Iterating through 2D arrays

As promised in our 2D array lesson, we're now going to cover how to iterate over 2D arrays using loops.

For Loop

Iterating across 2D arrays has trickier syntax, but it's very simple. Let's look at an example:

public class Learn {
    public static void main(String[] args) throws InterruptedException {

        int[][] arr = new int[3][6]; // 3x6.
        // Can be viewed as 3 rows and 6 columns
        // Or, 3 sets of data that contain 6 data entries

        System.out.println("Let's assign some values...");
        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr[i].length; j++) {
                arr[i][j] = j;
            }
        }

        // totally optional. Just makes output a little cooler!
        // This tells the program to 'sleep' for a number of milliseconds
        // In this example, 1000 millis, which is 1 second.
        Thread.sleep(1000);

        System.out.println("Elements are:");
        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr[i].length; j++) {
                System.out.println(i + " " + arr[i][j]);
            }
        }
    }
}

We've highlighted the lines that you should pay attention to. Remember how we used i to iterate over an array? Since it's just a variable name, you can name it whatever you want. i is just a convention. For inner loops, or loops inside loops, we need a different variable name, since they will iterate differently.

Here's the output of the above code. See if you can understand what it.

Let's assign some values...
Elements are:
0 0
0 1
0 2
0 3
0 4
0 5

1 0
1 1
1 2
1 3
1 4
1 5

2 0
2 1
2 2
2 3
2 4
2 5

The output tells us what the value of i and j are during each iteration. Essentially, the inner loop runs completely for every value of i. Once the inner loop reaches the end, i increases by 1, and the inner loop iterates again. And so on, until the outer loop is complete. That's why the second number (j) changes with every print, but the first number (i) changes only when the inner loop completes.

For-each loop

for-each syntax is different when iterating over 2D arrays. Let's look at a quick example and break it down. Rather than declare then initialize an array, we'll do it on the same line, which makes our explanation easier to understand.

public class Learn {
    public static void main(String[] args){

        int[][] arr = {{1,2,3}, {7,8,9}};

        for (int[] i: arr) {
            for (int j: i) {
                System.out.print(j + " ");
            }
        }
    }
}

Let's break this down line by line.

int[][] arr = {{1,2,3}, {7,8,9}}; declares and initializes a 2D array on the same line

int[] i: arr accesses element i of arr, where the type of i is an integer array, or int[]. Looking at the above line, we see that each element of the outer array is also an array, so this makes sense.

int j: i accesses element i of the array i, where the type of j is an integer. Why not type int[] like above? Simply because we are not looking at the elements of the inner arrays, which are simply ints.

Using for-each with 2D arrays is confusing for some, so it's okay to stick with regular for loops. But, if you understand how to write them, we recommend it. The syntax is much more succinct and readable.

Simple method to print 2D array

If you don't want to write out a loop, and simply want to print the values of your 2D array, there's a built-in method to help you with this.

import java.util.Arrays;

public class Learn {
    public static void main(String[] args){

        int[][] arr = {{1,2,3}, {7,8,9}};

        System.out.println(Arrays.deepToString(arr));
    }
}

// [[1, 2, 3], [7, 8, 9]]

.deepToString(arrName) is a quick and easy way to print the values of your 2D array. However, it's much less flexible that looping, and is only useful if you want a preformatted list of your array values.