Processing Two Dimensional Array

Nest for loops are often used to process a two dimensional array.

Suppose an array matrix is made as follows:

int[][] matrix = new int[10][10];

The following are some examples of processing two dimensional arrays:

1. Initializing arrays values by User Input.

The following loop initializes the array with user input values:

java.util.Scanner input = new Scanner(System.in);
System.out.println("Enter " + matrix.length + " rows and " + matrix[0].length + " columns: ");
for (int row = 0; row < matrix.length; row++) {
    for (int column = 0; column < matrix[row].length; column++) {
        matrix[row][column] = input.nextInt();
    }
}

2. Initializing arrays with random values.

The following loop initializes the array with random values between 0 and 99:

for (int row = 0; row < matrix.length; row++) {
    for (int column = 0; column < matrix[row].length; column++) {
        matrix[row][column] = (int)(Math.random() * 100);
    }
}

3. Printing arrays.

To print a two-dimensional array, you have to print each element in the array using a loop like the following:

for (int row = 0; row < matrix.length; row++) {
    for (int column = 0; column < matrix[row].length; column++) {
        System.out.print(matrix[row][column] + " ");
    }
    
    System.out.println();
}

4. Summing all elements.

Use a variable named total to store the sum. Initially total is 0. Add each element in the array to total using a loop like this:

int total = 0;
for (int row = 0; row < matrix.length; row++) {
    for (int column = 0; column < matrix[row].length; column++) {
    total += matrix[row][column];
    }
}

5. Summing elements by column.

For each column, use a variable named total to store its sum. Add each element in the column to total using using a loop like this:

for (int column = 0; column < matrix[0].length; column++) {
    int total = 0;
    for (int row = 0; row < matrix.length; row++)
        total += matrix[row][column];
    System.out.println("Sum for column " + column + " is " + total);
}

*Notice that column & row switched top & bottom order between examples 4. & 5. (this one)

6. Which row has the largest sum?

Us variables maxRow and indexOfMaxRow to track the largest sum and the index of the row. For each row, compute its sum and update maxRow and indexOfMaxRow if the new sum is greater.

int maxRow = 0;
int indexOfMaxRow 0;

// Get sum of the first row in maxRow
for (int column = 0; column < matrix[0].length; column++) {
    maxRow += matrix[0][column];
}

//Finding the total value of a row, for all rows
for (int row = 1; row < matrix.length; row++) {
    int totalOfThisRow = 0;
    for (int column = 0; column < matrix[row].length; column++)
        totalOfThisRow += matrix[row][column];
    
    //Compare totalOfThisRow to maxRow so that row with the largest total sum is selected after evaluation
    if (totalOfThisRow > maxRow) {
        maxRow = totalOfThisRow;
        indexOfMaxRow = row;
    }
}

System.out.println("Row " + indexOfMaxRow + " has the madimum sum of " + maxRow);

7. Random shuffling.

Shuffling the elements in a one-dimensional array was introduced in ___ . How do you shuffle all the elements in a two dimensional array? To accomplish this, for each element matrix[i][j], randomly generate indices i1 and j1 and swap matrix[i][j] with matrix[i1][j1], as follows:

for (int i = 0; i < matrix. length; i++) {
    for (int j = 0; j < matrix[i].length; j++) {
        int i1 = (int)(Math.random() * matrix.length);
        int j1 = (int)(Math.random() * matrix[i].length);
        
    // Swap matrix[i][j] with matrix[i1][j1]
    int temp = matrix[i][j];
    matrix[i][j] = matrix[i1][j1];
    matrix[i1][j1] = temp;
    }
}

Passing Two Dimensional Arrays to Methods

When passing a two dimensional array to a method, the reference of the array is passed to the method.

You can pass a two dimensional array to a method just as you pass a one dimensional array. You can also return an array from a method. The example below shows 2 methods. The first method, getArray(), returns a two dimensional array, and the second method, sum(int[][] m), returns the sum of all the elements in a matrix.

import java.util.Scanner;

public class PassTwoDimensionalArray {
    public static void main(String[] args) {
        int[][] m = getArray(); //Get an array

        // Display sum of elements
        System.out.println("\nSum of all elements is " + sum(m));
    }

    public static int[][] getArray() {
        // Make a Scanner
        Scanner input = new Scanner(System.in);

        // Enter array values
        int[][] m = new int[3][4];
        System.out.println("Enter " + m.length + " rows and " + m[0].length + " columns: ");
        for (int i = 0; i < m.length; i++)
            for (int j = 0; j < m[i].length; j++)
                m[i][j] = input.nextInt();
        return m;
    }

    public static int sum(int[][] m) {
        int total = 0;
        for (int row = 0; row < m.length; row++) {
            for (int column = 0; column < m[row].length; column++) {
                total += m[row][column];
            }
        }

        return total;
    }
}

Output

Further Explanation

The methods getArray prompts the user to enter values for the array:

public static int[][] getArray() {
        // Make a Scanner
        Scanner input = new Scanner(System.in);

        // Enter array values
        int[][] m = new int[3][4];
        System.out.println("Enter " + m.length + " rows and " + m[0].length + " columns: ");
        for (int i = 0; i < m.length; i++)
            for (int j = 0; j < m[i].length; j++)
                m[i][j] = input.nextInt();
        return m;
    }

and returns the array:

        return m;

The method sum:

public static int sum(int[][] m) {
        int total = 0;
        for (int row = 0; row < m.length; row++) {
            for (int column = 0; column < m[row].length; column++) {
                total += m[row][column];
            }
        }

        return total;
    }

has a two dimensional array argument. You can obtain the number of rows using m.length:

        for (int row = 0; row < m.length; row++)

and the number of columns in a specified row using m[row].length:

            for (int column = 0; column < m[row].length; column++)

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