Arrays of objects exercise 3 solution

Complete the Arrays of objects exercises before reviewing the solutions.

Review the Arrays of objects exercise 3 solution with AP CS Tutor Brandon Horn.

Original code

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

System.out.println(Arrays.deepToString(mat));
// prints:
// [[1, 2, 3],
//  [4, 5, 6],
//  [7, 8, 9]]

int[] row = mat[1];

row[0] = 0;

System.out.println(Arrays.deepToString(mat));

Output

Arrays.deepToString is a method to quickly produce formatted output. It is not covered on the AP CS A Exam. The format of the output below has been adjusted for readability.

[[1, 2, 3],
 [4, 5, 6],
 [7, 8, 9]]

[[1, 2, 3],
 [0, 5, 6],
 [7, 8, 9]]

Explanation

A 2D array is an array of 1D arrays. More precisely, a 2D array is an array of memory addresses of (references to) 1D arrays.

The statement int[] row = mat[1]; creates a variable named row that stores the same memory address as mat[1].

The statement row[0] = 0; changes the value at index 0 in the array to which row refers to 0. row and mat[1] refer to the same 1D array. The change affects the 2D array to which mat refers.

Step by step memory diagram

Step 1

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

System.out.println(Arrays.deepToString(mat));
// prints:
// [[1, 2, 3],
//  [4, 5, 6],
//  [7, 8, 9]]

// additional code not yet run

Memory diagram after Step 1

mat points ot a box representing an array. The array points to 3 boxes, each representing 1 row. The 1st row contains 1, 2, 3. The 2nd row contains 4, 5, 6. The 3rd row contains 7, 8, 9

A 2D array is an array of 1D arrays. More precisely, a 2D array is an array of memory addresses of (references to) 1D arrays. mat[0] stores the memory address of the array containing [1, 2, 3].

Output after Step 1

[[1, 2, 3],
 [4, 5, 6],
 [7, 8, 9]]

Step 2

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

System.out.println(Arrays.deepToString(mat));
// prints:
// [[1, 2, 3],
//  [4, 5, 6],
//  [7, 8, 9]]

int[] row = mat[1];

// additional code not yet run

Memory diagram after Step 2

mat points ot a box representing an array. The array points to 3 boxes, each representing 1 row. The 1st row contains 1, 2, 3. The 2nd row contains 4, 5, 6. The 3rd row contains 7, 8, 9. The variable row points to the box containing 4, 5, 6

mat[1] stores the memory address of the array containing [4, 5, 6]. The statement int[] row = mat[1]; creates a variable named row that stores the same memory address as mat[1].

Step 3

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

System.out.println(Arrays.deepToString(mat));
// prints:
// [[1, 2, 3],
//  [4, 5, 6],
//  [7, 8, 9]]

int[] row = mat[1];

row[0] = 0;

System.out.println(Arrays.deepToString(mat));

Memory diagram after Step 3

mat points ot a box representing an array. The array points to 3 boxes, each representing 1 row. The 1st row contains 1, 2, 3. The 2nd row contains 0, 5, 6. The 3rd row contains 7, 8, 9. The variable row points to the box containing 0, 5, 6

Output after Step 3

[[1, 2, 3],
 [4, 5, 6],
 [7, 8, 9]]

[[1, 2, 3],
 [0, 5, 6],
 [7, 8, 9]]

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