initialize a multidimensional C array of variable size to zero

Question

I want to initialize a two-dimensional array of variable size to zero. I know it can be done for a fixed-sized array:

int myarray[10][10] = {0};

but it is not working if I do this:

int i = 10;
int j = 10;
int myarray[i][j] = {0};

Is there a one-line way of doing this or do I have to loop over each member of the array?

Thanks

Answer 1

You cannot initialize it with an initializer, but you can memset() the array to 0.

#include <string.h>

int main(void) {
  int a = 13, b = 42;
  int m[a][b];
  memset(m, 0, sizeof m);
  return 0;
}

Note: this is C99. In C89 the declaration of m ( int m[a][b]; ) is an error.

Answer 2

Online C99 Standard (n1256 draft), Section 6.7.8, para 3:

The type of the entity to be initialized shall be an array of unknown size or an object type that is not a variable length array type.

Emphasis mine.

As everyone else has said, your best bet is to use memset().

Answer 3

If you get a pointer to your data structure, you could try memset.

Answer 4

Since C23 you can initialize variable length arrays (VLA) with the new intuitive empty {} initializer.

#include <string.h>

int main(void) {
  int a = 13, b = 42;
  int m[a][b] = {};
  return 0;
}

It can only be initialized with the empty initializer.

Answer 5

You can't create a static array using non-constant variables. Try using dynamic allocation:

int i = 10;
int j = 10;
size_t nbytes = i*j*sizeof(int);
int* myarray = (int*) malloc(nbytes);
memset(myarray,0,nbytes);

Answer 6

Variable size two dimensional arrays are not supported in C. One dimension (i can't remember if it is first or second) has to be fixed. I recommend looping over it once it is defined.