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Ace the AP Computer Science Exam for High Schoolers

Solution Review: Mirror a 2D Array

Understand how to create a mirror image of a 2D array by learning how to copy arrays correctly, traverse them in row-major order, and swap elements efficiently. This lesson breaks down the mirror function implementation step-by-step to help you confidently handle 2D array transformations in Java.

We'll cover the following...

Rubric criteria

Solution

Java
class MirrorImage 
{
    public static void main(String args[]) 
    {
        int[][] array = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
        int[][] result = new int[array.length][array[0].length];
        result = mirror(array);                                      // Calling the method
        // Printing the mirror image
        for(int i = 0; i < result.length; i++)          
        {
            for(int j = 0; j < result[i].length; j++)   
            {
                System.out.print(result[i][j]);         
                System.out.print(" ");                 
            }
            System.out.print("\n");                    
        }
    }
    // Method to mirror the 2D array
    public static int[][] mirror(int[][] origArray)
    {
        int[][] array = new int[origArray.length][origArray[0].length]; // Creating new array to avoid overriding                 
        // Copying the elements
        for(int i = 0; i < array.length; i++)
        {
            for(int j = 0; j < array[i].length; j++)
            {
                array[i][j] = origArray[i][j];
            }
        }
        for( int i = 0; i < array.length; i++ )             // Moving row-wise
        {
            for( int j = 0; j < (array[i].length)/2; j++)   // Moving upto the middle - columnn-wise in a row
            {
                if(j != array[i].length-1-j)                // Swapping the columns' values
                {
                    int temp = array[i][j];
                    array[i][j] = array[i][array[i].length-1-j];
                    array[i][array[i].length-1-j] = temp;
                }
            }
        }
        return array;
    }
}

Rubric-wise explanation

Point 1:

Look at line 23. We have the header of the mirror function. It takes an integer 2D array (int[][]) as a parameter and also returns an integer 2D array (int[][]).

Point 2:

In lines 28-34, we create a copy of the origArray (argument to the method) because we don’t want to change it.

❌ Note: We can’t do array = origArray. Because this statement will make array point to origArray. So, any change in array will also be reflected outside the function.

Point 3:

Look at line 26. We have an outer for ...