C++ calculate Row major and Column major access time - c++

Question

I want to calculate access time for these two ways : Row major and Column major
as we know C/C++ is Row major , so when we process in first way (Row major) we should be faster.
but look at this code in C++ language
#include <iostream>
#include <time.h>
#include <cstdio>
clock_t RowMajor()
{
char* buf =new char [20000,20000];
clock_t start = clock();
for (int i = 0; i < 20000; i++)
for (int j = 0; j <20000; j++)
{
++buf[i,j];
}
clock_t elapsed = clock() - start;
delete [] buf;
return elapsed ;
}
clock_t ColumnMajor()
{
char* buf =new char[20000,20000];
clock_t start = clock();
for (int i = 0; i < 20000; i++)
for (int j = 0; j < 20000; j++)
{
++buf[j,i];
}
clock_t elapsed = clock() - start;
delete [] buf;
return elapsed ;
}
int main()
{
std::cout << "Process Started." << std::endl;
printf( "ColumnMajor took %lu microSeconds. \n", ColumnMajor()*1000000/ (CLOCKS_PER_SEC) );
printf( "RowMajor took %lu microSeconds. \n", RowMajor() *1000000/ (CLOCKS_PER_SEC) );
std::cout << "done" << std::endl; return 0;
}
but whenever i run this code i get diffrent answers , sometimes Rowmajor time is grater than column major time and sometimes is opposite,
any help is apriciated.

Answer 1

in c++ the coma operator can't be used create/access matrix thing. To make a matrix you need to keep track of with and height and allocate all the memory as an array. Basically you need to create a vector with the number or elements equivalent to number of elements in the matrix and you get each element by taking the x + y * width.
clock_t RowMajor()
{
int width = 20000;
int height = 20000;
char* buf = new char[width * height];
clock_t start = clock();
for (int j = 0; j < height; j++)
for (int i = 0; i < width; i++)
{
++buf[i + width * j];
}
clock_t elapsed = clock() - start;
delete[] buf;
return elapsed;
}
for ColumnMajor the buf needs to be accessed with buf[j * width + i];
An alternative way to create a matrix (from comments, thanks to James Kanze) is to create the buffer like so: char (*buf)[20000] = new char[20000][200000]. In this case, accessing the buffer is like: buf[i][j]
The safest way to do this is to use std::vector or array, and avoid using new/delete. Use std::vector to prevent buffer write overflows:
clock_t RowMajor()
{
int width = 20000;
int height = 20000;
std::vector<char> buf;
buf.resize(width * height);
clock_t start = clock();
for (int j = 0; j <height; j++)
for (int i = 0; i <width; i++)
{
++buf[i + j * width];
}
clock_t elapsed = clock() - start;
return elapsed;
}

Answer 2

Thanks to Raxvan ,this is the final code works fine so far
#include <iostream>
#include <time.h>
#include <cstdio>
#include <windows.h>
int calc = 0;
clock_t RowMajor()
{
int width = 20000;
int height = 20000;
char* buf = new char[width * height];
clock_t start = clock();
for (int j = 0; j < height; j++)
for (int i = 0; i < width; i++)
{
++buf[i + width * j];
}
clock_t elapsed = clock() - start;
delete[] buf;
return elapsed;
}
clock_t ColumnMajor()
{
int width = 20000;
int height = 20000;
char* buf = new char[width * height];
clock_t start = clock();
for (int j = 0; j < height; j++)
for (int i = 0; i < width; i++)
{
++buf[j + width * i];
}
clock_t elapsed = clock() - start;
delete[] buf;
return elapsed;
}
int main()
{
std::cout << "Process Started." << std::endl;
calc= ColumnMajor() /CLOCKS_PER_SEC ;
printf( "ColumnMajor took %lu . \n", calc );
calc=RowMajor()/CLOCKS_PER_SEC ;
printf( "RowMajor took %lu . \n", calc );
std::cout << "done" << std::endl; return 0;
}