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# How to dynamically allocate a 2D array in C?

• Difficulty Level : Hard
• Last Updated : 13 Jul, 2021

Following are different ways to create a 2D array on heap (or dynamically allocate a 2D array).
In the following examples, we have considered ‘r‘ as number of rows, ‘c‘ as number of columns and we created a 2D array with r = 3, c = 4 and following values

```  1  2  3  4
5  6  7  8
9  10 11 12 ```

1) Using a single pointer:
A simple way is to allocate memory block of size r*c and access elements using simple pointer arithmetic.

## C

 `#include ``#include ` `void` `main()``{``    ``int` `r = 3, c = 4; ``//Taking number of Rows and Columns``    ``int` `*ptr, count = 0, i;``    ``ptr = (``int` `*)``malloc``((r * c) * ``sizeof``(``int``)); ``//Dynamically Allocating Memory``    ``for` `(i = 0; i < r * c; i++)``    ``{``        ``ptr[i] = i + 1; ``//Giving value to the pointer and simultaneously printing it.``        ``printf``(``"%d "``, ptr[i]);``        ``if` `((i + 1) % c == 0)``        ``{``            ``printf``(``"\n"``);``        ``}``    ``}``    ``free``(ptr);``}`

Output:

```1 2 3 4
5 6 7 8
9 10 11 12```

2) Using an array of pointers
We can create an array of pointers of size r. Note that from C99, C language allows variable sized arrays. After creating an array of pointers, we can dynamically allocate memory for every row.

## C

 `#include ``#include ` `int` `main()``{``    ``int` `r = 3, c = 4, i, j, count;` `    ``int` `*arr[r];``    ``for` `(i=0; i

Output:

`1 2 3 4 5 6 7 8 9 10 11 12`

3) Using pointer to a pointer
We can create an array of pointers also dynamically using a double pointer. Once we have an array pointers allocated dynamically, we can dynamically allocate memory and for every row like method 2.

## C

 `#include ``#include ` `int` `main()``{``    ``int` `r = 3, c = 4, i, j, count;` `    ``int` `**arr = (``int` `**)``malloc``(r * ``sizeof``(``int` `*));``    ``for` `(i=0; i

Output:

`1 2 3 4 5 6 7 8 9 10 11 12`

4) Using double pointer and one malloc call

## C

 `#include``#include` `int` `main()``{``    ``int` `r=3, c=4, len=0;``    ``int` `*ptr, **arr;``    ``int` `count = 0,i,j;` `    ``len = ``sizeof``(``int` `*) * r + ``sizeof``(``int``) * c * r;``    ``arr = (``int` `**)``malloc``(len);` `    ``// ptr is now pointing to the first element in of 2D array``    ``ptr = (``int` `*)(arr + r);` `    ``// for loop to point rows pointer to appropriate location in 2D array``    ``for``(i = 0; i < r; i++)``        ``arr[i] = (ptr + c * i);` `    ``for` `(i = 0; i < r; i++)``        ``for` `(j = 0; j < c; j++)``            ``arr[i][j] = ++count; ``// OR *(*(arr+i)+j) = ++count` `    ``for` `(i = 0; i < r; i++)``        ``for` `(j = 0; j < c; j++)``            ``printf``(``"%d "``, arr[i][j]);` `    ``return` `0;``}`

Output:

`1 2 3 4 5 6 7 8 9 10 11 12`

Thanks to Trishansh Bhardwaj for suggesting this 4th method.