Loop

sum.cpp

#include <iostream>
#include <string>
#include <ctime>

#define N 1000

double array_sum(double a[N][N])
{
    int i,j;
    double s;
    s=0;
    for (i=0;i<N;++i)
    {
        for (j=0;j<N;++j)
        {
            s += a[i][j];
        }
    }
    return s;
}

double array_sum2(double a[N][N])
{
    int i,j;
    double s;
    s=0;
    for (j=0;j<N;++j)
    {
        for (i=0;i<N;++i)
        {
            s += a[i][j];
        }
    }
    return s;
}

int main(int argc, char** argv)
{
    using namespace std;
    double a[N][N];
    int i,j;

    for (j=0;j<N;++j)
    {
        for (i=0;i<N;++i)
        {
            a[i][j] = 0.01;
        }
    }

    {
        double sum = 0.0;
        clock_t begin = clock();
        for (i=0;i<2000;++i)
        {
            sum+=array_sum(a);
        }
        clock_t end = clock();
        std::cout << "sum = " << sum << std::endl;
        double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
        std::cout << "elapsed_secs " << elapsed_secs << std::endl;
    }

    {
        double sum = 0.0;
        clock_t begin = clock();
        for (i=0;i<2000;++i)
        {
            sum+=array_sum2(a);
        }
        clock_t end = clock();
        std::cout << "sum = " << sum << std::endl;
        double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
        std::cout << "elapsed_secs " << elapsed_secs << std::endl;
    }

    return 0;
}