The Birthday Problem

Yesterday, in class, I asked the question “How many of you have the same birthday?” We went through the months of the year, and if a student had a birthday in that month they raised their hand. When we had multiple birthdays in the month we all shouted out our day to see if we had any matches. I had 23 students in the classroom, and we had no matches. Does this surprise you? Probably not, after all there are 365 days in the year and only 23 students in the room, so the probability of a match seems low, at least at first glance. This raises the question, what is the probability that in any given class of 23 students there will be at least two students with a birthday on the same day.

To answer this question we will avoid probability theory and use a Monte-Carlo simulation.,C6PBkg2KoOZSmqU-rme02DzVOcgdKId4xVbkByyjmwAlMK1UU7nrMH7rFd1fA4rNcmKjXK-D3dborhCQ64NfUERLsWIXLbxovaCZHue0lXKdyO8NiueTnd2Ui5ZFR_zIuhCAgltNNP5wYYjZKwioTxGDaI3acRYWhiyRWxV-7PP38DIqDR31vKbS46oNo43RslkUdcQ9

Monte Carlo is known for its racing, and its casinos, James bond: “Shaken not stirred.” Its the casino part that is important here, games of chance, and randomness. Monaco is the second smallest country in the world, second only to Vatican city, and is located on the French Riviera and, in fact, is surrounded by France.

The way we will figure out the probability that 2 or more people out of 23 share a birthday is to randomly generate 23 birthdays and see whether any of them match. If we do this many many times, then the number of times we get a matching birthday, divided by the total number of times we try gives us a good approximation of the probability. We will call each of these separate tests a trial. Lets say that we try 1000 trials. In 490 of the trials we find a duplicate birthday. So, $490/1000 = 0.49$

So, how are we going to approach this calculation. If we were a statistician we would go a completely different direction, but since we are computer scientists, we are going to do a simulation. The first question we need to ask is how are we going to represent, and remember each of the birthdays for each class member? Representation is an important aspect of problem solving, so we’ll spend a few words talking about our representation for this problem.

It would be difficult to try to represent the birthdays in normal month/day format, so we will use an easier representation. That is the number of the day of the year. If we think of January 1 as day 0 (Yes, we computer scientists insist on starting at 0) then December 31 is day 364. So we can represent each birthday as a single integer number.

The next representation question is how are we going to remember all of these birthdays? The answer is that we will use a Python list. If you are completely new to Python lists, you may want to look at this chapter. In any case we can think of a Python list just like any list we might write down for ourselves. If we were going to make up 23 birthdates and we wanted to remember them, we would write them on a sheet of paper, one birthday per line. A Python list is analogous to our paper list but we will refer to each line of our list as an entry in a Python list.

Counting Birthdays using Count

Let us start out simple, and look at how we can use the random module along with a single list method, append to construct a list of birthdays. Here is the code:

So, now we have a list of randomly generated birthdays, but chances are that finding a duplicate on this list is still not an easy task, even using our human powers of observation. It would be even worse if we used a class size of 100. So how can we have Python do the work for us of deciding whether there is a shared birthday on the list. One easy solution is to use the list count method. All we have to do is ask python to systematically go through the list and count the number of times each birthday appears on the list. If we ever find one that appears more than once then we know we have found a duplicate.

Here’s the code for this part of the problem:

foundDupe = False
for num in birthdayList:
    if birthdayList.count(num) > 1:
       foundDupe = True

Now this is a very interesting pattern that you will use to solve programming problems many times. The pattern is: Assume something is False, then systematically check to see if what we assumed can be found to be True. In this case we assume that there are no duplicates foundDupe = False, but then we check every number on the birthday list to see if it appears more than once. If it does then we know our original assumtion was wrong and we remember that by foundDupe = True.

What remains to do is to wrap both the birthday list creation loop, and the duplicate checking loop inside an outer loop that can do this experiment many times. Each time foundDupe is True we know we have a shared birthday, so for a large number of trials foundDupe/numTrials gives us an estimate of the probability that two people share the same birthday.


  1. Try running this program for different class sizes to see what kind of results you get. How large would the class need to be in order for the probability to exceed 0.9 ?
  2. Try the program above using a larger number of trials and a smaller number of trials Run it several each way. What do you notice about the consistency of the answers as you increase the number of trials?
  3. Modify the program above to print out a table of class size and probability. You will need to add yet another for loop around the for trial in range(numTrials) loop.

Counting Birthdays using Indexing

Lets look at another way of keeping track of the birthdays in our class. Rather than keeping a list of the day numbers, lets suppose we make a list that has 365 slots. Each of the slots in the list represents a day of the year. In fact lets call this list year. Now year[0] represents January 1, the first day of the year. similarly year[364] represents December 31. The square brackets after the list name are the index operator and allow us to access the value that is stored in that slot of the list. For this problem we will start out with a zero in every location. When we generate a random birthday we will update the count of birthdays on that day in the list by one. Lets look at the new code for generating a random birthday list using this method.

This approach makes it easier for us humans to quickly scan the list for a duplicate. If we spot a number 2 or larger its easy to see that there is. The line year = [0]*365 uses the Python repetition operator to create a list with 365 zeros.

We can continue our two phase approach to finding a duplicate birthday by simply iterating over every number in year looking for a number larger than 1.

We can actually make our program much shorter, and find a duplicate in a single pass by rearranging the code just slightly.

Using this new representation, allows us to check for a duplicate while we are generating the random birthdays! This is a bit more efficient than our previous approach. We could make this approach even more efficient by adding a break statement right after the foundDupe = True line. The break statement breaks the loop essentially causing it to skip the rest of the class once we have determined that there is a birthday. Personally, I’m not a big believer in using breaks, so I’m not going to include it in the body of the code. I think they are confusing, and I can never remember what exactly they break. If you know about while loops, you could re-write the above example to get the same behavior as using a break by using a compound condition on the while loop. This is much more clear, and is always my preferred way of doing things.

Calculating the Probability using.... Probability

Although this post has been mostly about using the Monte-Carlo simulation method to approximate the probability of a duplicate birthday, lets look at just a tiny bit of probability theory, and apply the accumulator pattern in another setting to check our simulation.

Now, calculating the probability of a duplicate birthday may seem like a daunting task. But what about calculating the probability that there is not a duplicate birthday? This is actually an easier task. Especially if we simplify the problem to a very small class.

Let us assume that the class only has one student. There is a 100% chance that this person does not share a birthday since there is not anyone else in the class. But now lets add a second person to the class. What are the chances that they have a different birthday that person one? In fact this is pretty easy, there are 364 other days in the year so the chances are 364/365. How about we add a third person to the class? Now there are 363/365 days. To get the overall probability that there are no shared birthdays we just multiply the individual probabilties together. So for a class of three the probability of no shared birthdays is 365/365 * 364/365 * 363/365 which is .99 or a 99% chance that there are no shared birthdays among the three classmates.

The important thing is the pattern. We can now easily calculate the probability of no shared birthdays using a for loop.

Ok, but what about the probability that there is a shared birthday? In fact this is quite easy as well. Because remember that there are only two possibilities here. 1 is that there is a shared birthday. The other that there is not a shared birthday. By definition these two probabilities must add up to 1.0. So once we have calculated the probability that there is not a shared birthday we can easily calculate the probability that there is as 1 - prob.


  1. Modify the code above to print out the probability that there is a shared birthday.
  2. Modify the code above to print out a table of probabilities.
  3. Modify the code above to compute the probabilities using either of the first two methods, along with the last method, and compare the results. How close does our Monte-carlo simulation come to the value using probability theory?
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