convert itself into a string so that the string can be written to the output. The only choice myf has is to show the actual reference that is stored in the variable (the address itself). This is not what we want.

There are two ways we can solve this problem. One is to define a method called show that will allow the Fraction object to print itself as a string. We can implement this method as shown in Listing 3. If we create a Fraction object as before, we can ask it to show itself, in other words, print itself in the proper format. Unfortunately, this does not work in general. In order to make printing work properly, we need to tell the Fraction class how to convert itself into a string. This is what the print function needs in order to do its job.

Listing 3

def show(self):
>>> myf = Fraction(3,5)
3 / 5
>>> print(myf)
<__main__.Fraction instance at 0x40bce9ac>

In Python, all classes have a set of standard methods that are provided but may not work properly. One of these, __str__, is the method to convert an object into a string. The default implementation for this method is to return the instance address string as we have already seen. What we need to do is provide a “better” implementation for this method. We will say that this implementation overrides the previous one, or that it redefines the method’s behavior.

To do this, we simply define a method with the name __str__ and give it a new implementation as shown in Listing 4. This definition does not need any other information except the special parameter self. In turn, the method will build a string representation by converting each piece of internal state data to a string and then placing a / character in between the strings using string concatenation. The resulting string will be returned any time a Fraction object is asked to convert itself to a string. Notice the various ways that this function is used.

Listing 4

def __str__(self):
    return str(self.num)+"/"+str(self.den)
>>> myf = Fraction(3,5)
>>> print(myf)
>>> print("I ate", myf, "of the pizza")
I ate 3/5 of the pizza
>>> myf.__str__()
>>> str(myf)

We can override many other methods for our new Fraction class. Some of the most important of these are the basic arithmetic operations. We would like to be able to create two Fraction objects and then add them together using the standard “+” notation. At this point, if we try to add two fractions, we get the following:

>>> f1 = Fraction(1,4)
>>> f2 = Fraction(1,2)
>>> f1+f2

Traceback (most recent call last):
  File "<pyshell#173>", line 1, in -toplevel-
TypeError: unsupported operand type(s) for +:
          'instance' and 'instance'

If you look closely at the error, you see that the problem is that the “+” operator does not understand the Fraction operands.

We can fix this by providing the Fraction class with a method that overrides the addition method. In Python, this method is called __add__ and it requires two parameters. The first, self, is always needed, and the second represents the other operand in the expression. For example,


would ask the Fraction object f1 to add the Fraction object f2 to itself. This can be written in the standard notation, f1+f2.

Two fractions must have the same denominator to be added. The easiest way to make sure they have the same denominator is to simply use the product of the two denominators as a common denominator so that \(\frac {a}{b} + \frac {c}{d} = \frac {ad}{bd} + \frac {cb}{bd} = \frac{ad+cb}{bd}\) The implementation is shown in Listing 5. The addition function returns a new Fraction object with the numerator and denominator of the sum. We can use this method by writing a standard arithmetic expression involving fractions, assigning the result of the addition, and then printing our result.

Listing 5

def __add__(self,otherfraction):

     newnum = self.num*otherfraction.den + self.den*otherfraction.num
     newden = self.den * otherfraction.den

     return Fraction(newnum,newden)
>>> f1=Fraction(1,4)
>>> f2=Fraction(1,2)
>>> f3=f1+f2
>>> print(f3)

The addition method works as we desire, but one thing could be better. Note that \(6/8\) is the correct result (\(\frac {1}{4} + \frac {1}{2}\)) but that it is not in the “lowest terms” representation. The best representation would be \(3/4\). In order to be sure that our results are always in the lowest terms, we need a helper function that knows how to reduce fractions. This function will need to look for the greatest common divisor, or GCD. We can then divide the numerator and the denominator by the GCD and the result will be reduced to lowest terms.

The best-known algorithm for finding a greatest common divisor is Euclid’s Algorithm, which will be discussed in detail in Chapter 8. Euclid’s Algorithm states that the greatest common divisor of two integers \(m\) and \(n\) is \(n\) if \(n\) divides \(m\) evenly. However, if \(n\) does not divide \(m\) evenly, then the answer is the greatest common divisor of \(n\) and the remainder of \(m\) divided by \(n\). We will simply provide an iterative implementation here (see ActiveCode 11). Note that this implementation of the GCD algorithm only works when the denominator is positive. This is acceptable for our fraction class because we have said that a negative fraction will be represented by a negative numerator.

The Greatest Common Divisor Function (gcd_cl)

Now we can use this function to help reduce any fraction. To put a fraction in lowest terms, we will divide the numerator and the denominator by their greatest common divisor. So, for the fraction \(6/8\), the greatest common divisor is 2. Dividing the top and the bottom by 2 creates a new fraction, \(3/4\) (see Listing 6).

Listing 6

def __add__(self,otherfraction):
    newnum = self.num*otherfraction.den + self.den*otherfraction.num
    newden = self.den * otherfraction.den
    common = gcd(newnum,newden)
    return Fraction(newnum//common,newden//common)
>>> f1=Fraction(1,4)
>>> f2=Fraction(1,2)
>>> f3=f1+f2
>>> print(f3)

Figure 6: An Instance of the Fraction Class with Two Methods

Our Fraction object now has two very useful methods and looks like Figure 6. An additional group of methods that we need to include in our example Fraction class will allow two fractions to compare themselves to one another. Assume we have two Fraction objects, f1 and f2. f1==f2 will only be True if they are references to the same object. Two different objects with the same numerators and denominators would not be equal under this implementation. This is called shallow equality (see Figure 7).


Figure 7: Shallow Equality Versus Deep Equality

We can create deep equality (see Figure 7)–equality by the same value, not the same reference–by overriding the __eq__ method. The __eq__ method is another standard method available in any class. The __eq__ method compares two objects and returns True if their values are the same, False otherwise.

In the Fraction class, we can implement the __eq__ method by again putting the two fractions in common terms and then comparing the numerators (see Listing 7). It is important to note that there are other relational operators that can be overridden. For example, the __le__ method provides the less than or equal functionality.

Listing 7

def __eq__(self, other):
    firstnum = self.num * other.den
    secondnum = other.num * self.den

    return firstnum == secondnum

The complete Fraction class, up to this point, is shown in ActiveCode 12. We leave the remaining arithmetic and relational methods as exercises.

The Fraction Class (fraction_class)

Self Check

To make sure you understand how operators are implemented in Python classes, and how to properly write methods, write some methods to implement *, /, and - . Also implement comparison operators > and <


Inheritance: Logic Gates and Circuits

Our final section will introduce another important aspect of object-oriented programming. Inheritance is the ability for one class to be related to another class in much the same way that people can be related to one another. Children inherit characteristics from their parents. Similarly, Python child classes can inherit characteristic data and behavior from a parent class. These classes are often referred to as subclasses and superclasses.

Figure 8 shows the built-in Python collections and their relationships to one another. We call a relationship structure such as this an inheritance hierarchy. For example, the list is a child of the sequential collection. In this case, we call the list the child and the sequence the parent (or subclass list and superclass sequence). This is often referred to as an IS-A Relationship (the list IS-A sequential collection). This implies that lists inherit important characteristics from sequences, namely the ordering of the underlying data and operations such as concatenation, repetition, and indexing.


Figure 8: An Inheritance Hierarchy for Python Collections

Lists, tuples, and strings are all types of sequential collections. They all inherit common data organization and operations. However, each of them is distinct based on whether the data is homogeneous and whether the collection is immutable. The children all gain from their parents but distinguish themselves by adding additional characteristics.

By organizing classes in this hierarchical fashion, object-oriented programming languages allow previously written code to be extended to meet the needs of a new situation. In addition, by organizing data in this hierarchical manner, we can better understand the relationships that exist. We can be more efficient in building our abstract representations.

To explore this idea further, we will construct a simulation, an application to simulate digital circuits. The basic building block for this simulation will be the logic gate. These electronic switches represent boolean algebra relationships between their input and their output. In general, gates have a single output line. The value of the output is dependent on the values given on the input lines.

AND gates have two input lines, each of which can be either 0 or 1 (representing False or True, repectively). If both of the input lines have the value 1, the resulting output is 1. However, if either or both of the input lines is 0, the result is 0. OR gates also have two input lines and produce a 1 if one or both of the input values is a 1. In the case where both input lines are 0, the result is 0.

NOT gates differ from the other two gates in that they only have a single input line. The output value is simply the opposite of the input value. If 0 appears on the input, 1 is produced on the output. Similarly, 1 produces 0. Figure 9 shows how each of these gates is typically represented. Each gate also has a truth table of values showing the input-to-output mapping that is performed by the gate.


Figure 9: Three Types of Logic Gates

By combining these gates in various patterns and then applying a set of input values, we can build circuits that have logical functions. Figure 10 shows a circuit consisting of two AND gates, one OR gate, and a single NOT gate. The output lines from the two AND gates feed directly into the OR gate, and the resulting output from the OR gate is given to the NOT gate. If we apply a set of input values to the four input lines (two for each AND gate), the values are processed and a result appears at the output of the NOT gate. Figure 10 also shows an example with values.


Figure 10: Circuit

In order to implement a circuit, we will first build a representation for logic gates. Logic gates are easily organized into a class inheritance hierarchy as shown in Figure 11. At the top of the hierarchy, the LogicGate class represents the most general characteristics of logic gates: namely, a label for the gate and an output line. The next level of subclasses breaks the logic gates into two families, those that have one input line and those that have two. Below that, the specific logic functions of each appear.


Figure 11: An Inheritance Hierarchy for Logic Gates

We can now start to implement the classes by starting with the most general, LogicGate. As noted earlier, each gate has a label for identification and a single output line. In addition, we need methods to allow a user of a gate to ask the gate for its label.

The other behavior that every logic gate needs is the ability to know its output value. This will require that the gate perform the appropriate logic based on the current input. In order to produce output, the gate needs to know specifically what that logic is. This means calling a method to perform the logic computation. The complete class is shown in Listing 8.

Listing 8

class LogicGate:

    def __init__(self,n):
        self.label = n
        self.output = None

    def getLabel(self):
        return self.label

    def getOutput(self):
        self.output = self.performGateLogic()
        return self.output

At this point, we will not implement the performGateLogic function. The reason for this is that we do not know how each gate will perform its own logic operation. Those details will be included by each individual gate that is added to the hierarchy. This is a very powerful idea in object-oriented programming. We are writing a method that will use code that does not exist yet. The parameter self is a reference to the actual gate object invoking the method. Any new logic gate that gets added to the hierarchy will simply need to implement the performGateLogic function and it will be used at the appropriate time. Once done, the gate can provide its output value. This ability to extend a hierarchy that currently exists and provide the specific functions that the hierarchy needs to use the new class is extremely important for reusing existing code.

We categorized the logic gates based on the number of input lines. The AND gate has two input lines. The OR gate also has two input lines. NOT gates have one input line. The BinaryGate class will be a subclass of LogicGate and will add two input lines. The UnaryGate class will also subclass LogicGate but will have only a single input line. In computer circuit design, these lines are sometimes called “pins” so we will use that terminology in our implementation.

Listing 9

class BinaryGate(LogicGate):

    def __init__(self,n):

        self.pinA = None
        self.pinB = None

    def getPinA(self):
        return int(input("Enter Pin A input for gate "+ self.getLabel()+"-->"))

    def getPinB(self):
        return int(input("Enter Pin B input for gate "+ self.getLabel()+"-->"))

Listing 10

class UnaryGate(LogicGate):

    def __init__(self,n):
        LogicGate.__init__(self,n) = None

    def getPin(self):
        return int(input("Enter Pin input for gate "+ self.getLabel()+"-->"))

Listing 9 and Listing 10 implement these two classes. The constructors in both of these classes start with an explicit call to the constructor of the parent class using the super function. When creating an instance of the BinaryGate class, we first want to initialize any data items that are inherited from LogicGate. In this case, that means the label for the gate. The constructor then goes on to add the two input lines (pinA and pinB). This is a very common pattern that you should always use when building class hierarchies. Child class constructors need to call parent class constructors and then move on to their own distinguishing data.

The only behavior that the BinaryGate class adds is the ability to get the values from the two input lines. Since these values come from some external place, we will simply ask the user via an input statement to provide them. The same implementation occurs for the UnaryGate class except that there is only one input line.

Now that we have a general class for gates depending on the number of input lines, we can build specific gates that have unique behavior. For example, the AndGate class will be a subclass of BinaryGate since AND gates have two input lines. As before, the first line of the constructor calls upon the parent class constructor (BinaryGate), which in turn calls its parent class constructor (LogicGate). Note that the AndGate class does not provide any new data since it inherits two input lines, one output line, and a label.

Listing 11

class AndGate(BinaryGate):

    def __init__(self,n):

    def performGateLogic(self):

        a = self.getPinA()
        b = self.getPinB()
        if a==1 and b==1:
            return 1
            return 0

The only thing AndGate needs to add is the specific behavior that performs the boolean operation that was described earlier. This is the place where we can provide the performGateLogic method. For an AND gate, this method first must get the two input values and then only return 1 if both input values are 1. The complete class is shown in Listing 11.

We can show the AndGate class in action by creating an instance and asking it to compute its output. The following session shows an AndGate object, g1, that has an internal label "G1". When we invoke the getOutput method, the object must first call its performGateLogic method which in turn queries the two input lines. Once the values are provided, the correct output is shown.

>>> g1 = AndGate("G1")
>>> g1.getOutput()
Enter Pin A input for gate G1-->1
Enter Pin B input for gate G1-->0

The same development can be done for OR gates and NOT gates. The OrGate class will also be a subclass of BinaryGate and the NotGate class will extend the UnaryGate class. Both of these classes will need to provide their own performGateLogic functions, as this is their specific behavior.

We can use a single gate by first constructing an instance of one of the gate classes and then asking the gate for its output (which will in turn need inputs to be provided). For example:

>>> g2 = OrGate("G2")
>>> g2.getOutput()
Enter Pin A input for gate G2-->1
Enter Pin B input for gate G2-->1
>>> g2.getOutput()
Enter Pin A input for gate G2-->0
Enter Pin B input for gate G2-->0
>>> g3 = NotGate("G3")
>>> g3.getOutput()
Enter Pin input for gate G3-->0

Now that we have the basic gates working, we can turn our attention to building circuits. In order to create a circuit, we need to connect gates together, the output of one flowing into the input of another. To do this, we will implement a new class called Connector.

The Connector class will not reside in the gate hierarchy. It will, however, use the gate hierarchy in that each connector will have two gates, one on either end (see Figure 12). This relationship is very important in object-oriented programming. It is called the HAS-A Relationship. Recall earlier that we used the phrase “IS-A Relationship” to say that a child class is related to a parent class, for example UnaryGate IS-A LogicGate.


Figure 12: A Connector Connects the Output of One Gate to the Input of Another

Now, with the Connector class, we say that a Connector HAS-A LogicGate meaning that connectors will have instances of the LogicGate class within them but are not part of the hierarchy. When designing classes, it is very important to distinguish between those that have the IS-A relationship (which requires inheritance) and those that have HAS-A relationships (with no inheritance).

Listing 12 shows the Connector class. The two gate instances within each connector object will be referred to as the fromgate and the togate, recognizing that data values will “flow” from the output of one gate into an input line of the next. The call to setNextPin is very important for making connections (see Listing 13). We need to add this method to our gate classes so that each togate can choose the proper input line for the connection.

Listing 12

class Connector:

    def __init__(self, fgate, tgate):
        self.fromgate = fgate
        self.togate = tgate


    def getFrom(self):
        return self.fromgate

    def getTo(self):
        return self.togate

In the BinaryGate class, for gates with two possible input lines, the connector must be connected to only one line. If both of them are available, we will choose pinA by default. If pinA is already connected, then we will choose pinB. It is not possible to connect to a gate with no available input lines.

Listing 13

def setNextPin(self,source):
    if self.pinA == None:
        self.pinA = source
        if self.pinB == None:
            self.pinB = source
           raise RuntimeError("Error: NO EMPTY PINS")

Now it is possible to get input from two places: externally, as before, and from the output of a gate that is connected to that input line. This requires a change to the getPinA and getPinB methods (see Listing 14). If the input line is not connected to anything (None), then ask the user externally as before. However, if there is a connection, the connection is accessed and fromgate’s output value is retrieved. This in turn causes that gate to process its logic. This continues until all input is available and the final output value becomes the required input for the gate in question. In a sense, the circuit works backwards to find the input necessary to finally produce output.

Listing 14

def getPinA(self):
    if self.pinA == None:
        return input("Enter Pin A input for gate " + self.getName()+"-->")
        return self.pinA.getFrom().getOutput()

The following fragment constructs the circuit shown earlier in the section:

>>> g1 = AndGate("G1")
>>> g2 = AndGate("G2")
>>> g3 = OrGate("G3")
>>> g4 = NotGate("G4")
>>> c1 = Connector(g1,g3)
>>> c2 = Connector(g2,g3)
>>> c3 = Connector(g3,g4)

The outputs from the two AND gates (g1 and g2) are connected to the OR gate (g3) and that output is connected to the NOT gate (g4). The output from the NOT gate is the output of the entire circuit. For example:

>>> g4.getOutput()
Pin A input for gate G1-->0
Pin B input for gate G1-->1
Pin A input for gate G2-->1
Pin B input for gate G2-->1

Try it yourself using ActiveCode 14.

The Complete Circuit Program. (complete_cuircuit)

Self Check

Create a two new gate classes, one called NorGate the other called NandGate. NandGates work like AndGates that have a Not attached to the output. NorGates work lake OrGates that have a Not attached to the output.

Create a series of gates that prove the following equality NOT (( A and B) or (C and D)) is that same as NOT( A and B ) and NOT (C and D). Make sure to use some of your new gates in the simulation.



  • Computer science is the study of problem solving.
  • Computer science uses abstraction as a tool for representing both processes and data.
  • Abstract data types allow programmers to manage the complexity of a problem domain by hiding the details of the data.
  • Python is a powerful, yet easy-to-use, object-oriented language.
  • Lists, tuples, and strings are built in Python sequential collections.
  • Dictionaries and sets are nonsequential collections of data.
  • Classes allow programmers to implement abstract data types.
  • Programmers can override standard methods as well as create new methods.
  • Classes can be organized into hierarchies.
  • A class constructor should always invoke the constructor of its parent before continuing on with its own data and behavior.

Key Terms

abstract data type abstraction algorithm
class computable data abstraction
data structure data type deep equality
dictionary encapsulation exception
format operator formatted strings HAS-A relationship
implementation-independent information hiding inheritance
inheritance hierarchy interface IS-A relationship
list list comprehension method
mutability object procedural abstraction
programming prompt self
shallow equality simulation string
subclass superclass truth table

Discussion Questions

  1. Construct a class hierarchy for people on a college campus. Include faculty, staff, and students. What do they have in common? What distinguishes them from one another?
  2. Construct a class hierarchy for bank accounts.
  3. Construct a class hierarchy for different types of computers.
  4. Using the classes provided in the chapter, interactively construct a circuit and test it.

Programming Exercises

  1. Implement the simple methods getNum and getDen that will return the numerator and denominator of a fraction.
  2. In many ways it would be better if all fractions were maintained in lowest terms right from the start. Modify the constructor for the Fraction class so that GCD is used to reduce fractions immediately. Notice that this means the __add__ function no longer needs to reduce. Make the necessary modifications.
  3. Implement the remaining simple arithmetic operators (__sub__, __mul__, and __truediv__).
  4. Implement the remaining relational operators (__gt__, __ge__, __lt__, __le__, and __ne__)
  5. Modify the constructor for the fraction class so that it checks to make sure that the numerator and denominator are both integers. If either is not an integer the constructor should raise an exception.
  6. In the definition of fractions we assumed that negative fractions have a negative numerator and a positive denominator. Using a negative denominator would cause some of the relational operators to give incorrect results. In general, this is an unnecessary constraint. Modify the constructor to allow the user to pass a negative denominator so that all of the operators continue to work properly.
  7. Research the __radd__ method. How does it differ from __add__? When is it used? Implement __radd__.
  8. Repeat the last question but this time consider the __iadd__ method.
  9. Research the __repr__ method. How does it differ from __str__? When is it used? Implement __repr__.
  10. Research other types of gates that exist (such as NAND, NOR, and XOR). Add them to the circuit hierarchy. How much additional coding did you need to do?
  11. The most simple arithmetic circuit is known as the half-adder. Research the simple half-adder circuit. Implement this circuit.
  12. Now extend that circuit and implement an 8 bit full-adder.
  13. The circuit simulation shown in this chapter works in a backward direction. In other words, given a circuit, the output is produced by working back through the input values, which in turn cause other outputs to be queried. This continues until external input lines are found, at which point the user is asked for values. Modify the implementation so that the action is in the forward direction; upon receiving inputs the circuit produces an output.
  14. Design a class to represent a playing card. Now design a class to represent a deck of cards. Using these two classes, implement a favorite card game.
  15. Find a Sudoku puzzle in the local newspaper. Write a program to solve the puzzle.