Re: C++ is Slow?

From:

Ioannis Vranos <ivranos@nospam.no.spamfreemail.gr>

Newsgroups:

comp.lang.c++

Date:

Tue, 05 Feb 2008 03:27:12 +0200

Message-ID:

<fo8e1k$1bja$1@ulysses.noc.ntua.gr>

nw wrote:

More specifically vector. The argument goes like this:>
"Multidimensional arrays should be allocated as large contiguous
blocks. This is so that when you are accessing the array and reach the
end of a row, the next row will already be in the cache. You also
don't need to spend time navigating pointers when accessing the array.
So a 2 dimensional array of size 100x100 should be created like this:

const int xdim=100;
const int ydim=100;

int *myarray = malloc(xdim*ydim*sizeof(int));

and accessed like this:

myarray[xdim*ypos+xpos] = avalue;

Is this argument reasonable? (Sounds reasonable to me, though the
small tests I've performed don't usually show any significant
difference).

To me this syntax looks horrible, am I wrong? Is vector the wrong
container to use? (My usual solution would be a vector<vector<int> >).
Would using a valarray help?

I think your code is incorrect. Your approach corrected:

#include <cstdlib>

int main()
{
   using namespace std;

   const int XDIM= 100;

   const int YDIM= 200;

    int (*my_array)[YDIM]= static_cast<int (*)[200]> ( malloc(XDIM* YDIM
* **my_array) );

    if(my_array== 0)
        return EXIT_FAILURE;

    for(size_t i= 0; i< XDIM; ++i)
        for(size_t j= 0; j< YDIM; ++j)
       my_array[i][j]= i+j;

    // ...

    free(my_array);

    // ...
}

The equivalent C++ style:

include <cstdlib>

int main()
{
   using namespace std;

   const int XDIM= 100;

   const int YDIM= 200;

    int (*my_array)[YDIM]= new int[XDIM][YDIM];

    for(size_t i= 0; i< XDIM; ++i)
        for(size_t j= 0; j< YDIM; ++j)
       my_array[i][j]= i+j;
}

The proper C++ approach:

#include <cstdlib>
#include <vector>

int main()
{
   using namespace std;

   const int XDIM= 100;

   const int YDIM= 200;

   vector<vector<int> > my_array(XDIM, vector<int>(YDIM));

   for(vector<vector<int> >::size_type i= 0; i< my_array.size(); ++i)
        for(vector<int>::size_type j= 0; j< my_array[i].size(); ++j)
       my_array[i][j]= i+j;

      // ...

   // No need to clean up your memory or any other resource
   // - RAII (Resource Acquisition Is Initialisation)

}