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

• Difficulty Level : Hard
• Last Updated : 12 Jan, 2022

Following are different ways to create a 2D array on the 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 the following values

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

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

## C

 `#include ``#include ` `int` `main(``void``)``{``    ``int` `r = 3, c = 4;` `    ``int``* ptr = ``malloc``((r * c) * ``sizeof``(``int``));` `    ``/* Putting 1 to 12 in the 1D array in a sequence */``    ``for` `(``int` `i = 0; i < r * c; i++)``        ``ptr[i] = i + 1;` `    ``/* Accessing the array values as if it was a 2D array */``    ``for` `(``int` `i = 0; i < r; i++) {``        ``for` `(``int` `j = 0; j < c; j++)``            ``printf``(``"%d "``, ptr[i * c + j]);``        ``printf``(``"\n"``);``    ``}` `    ``free``(ptr);` `    ``return` `0;``}`

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 < r; i++)``        ``arr[i] = (``int``*)``malloc``(c * ``sizeof``(``int``));` `    ``// Note that arr[i][j] is same as *(*(arr+i)+j)``    ``count = 0;``    ``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]);` `    ``/* Code for further processing and free the``      ``dynamically allocated memory */` `    ``for` `(``int` `i = 0; i < r; i++)``        ``free``(arr[i]);` `    ``return` `0;``}`

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 < r; i++)``        ``arr[i] = (``int``*)``malloc``(c * ``sizeof``(``int``));` `    ``// Note that arr[i][j] is same as *(*(arr+i)+j)``    ``count = 0;``    ``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]);` `    ``/* Code for further processing and free the``       ``dynamically allocated memory */` `    ``for` `(``int` `i = 0; i < r; i++)``        ``free``(arr[i]);` `    ``free``(arr);` `    ``return` `0;``}`

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.

5) Using a pointer to Variable Length Array.

The dimensions of VLA are bound to the type of the variable. Therefore one form a pointer to an array with run-time defined shape.
The pointer has to be dereferenced before subscripting with syntax (*arr)[i][j].

## C

 `#include ``#include ` `int` `main()``{``    ``int` `row = 3, col = 4, i, j, count;` `    ``int` `(*arr)[row][col] = ``malloc``(``sizeof` `*arr);``    ` `    ``count = 0;``    ``for` `(i = 0; i < row; i++)``        ``for` `(j = 0; j < col; j++)``            ``(*arr)[i][j] = ++count;` `    ``for` `(i = 0; i < row; i++)``        ``for` `(j = 0; j < col; j++)``            ``printf``(``"%d "``, (*arr)[i][j]);` `    ``free``(arr);``    ` `    ``return` `0;``}`

6) Using a pointer to the first row of VLA

Similar to 5 but allows arr[i][j] syntax.

## C

 `#include ``#include ` `int` `main()``{``    ``int` `row = 3, col = 4, i, j, count;` `    ``int` `(*arr)[col] = ``calloc``(row, ``sizeof` `*arr);``    ` `    ``count = 0;``    ``for` `(i = 0; i < row; i++)``        ``for` `(j = 0; j < col; j++)``            ``arr[i][j] = ++count;` `    ``for` `(i = 0; i < row; i++)``        ``for` `(j = 0; j < col; j++)``            ``printf``(``"%d "``, arr[i][j]);` `    ``free``(arr);``    ` `    ``return` `0;``}`

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