how to return 2 dim char array from function in C

Question

I have some C code that works as:

int main(void)
{
   printf("Height: ");
   int height;
   do
   {
     height = get_int();
   } while (height < 0 || height > 23);

   char mtn[height][2*height+2];

   for(int i = 0;i <height+1;i++){
       for (int j = 0;j<2+height+3;j++){            
          mtn[i][j] = ' ';            
       }
   }    
}

but would like to make the double for loops a function, something like:

char space_mtn(int height);
int main(void)
{
  printf("Height: ");
  int height;
  do
  {
    height = get_int();
  } while (height < 0 || height > 23);

  char mtn[height][2*height+2];
  strcpy(mtn,space_mtn(height);

}

char space_mtn(int height){
char mtn[height][2*height+2];
for(int i = 0;i <height+1;i++){
    for (int j = 0;j<2+height+3;j++){            
        mtn[i][j] = ' ';            
    }
}

}

but it's not working.

Pass `mtn` to the function, and have the function fill it in. — Barmar, Jan 10 '17 at 01:35
See http://stackoverflow.com/questions/14548753/passing-a-multidimensional-variable-length-array-to-a-function — Barmar, Jan 10 '17 at 01:45
It shows how to pass the 2-dimensional array to the function, and how the function can then access the elements. What more do you need to know? — Barmar, Jan 10 '17 at 01:49
@M.M You're right, I now see that in the question I linked to. — Barmar, Jan 10 '17 at 01:54

score 1 · Accepted Answer · answered Jan 10 '17 at 01:55

Arrays cannot be returned by value in C. One way is to pass the array as a parameter. (Array parameters are adjusted to pointers so that it appears as if the array has been passed by reference, so the function can modify the original array).

void space_mtn(int height, char mtn[height][2*height+2])
{
    for(int i = 0;i < height;i++)
        for (int j = 0; j < 2*height+2 ;j++){           
            mtn[i][j] = ' ';            
}

int main(void)
{
    // ...
    char mtn[height][2*height+2];
    space_mtn(height, mtn);
}

Note, I adjusted the loop bounds on your filler loop - they were nonsense and/or writing out of bounds.

Also, passing arrays of runtime bound to functions does not have compile-time bounds checking available of course -- so you need to take extra care that the bound in the parameter matches the array you are passing.

how to return 2 dim char array from function in C

1 Answers1