How to create at runtime a two dimensional array in C

Question

I cannot create a 2D array from 2 variables (eg int arr[i][j] not allowed) so how would I create a dynamically sized 2D array?

The dimensions of the array are only known at runtime in my program. The array is to represent a grid. How would I code this in C?

Solution 1

You have to allocate a 1-dimensional array:

int* array = calloc(m*n, sizof(int));

And access it like this:

array[i*n + j]

The compiler does exactly this when accessing two-dimensional arrays, and will probably output the same code when n can be guessed at compile time.

Solution 2

First allocate an array of pointers.

/* size_x is the width of the array */
int **array = (int**)calloc(size_x, sizeof(int*));

Then allocate each column.

for(int i = 0; i < size_x; i++) 
{
    /* size_y is the height */
    array[i] = (int*)calloc(size_y, sizeof(int));
}

You can access the elements with array[i][j]. Freeing the memory is done in 'reverse' order:

for(int i = 0; i < size_x; i++) 
{
    free(array[i]);
}
free(array);

Solution 3

This is a FAQ on comp.lang.c (I took the liberty to add the c-faq tag), it even has a FGA (frequently given answer :-) See http://c-faq.com/aryptr/index.html, 6.16 How can I dynamically allocate a multidimensional array?