Question

I need the equivalent of the Matlab find function (reference) in C for arrays:

ind = find(X) locates all nonzero elements of array X, and returns the
linear indices of those elements in vector ind. If X is a row vector,
then ind is a row vector; otherwise, ind is a column vector. If X
contains no nonzero elements or is an empty array, then ind is an
empty array.

A trivial untested example:

#include <stdlib.h>
#include <time.h>
int main()
{
   /* Initialize variables */
   int x[25];
   int y[25];

   /* initialize random seed: */
   srand ( time(NULL) );

   /* Fill arrays with stuff */
   for (i=0;i<25;i++)
   {
      x[i] = rand();
      y[i] = rand();
   }

   /* Find */
   ind = find((x-y) > 0); // Need to figure out how to do this
   }
}

Now the kicker is I can’t use Boost or C++ containers like vector due to project constraints.

Answer 1

If you are restricted to “vanilla C” (which seems to be the case, from your question) there’s nothing like that built-in, you have to write your own version of such function.

But from what I see from your example you need something different than a find function, you want to find the elements differing into x and y. If you strive for flexibility it could be a good idea to write a generic function that checks the given predicate (passed as a function pointer) on two arrays. On the other hand, since in C we have only function pointers (and not functors) performance may suffer.