I always do it like this:

Code:
void test()
{
  var dst, smallest = 99999;
  int i, smallest_i = 0;

  for (i = 0; i < array_size; i++)
  {
    dst = vec_dist((ent[i]).x, mouse_hit.x);
    if (dst < smallest)
    {
      smallest = dst;
      smallest_i = i;
    }
   }
}