Multi-dimensional arrays

Figure 450. Two-dimensional arrays Slide presentation 
final int[][] matrix = new int[2][3];

for (int row = 0; row < 2; row++) {
  for (int col = 0; col < 3; col++) {
    matrix[row][col] = col + row;
  }
}
for (int row = 0; row < 2; row++) {
  System.out.println(Arrays.toString(matrix[row]));
}

Figure 451. Behind the scenes Slide presentation 
final int[][] matrix = new int[2][]; // Array containing two int arrays
matrix[0] = new int[3];              // first int array
matrix[1] = new int[3];              // second int array

Figure 452. Memory allocation Slide presentation
  • Sd1/Arrays/Fig/array2dim-0
  • Sd1/Arrays/Fig/array2dim-1
  • Sd1/Arrays/Fig/array2dim-2
  • Sd1/Arrays/Fig/array2dim-3
  • Sd1/Arrays/Fig/array2dim-4
  • Sd1/Arrays/Fig/array2dim-5
  • Sd1/Arrays/Fig/array2dim-6
  • Sd1/Arrays/Fig/array2dim-7
  • Sd1/Arrays/Fig/array2dim-8
  • Sd1/Arrays/Fig/array2dim-9
  • Sd1/Arrays/Fig/array2dim-10
  • Sd1/Arrays/Fig/array2dim-11
  • Sd1/Arrays/Fig/array2dim-12
  • Sd1/Arrays/Fig/array2dim-13
  • Sd1/Arrays/Fig/array2dim-14
  • Sd1/Arrays/Fig/array2dim-15
  • Sd1/Arrays/Fig/array2dim-16
  • Sd1/Arrays/Fig/array2dim-17
  • Sd1/Arrays/Fig/array2dim-18
  • Sd1/Arrays/Fig/array2dim-19
  • Sd1/Arrays/Fig/array2dim-20
‹›


exercise No. 151

2-dimensional arrays and .length

Q:

  1. In Figure 425, “Safer: Using length ” you were advised to favour .length over hard coded int values like e.g. “5”. Replace the error prone limits 2 and 3 in Figure 450, “Two-dimensional arrays ” accordingly.

  2. Replace the second loop in Figure 450, “Two-dimensional arrays ” by a for-each style loop.

    Tip

    matrix[0] and matrix[1] are of type int[].

A:

Two-dimensional arrays in Java™ are nested arrays of arrays. We thus choose appropriate data types: 

final int[][] matrix = new int[2][3];

for (int row = 0; row < matrix.length ❶; row++) {
  for (int col = 0; col < matrix[row].length ❷; col++) {
    matrix[row][col] = col + row;
  }
}
for (final int[] ❸ row : matrix) {
  System.out.println(Arrays.toString(row));
}

❶

The «outer» array's dimension: This int[][] array is the visible entry point containing references to rows each being implemented as a «simple» int[] array.

❷

All rows are having identical length. We could thus use either representative matrix[0].length or matrix[1].length as well. However choosing matrix[row].length also works in case of different row lengths.

❸

The variable matrix is of type int[][]. Each matrix[row] entry thus represents a reference to an «ordinary» int[] array. So row iteration requires a loop variable of type int[].

Figure 453. Static array initialization Slide presentation 
final int[][] matrix = new int[][] {
  {0, 1, 2},
  {1, 2, 3}
};

Figure 454. Static array initialization, variable lengths Slide presentation 
final String[][] groups = new String[][] {
  {"Jill", "Tom"},
  {"Jane", "Smith", "Joe"},
  {"Jeff"}
};

for (int row = 0; row < groups.length; row++) {
  System.out.println(Arrays.toString(groups[row]));
}
 
[Jill, Tom]
[Jane, Smith, Joe]
[Jeff]

exercise No. 152

External array and string exercises

Q:

Solve all examples from the following sections:
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