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?

C99 allows you to declare VLA - variable-length arrays - so if you have a C99 compiler and no artificial (homework) constraints on your work, you can write precisely `int arr[i][j];`. (If you don't have a C99 compiler, it's time to get one.) With that said, how do you plan to access the array? You can either be hoping to use `arr[i][j]` to access an element, or you can be prepared to use a calculation like `arr[i*n+j]` to access it. The memory allocation patterns required are quite different for the two cases. — Jonathan Leffler, Sep 17 '11 at 17:56

score 11 · Answer 1 · edited May 09 '21 at 14:50

11

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);

edited May 09 '21 at 14:50

Adrian Mole

49,934
160
51
83

answered Sep 17 '11 at 17:48

Staven

3,113
16
20

I got an error with VS2010 which said that it can't cast void* to int*. So I had to explixitly cast to `int**` with `(int**)`. So that the first line becomes `int **array = (int**)calloc(size_x, sizeof(int*));` And `(int*)` at the second line. – JaBe Mar 12 '15 at 21:19
1

@JaBe You have to cast explicitly in C++. But then, why are you using calloc in C++? – Staven Mar 13 '15 at 03:56
I'm using C++ and calling a lot of C code which I can't change. Calling it with C++ vec does not look much better: http://stackoverflow.com/questions/6701816/how-to-use-a-stdvector-in-a-c-function – JaBe Mar 24 '15 at 07:45
1

Should the limit on that for loop be size_x not size_y? – jterm Dec 09 '16 at 21:08

Arnaud Le Blanc · Accepted Answer · 2011-09-17T18:53:17.227

8

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.

edited Sep 17 '11 at 18:53

answered Sep 17 '11 at 17:42

Arnaud Le Blanc

98,321
23
206
194

score 1 · Answer 3 · answered Sep 17 '11 at 18:03

1

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?

answered Sep 17 '11 at 18:03

Jens

69,818
15
125
179

score 0 · Answer 4 · edited Sep 17 '11 at 17:52

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.

score 0 · Answer 5 · answered Sep 17 '11 at 18:13

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);

How to create at runtime a two dimensional array in C

5 Answers5

Linked