This example fills a two-dimensional array with the sum of the row and column indexes

```
class FillArray {
public static void main (String args[]) {
int[][] matrix;
matrix = new int[4][5];
for (int row=0; row < 4; row++) {
for (int col=0; col < 5; col++) {
matrix[row][col] = row+col;
}
}
}
}
```

Of course the algorithm you use to fill the array depends completely on the use to which the array is to be put. The next example calculates the identity matrix for a given dimension. The identity matrix of dimension N is a square matrix which contains ones along the diagonal and zeros in all other positions.

```
class IDMatrix {
public static void main (String args[]) {
double[][] id;
id = new double[4][4];
for (int row=0; row < 4; row++) {
for (int col=0; col < 4; col++) {
if (row != col) {
id[row][col]=0.0;
}
else {
id[row][col] = 1.0;
}
}
}
}
}
```

In two-dimensional arrays `ArrayIndexOutOfBoundsException`

s occur whenever you exceed the maximum column index or row index. You can also declare, allocate, and initialize a a two-dimensional array at the same time by providing a list of the initial values inside nested brackets. For instance the three by three identity matrix could be set up like this:

```
double[][] ID3 = {
{1.0, 0.0, 0.0},
{0.0, 1.0, 0.0},
{0.0, 0.0, 1.0}
};
```

The spacing and the line breaks used above are purely for the programmer. The compiler doesn't care. The following works equally well:

```
double[][] ID3 = {{1.0, 0.0, 0.0},{0.0, 1.0, 0.0},{0.0, 0.0, 1.0}};
```

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Last Modified September 24, 1999`

Copyright 1997, 1999 Elliotte Rusty Harold

elharo@metalab.unc.edu