Getting the Number of Rows and Columns

Arrays know their length (how many elements they can store). It is a public read-only field so you can use dot-notation to access the field (arrayName.length). The length of the outer array is the number of rows and the length of one of the inner arrays is the number of columns.

Note

Note that length is a field and not a method, so you don’t add parentheses after length. However, if you use parentheses after length during the exam, you won’t lose any points.

ticketInfo.length // returns the number of rows
ticketInfo[0].length // returns the number of columns

Note

Since for the AP CS A exam all two-dimensional arrays are rectangular arrays (arrays that have the same number of columns in each row) you can just use the length of the first inner array as the number of columns as shown by ticketInfo[0].length.

Check your understanding

Looping Through a 2D Array

Since you can find out the number of rows and columns in a 2D array you can use a nested for loop (one loop inside of another loop) to loop through all of the elements of a 2D array.

Some key things to notice about this code are:

You can step through the code by clicking on this link1

Mixed up programs

        9-3-3: The following has the correct code to find the largest value in a 2D array. Drag the blocks from the left into the correct order on the right and indent them as well. Check your solution by clicking on the Check Me button.  You will be told if any of the blocks are in the wrong order or have the wrong indention.public static int getLargest(int[][] arr)  {
---
 int largest = arr[0][0];
 int current = 0;
 for (int r = 0; r < arr.length; r++)  {
 ---
   for (int c = 0; c < arr[0].length; c++)  {
 ---
     current = arr[r][c];
     if (current > largest)  {
 ---
       largest = current;
 ---
     } // end if
 ---
   } // end column loop
 ---
 } // end row loop
 return largest;
---
} // end method

You can step through this code using the Java Visualizer by clicking on the following link2

Use a For-Each to Loop Through an Array

Since 2D arrays are really arrays of arrays you can also use a nested for-each loop to loop through all elements in an array. Loop through each of the inner arrays and loop through all the values in each inner array.

In this case the for (int[] colArray : a) means to loop through each element of the outer array which will set colArray to the current column array. Then you can loop through the value in the column array.

You can step through this code using the Java Visualizer by clicking on the following link3

Next Section - Loop Through Part of a 2D Array