C vs C++ code optimization for simple array creation and i/o

Question

I've been trying to convince a friend of mine to avoid using dynamically allocated arrays and start moving over to the STL vectors. I sent him some sample code to show a couple things that could be done with STL and functors/generators:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

#define EVENTS 10000000

struct random_double {
  double operator() () { return (double)rand()/RAND_MAX; }
};  

int main(int argc, char **argv){

  std::vector<double> vd (EVENTS);

  generate(vd.begin(), vd.end(), random_double());
  copy(vd.begin(), vd.end(), std::ostream_iterator<double>(std::cout, "\n"));

  return 0;
} 

His reply to this, although he feels it's more elegant, is that his own code is faster (by almost a factor of 2!) Here's the C code he replied with:

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>

#define EVENTS 10000000

__inline double random_double() {
  return (double)rand()/RAND_MAX;
}


int main(int argc, char **argv){
  unsigned int i;
  double *vd;
  vd = (double *) malloc(EVENTS*sizeof(double));

  for(i=0;i<EVENTS;i++){ vd[i]=random_double(); }

  for(i=0;i<EVENTS;i++){ printf("%lf\n",vd[i]); }

  free(vd);

  return 0;
}

So I ran the simple timing test to see just what happens, and here's what I got:

> time ./c++test > /dev/null
real    0m14.665s
user    0m14.577s
sys     0m0.092s

> time ./ctest > /dev/null
real    0m8.070s
user    0m8.001s
sys     0m0.072s

The compiler options, using g++ were: g++ -finline -funroll-loops. Nothing too special. Can anyone tell me why the C++/STL version is slower in this case? Where is the bottleneck, and will I ever be able to sell my friend on using STL containers?

Answer 1

Almost certainly the use of the iostream library versus printf(). If you want to time the algorithm, you should do your output outside the loop.

Answer 2

Using printf:

  for (std::vector<double>::iterator i = vd.begin(); i != vd.end(); ++i)
     printf("%lf\n", *i);

results are:

koper@elisha ~/b $ time ./cpp > /dev/null
real    0m4.985s
user    0m4.930s
sys     0m0.050s
koper@elisha ~/b $ time ./c > /dev/null
real    0m4.973s
user    0m4.920s
sys     0m0.050s

Flags used: -O2 -funroll-loops -finline