Re: C++ is Slow?

From:

terminator <farid.mehrabi@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 4 Feb 2008 21:27:08 -0800 (PST)

Message-ID:

<1f4435af-9ccb-47b6-b161-f474455c8e35@b2g2000hsg.googlegroups.com>

On Feb 5, 4:27 am, Ioannis Vranos <ivra...@nospam.no.spamfreemail.gr>
wrote:

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)

}- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

I would rather:

class myclass{
   vector <int> data;
public:
   myclass(Xdim,Ydim):
     data(Xdim*Ydim)//*int data[Ydim*Xdim];*/
     {};
     ...
};

regards,
FM.