So I'm a bit confused about how to use a 2D Array in Java if you only declare the number of rows at first. For example:
int[][] x = new int[5][];
But how would you go on about filling those 5 rows? I'm thinking you need to declare the size of each one of the rows first, but I can't figure out a way to do so. If I try to do something like:
x[0][0] = 5;
The compiler tells me that x[0] is null.
Would appreciate some help, thanks in advance.
you tried to define a 2d array: firstly you specified the numbers of row, but you didn't specified the number of columns and its necessary.
for example I defined the 2 columns for the first row and assigned the value:
int[][] x = new int[5][];
x[0] = new int[2];
x[0][0] = 5;
System.out.println(x[0][0]);
for better undestanding of 2d-arrays you need to read more:
https://www.programiz.com/java-programming/multidimensional-array
Simply do it:
public class ClassName {
public static void main(String[] args) {
//Assignment of multi-dimensional array
int[][] multiarray = new int[2][3];
// assign value to row "0" and col "0" index
multiarray[0][0] = 23;
System.out.println("My Age is:" + multiarray[0][0]);
}
}
An element of a 2d array is a 1d array. If you define an array directly new int[5][4] then length of each row is 4, otherwise it may vary. When you create a 2d array with undefined row length, each element of it, actually 1d array, is not initialized yet and is null. When you create 1d array of int[] it is initialized with zeros by default. If you define 2d array directly the elements of its rows are initialized with zeros.
You can define column length without row length:
int[][] arr = new int[5][];
arr[0] = new int[]{1, 2, 3};
arr[3] = new int[2];
arr[3][1] = 9;
arr[4] = new int[]{3, 3, 2, 1, 6, 7};
// output:
[1, 2, 3]
null
null
[0, 9]
[3, 3, 2, 1, 6, 7]
You can define column and row length:
int[][] arr2 = new int[3][3];
arr2[0][0] = 1;
// output:
[1, 0, 0]
[0, 0, 0]
[0, 0, 0]
You can define each element of the 2d array at creation time, optionally with reserved rows for further operations:
int[][] arr3 = new int[][]{{1, 2, 3}, {4, 5}, {6}, null};
// output:
[1, 2, 3]
[4, 5]
[6]
null
You can't access x[0][0] because no columns exists at row x[0].
You will need to initialize every column in a row before using it:
x[0] = new int[5];
x[1] = new int[5];
or use a loop:
for (int i = 0; i < 5; i++) {
x[i] = new int[5];
}
int ROWS = 5;
int COLUMNS = 12;
int[][] x = new int[ROWS][];
Initialize the values of all ROWS to zero:
IntStream.range(0, ROWS).forEach(i -> {
x[i] = new int[COLUMNS];
});
If You want to initialize to a different value eg. -1:
IntStream.range(0, ROWS).forEach(i -> {
x[i] = new int[COLUMNS];
Arrays.fill(x[i], -1);
});
