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Syntax for creating a two-dimensional array


Question

Consider:

int[][] multD = new int[5][];
multD[0] = new int[10];

Is this how you create a two-dimensional array with 5 rows and 10 columns?

I saw this code online, but the syntax didn't make sense.

2015/07/09
1
443
7/9/2015 10:30:33 PM

Accepted Answer

Try the following:

int[][] multi = new int[5][10];

... which is a short hand for something like this:

int[][] multi = new int[5][];
multi[0] = new int[10];
multi[1] = new int[10];
multi[2] = new int[10];
multi[3] = new int[10];
multi[4] = new int[10];

Note that every element will be initialized to the default value for int, 0, so the above are also equivalent to:

int[][] multi = new int[][]{
  { 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, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 },
  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }
};
2013/05/21
823
5/21/2013 6:04:34 PM

We can declare a two dimensional array and directly store elements at the time of its declaration as:

int marks[][]={{50,60,55,67,70},{62,65,70,70,81},{72,66,77,80,69}};

Here int represents integer type elements stored into the array and the array name is 'marks'. int is the datatype for all the elements represented inside the "{" and "}" braces because an array is a collection of elements having the same data type.

Coming back to our statement written above: each row of elements should be written inside the curly braces. The rows and the elements in each row should be separated by a commas.

Now observe the statement: you can get there are 3 rows and 5 columns, so the JVM creates 3 * 5 = 15 blocks of memory. These blocks can be individually referred ta as:

marks[0][0]  marks[0][1]  marks[0][2]  marks[0][3]  marks[0][4]
marks[1][0]  marks[1][1]  marks[1][2]  marks[1][3]  marks[1][4]
marks[2][0]  marks[2][1]  marks[2][2]  marks[2][3]  marks[2][4]


NOTE:
If you want to store n elements then the array index starts from zero and ends at n-1. Another way of creating a two dimensional array is by declaring the array first and then allotting memory for it by using new operator.

int marks[][];           // declare marks array
marks = new int[3][5];   // allocate memory for storing 15 elements

By combining the above two we can write:

int marks[][] = new int[3][5];
2013/08/24

You can create them just the way others have mentioned. One more point to add: You can even create a skewed two-dimensional array with each row, not necessarily having the same number of collumns, like this:

int array[][] = new int[3][];
array[0] = new int[3];
array[1] = new int[2];
array[2] = new int[5];
2015/07/09

The most common idiom to create a two-dimensional array with 5 rows and 10 columns is:

int[][] multD = new int[5][10];

Alternatively, you could use the following, which is more similar to what you have, though you need to explicitly initialize each row:

int[][] multD = new int[5][];
for (int i = 0; i < 5; i++) {
  multD[i] = new int[10];
}
2012/09/01

It is also possible to declare it the following way. It's not good design, but it works.

int[] twoDimIntArray[] = new int[5][10];
2015/07/09

Try:

int[][] multD = new int[5][10];

Note that in your code only the first line of the 2D array is initialized to 0. Line 2 to 5 don't even exist. If you try to print them you'll get null for everyone of them.

2015/07/09