transpose.cc

// http://stackoverflow.com/questions/11413855/why-is-transposing-a-matrix-of-512x512-much-slower-than-transposing-a-matrix-of
#define SAMPLES 100
#ifndef MATSIZE
#define MATSIZE 512
#endif

#include <ctime>
#include <iostream>
int mat[MATSIZE][MATSIZE];

void transpose()
{
   for ( int i = 0 ; i < MATSIZE ; i++ )
   for ( int j = 0 ; j < MATSIZE ; j++ )
   {
       int aux = mat[i][j];
       mat[i][j] = mat[j][i];
       mat[j][i] = aux;
   }
}

int main()
{
   //initialize matrix
   for ( int i = 0 ; i < MATSIZE ; i++ )
   for ( int j = 0 ; j < MATSIZE ; j++ )
       mat[i][j] = i+j;

   int t = std::clock();
   for ( int i = 0 ; i < SAMPLES ; i++ )
       transpose();
   int elapsed = std::clock() - t;

   std::cout << "Average for a matrix of " << MATSIZE << ": " << elapsed / SAMPLES << std::endl;
}