AP Computer Science A Unit 8: 2D Array

Which declaration correctly creates a 2D array of integers with 4 rows and 3 columns?

What is the output of the following code?

int[][] m = {{2, 4}, {6, 8}};
for (int r = 0; r < m.length; r++) {
    for (int c = 0; c < m[r].length; c++) {
        System.out.print(m[r][c] + " ");
    }
}

Consider a 2D array representing a grid where each row may have a different length (a jagged array). Which expression safely gives the number of columns in row i?

A student writes the following method to count elements equal to a target value in a 2D array:

public static int count(int[][] arr, int target) {
    int cnt = 0;
    for (int r = 0; r < arr.length; r++) {
        for (int c = 0; c < arr[0].length; c++) {
            if (arr[r][c] == target) cnt++;
        }
    }
    return cnt;
}

For which input would this method produce incorrect results?

What is the state of array a after the following code executes?

int[][] a = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
int[] temp = a[0];
a[0] = a[2];
a[2] = temp;

A method receives a 2D array and should return the largest value in the array. Which implementation is correct?

public static int findMax(int[][] arr) {
    int max = arr[0][0];
    for (int[] row : arr) {
        for (int val : row) {
            if (val > max) max = val;
        }
    }
    return max;
}

What value does the following method return when called with the array {{1, 2}, {3, 4}}?

public static int total(int[][] arr) {
    int sum = 0;
    for (int r = 0; r < arr.length; r++) {
        for (int c = 0; c < arr[r].length; c++) {
            sum += arr[r][c];
        }
    }
    return sum;
}

What is the output of the following code?

int[][] t = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
for (int c = 0; c < t[0].length; c++) {
    for (int r = 0; r < t.length; r++) {
        System.out.print(t[r][c] + " ");
    }
}

Given the following code snippet:

int[][] grid = new int[3][4];
System.out.println(grid.length + " " + grid[0].length);

What is printed?

The following code attempts to print the sum of the main diagonal of a 3×3 array:

int[][] m = {{1, 0, 0}, {0, 5, 0}, {0, 0, 9}};
int diag = 0;
for (int i = 0; i < m.length; i++) {
    diag += m[i][i];
}
System.out.println(diag);

What does it print?

Consider the following 2D array declaration:

int[][] arr = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};

What is the value of arr[1][2]?

What is the output of the following code?

int[][] g = new int[2][3];
for (int r = 0; r < g.length; r++) {
    for (int c = 0; c < g[r].length; c++) {
        g[r][c] = r + c;
    }
}
System.out.println(g[1][2]);