📓4.12: 2D Array Traversals

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 4.12

2D Array Traversals: Nested Loops

In this lesson, you will learn how to use nested loops to traverse 2D arrays.

Nested Loops

Nested iteration statements (loops) are used to traverse and access all or an ordered sequence of elements in a 2D array.
Since 2D arrays are stored as arrays of arrays, the way 2D arrays are traversed using for loops and enhanced for loops is similar to 1D arrays.

Here is an example of a nested for loop to traverse all elements of a 2D array:

int[][] array = { {1, 2, 3}, {4, 5, 6} };
for (int row = 0; row < array.length; row++) {
    for (int col = 0; col < array[0].length; col++) {
        System.out.println(array[row][col]);
    }
}

Coding Exercise: Calculating Average in a 2D Array

Instructions:

  1. Review the code below — it calculates the average of all elements in a 2D array.
  2. Add another row of numbers to matrix. Will the loops traverse this row too?
  3. Run and observe.
  4. Use CodeLens to trace through the code and confirm.
public class Test1 {
    public static double getAverage(int[][] a) {
        double total = 0;
        int value = 0;
        for (int row = 0; row < a.length; row++) {
            for (int col = 0; col < a[0].length; col++) {
                value = a[row][col];
                total = total + value;
            }
        }
        return total / (a.length * a[0].length);
    }

    public static void main(String[] args) {
        int[][] matrix = { {1, 2, 3}, {4, 5, 6} };
        System.out.println(getAverage(matrix));
    }
}

Traversing with Enhanced For Loops

You can also use enhanced for loops to traverse 2D arrays. Each row is itself a 1D array, and the inner loop iterates over each element in that row.

int[][] array = { {1, 2, 3}, {4, 5, 6} };
for (int[] row : array) {
    for (int value : row) {
        System.out.println(value);
    }
}

Challenge: Find the Maximum Value

Instructions: Write a method findMax that takes a 2D array of integers and returns the largest value.

public static int findMax(int[][] arr) {
    // Your code here
}

Test with:

int[][] nums = { {3, 5, 1}, {4, 8, 2} };
System.out.println(findMax(nums)); // Expected: 8

Summary

  • Nested loops are used to traverse 2D arrays.
  • Outer loop goes over rows, inner loop goes over columns.
  • Enhanced for loops can simplify traversal when you don’t need row/column indices.
  • Be aware of the dimensions: array.length gives rows, array[0].length gives columns.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.