Question

I have this code:

void
fill_array (unsigned int *iarray, char *string, int max)
{
  int ipos = 0;
  char *iholder;

  iholder = strtok (string, ",");
  while (iholder != NULL)
    {
      iarray[ipos++] = atoi (iholder);
      if (ipos == max)
    {
      return;
    }
      iholder = strtok (NULL, ",");
    }

}

It takes a string “1,2,3,4” for example, and enters the numbers into an array.
I put this in a loop and got 3.3 seconds runtime.

With this code:

void
fill_array (unsigned int *iarray, char *string, int max)
{
  int ipos = 0;
  char *iholder;
  if (!strchr (string, ','))
    {
      iarray[0] = atoi (string);
      return;
    }
  iholder = strtok (string, ",");
  while (iholder != NULL)
    {
      iarray[ipos++] = atoi (iholder);
      if (ipos == max)
    {
      return;
    }
      iholder = strtok (NULL, ",");
    }

}

It took about 1.4 seconds to execute.

The only difference is the strchr which I inserted just to see if it would run faster on single numbers, but it runs much faster on longer lists for some reason.

Can anyone explain why?

I am testing with this code:

int main ()
{
  unsigned int iarray[5];
  char str_test[] = "56,75,22,83";
  int i;
  struct timeval start;
  struct timeval end;

  gettimeofday (&start, NULL);


  for (i = 0; i < 10000000; i++)
    {
      fill_array (iarray, str_test, 5);
    }

  gettimeofday (&end, NULL);
  if (end.tv_usec - start.tv_usec < 0)
    {
      end.tv_usec += 1000000L;
      end.tv_sec -= 1;
    }

  printf ("Runtime: %ld s %03ld ms\n",
      end.tv_sec - start.tv_sec, (end.tv_usec - start.tv_usec) / 1000);

  return 0;
}

Answer 1

Presumably that first strchr is making your code drop out earlier so that it doesn’t have to do as much processing?