How to create at runtime a two dimensional array in C

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 p开发者_如何学JAVArogram. The array is to represent a grid. How would I code this in C?

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);

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.

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?

In C a multidimensional array is just an array for which each element is another array.

So you need to first allocate memory for one array (the rows). You can use the malloc() function which will return a pointer to the array.

Then you iterate through the array and for each element you allocate memory for the number of columns.

NOTE: don't forget to free the memory you manually allocate with the free() function in the same way you used malloc() to allocate it.

Some of the examples show multiple (more than 2) allocations for the array; it is perfectly feasible to do it in just two allocations (error checking omitted) for an n × m array:

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

for (int i = 0; i < m; i++)
    array[i] = &data[i * n];


...use array[i][j]...

free(array[0]);
free(array);