newbie C question

[timfenn]

I'm still a bit new to C, and attempting to master pointers. I have a function that allocates a 2d pointer array, like so:

int main(int argc, char *argv[]){
  float **data;

  alloc_data(&data);
}

void alloc_data(float ***data){
  int i, nx, ny;
  float **tmp;

  nx = 3;
  ny = 3;

  tmp = (float **) calloc(nx, sizeof(float *));
  for (i=0; i<nx; i++)
    tmp[i] = (float *) calloc(ny, sizeof(float));

  *data = tmp;
}

works just fine, but how can I allocate data without having to alloc tmp and have data point to tmp? For example, if I do:

  *data = (float **) calloc(nx, sizeof(float *));
  for (i=0; i<nx; i++)
    *data[i] = (float *) calloc(ny, sizeof(float));

it doesn't work as I expect - what am I missing here?

thanks for any help.

-Tim

[evilsaltine]

[] has higher precedence than *, so the compiler interperets *data[i] as *(data[i]) when what you want is (*data)[i].

[timfenn]

Aha! Thanks! Now I'll go learn my C precedence rules.

-Tim

[wildfrog]

Another way is to pass the double pointer as a reference:

void alloc_data(float** &data) // pass reference (to the double pointer)
{
    int i, nx, ny;

    nx = 3;
    ny = 3;

    data = (float **) calloc(nx, sizeof(float *));
    for (i=0; i<nx; i++)
        data[i] = (float *) calloc(ny, sizeof(float));
}

int main(int argc, char *argv[])
{
    float **data;
    alloc_data(data);
}

- petter

[Eli Gassert]

I agree with Wildfrog; that is the method of choice in my opinion

[NoHero]

I agree with Wildfrog; that is the method of choice in my opinion

The only problem: C++ might understand references, but C does not. The OP stated to use C so your solution won't help him at all.

[wildfrog]

The only problem: C++ might understand references, but C does not. The OP stated to use C so your solution won't help him at all.

Oh, I thought he was just another lazy C++ programmer (and dropped the obvious ++)

Well, then I'd go for evilsaltines solution:

void alloc_data(float*** data) // pass pointer (to the double pointer)
{
    int i, nx, ny;

    nx = 3;
    ny = 3;

    *data = (float **) calloc(nx, sizeof(float *));
    for (i=0; i<nx; i++)
        (*data)[i] = (float *) calloc(ny, sizeof(float));
}

int main(int argc, char *argv[])
{
    float **data = 0;
    alloc_data(&data);
}

- petter

[timfenn]

The code is currently in C, but I am fiddling a bit with C++ as well, so thanks for pointing out the handy pointer reference trick for C++.

-Tim