Getting the array length of a 2D array in Java
Java allows you to create "ragged arrays" where each "row" has different lengths. If you know you have a square array, you can use your code modified to protect against an empty array like this:
if (row > 0) col = test[0].length;
It was really hard to remember that
int numberOfColumns = arr.length;
int numberOfRows = arr[0].length;
Let's understand why this is so and how we can figure this out when we're given an array problem. From the below code we can see that rows = 4 and columns = 3:
int[][] arr = { {1, 1, 1, 1},
{2, 2, 2, 2},
{3, 3, 3, 3} };
arr
has multiple arrays in it, and these arrays can be arranged in a vertical manner to get the number of columns. To get the number of rows, we need to access the first array and consider its length. In this case, we access [1, 1, 1, 1] and thus, the number of rows = 4. When you're given a problem where you can't see the array, you can visualize the array as a rectangle with n X m dimensions and conclude that we can get the number of rows by accessing the first array then its length. The other one (arr.length
) is for the columns.
Consider
public static void main(String[] args) {
int[][] foo = new int[][] {
new int[] { 1, 2, 3 },
new int[] { 1, 2, 3, 4},
};
System.out.println(foo.length); //2
System.out.println(foo[0].length); //3
System.out.println(foo[1].length); //4
}
Column lengths differ per row. If you're backing some data by a fixed size 2D array, then provide getters to the fixed values in a wrapper class.
A 2D array is not a rectangular grid. Or maybe better, there is no such thing as a 2D array in Java.
import java.util.Arrays;
public class Main {
public static void main(String args[]) {
int[][] test;
test = new int[5][];//'2D array'
for (int i=0;i<test.length;i++)
test[i] = new int[i];
System.out.println(Arrays.deepToString(test));
Object[] test2;
test2 = new Object[5];//array of objects
for (int i=0;i<test2.length;i++)
test2[i] = new int[i];//array is a object too
System.out.println(Arrays.deepToString(test2));
}
}
Outputs
[[], [0], [0, 0], [0, 0, 0], [0, 0, 0, 0]]
[[], [0], [0, 0], [0, 0, 0], [0, 0, 0, 0]]
The arrays test
and test2
are (more or less) the same.