Multidimensional Arrays
In Java, multidimensional arrays are actually arrays of arrays.
For example, the following declares a two-dimensional array variable called twoD.
int twoD[][] = new int[4][5];
This allocates a 4-by-5 array and assigns it to twoD. This array will look like the one shown in the following:
[leftIndex][rightIndex]
[0][0] [0][1] [0][2] [0][3] [0][4]
[1][0] [1][1] [1][2] [1][3] [1][4]
[2][0] [2][1] [2][2] [2][3] [2][4]
[3][0] [3][1] [3][2] [3][3] [3][4]
+----+----+----+
| 1| 2| 3|
+----+----+----+
| 4| 5| 6|
+----+----+----+
| 7| 8| 9|
+----+----+----+
+--+ +----+----+----+
| |--------| 1| 2| 3|
+--+ +----+----+----+ +----+----+----+
| |-----------------------------| 4| 5| 6|
+--+ +----+----+----+ +----+----+----+
| |---| 7| 8| 9|
+--+ +----+----+----+
+--+ +----+----+
| |--------| 1| 2|
+--+ +----+----+ +----+----+----+
| |-----------------------------| 4| 5| 6|
+--+ +----+----+----+----+ +----+----+----+
| |---| 7| 8| 9| 10|
+--+ +----+----+----+----+
The following code use nested for loop to assign values to a two-dimensional array.
public class Main {
public static void main(String args[]) {
int twoD[][] = new int[4][5];
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 5; j++) {
twoD[i][j] = i*j;
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 5; j++) {
System.out.print(twoD[i][j] + " ");
}
System.out.println();
}
}
}
This program generates the following output:
0 0 0 0 0
0 1 2 3 4
0 2 4 6 8
0 3 6 9 12
Three-dimensional array
The following program creates a 3 by 4 by 5, three-dimensional array.
public class Main {
public static void main(String args[]) {
int threeD[][][] = new int[3][4][5];
for (int i = 0; i < 3; i++)
for (int j = 0; j < 4; j++)
for (int k = 0; k < 5; k++)
threeD[i][j][k] = i * j * k;
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 4; j++) {
for (int k = 0; k < 5; k++)
System.out.print(threeD[i][j][k] + " ");
System.out.println();
}
System.out.println();
}
}
}
This program generates the following output:
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 1 2 3 4
0 2 4 6 8
0 3 6 9 12
0 0 0 0 0
0 2 4 6 8
0 4 8 12 16
0 6 12 18 24
Jagged array
When you allocate memory for a multidimensional array, you can allocate the remaining dimensions separately. For example, the following code allocates the second dimension manually.
public class Main {
public static void main(String[] argv) {
int twoD[][] = new int[4][];
twoD[0] = new int[5];
twoD[1] = new int[5];
twoD[2] = new int[5];
twoD[3] = new int[5];
}
}
When allocating dimensions manually, you do not need to allocate the same number of elements for each dimension.
The following program creates a two-dimensional array in which the sizes of the second dimension are unequal.
public class Main {
public static void main(String args[]) {
int twoD[][] = new int[4][];
twoD[0] = new int[1];
twoD[1] = new int[2];
twoD[2] = new int[3];
twoD[3] = new int[4];
for (int i = 0; i < 4; i++){
for (int j = 0; j < i + 1; j++) {
twoD[i][j] = i + j;
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < i + 1; j++)
System.out.print(twoD[i][j] + " ");
System.out.println();
}
}
}
This program generates the following output:
0
1 2
2 3 4
3 4 5 6
The array created by this program looks like this:
[0][0]
[1][0] [1][1]
[2][0] [2][1] [2][2]
[3][0] [3][1] [3][2] [3][3]
Initialize multidimensional arrays
Enclose each dimension's initializer within its own set of curly braces.
public class Main{
public static void main(String args[]) {
double m[][] = {
{ 0, 1, 2, 3 },
{ 0, 1, 2, 3 },
{ 0, 1, 2, 3 },
{ 0, 1, 2, 3 }
};
for(int i=0; i<4; i++) {
for(int j=0; j<4; j++){
System.out.print(m[i][j] + " ");
}
System.out.println();
}
}
}
When you run this program, you will get the following output:
0.0 1.0 2.0 3.0
0.0 1.0 2.0 3.0
0.0 1.0 2.0 3.0
0.0 1.0 2.0 3.0