random not working + counting time in C++

Question

When running the code below I get the following errors. Can you please explain to me why and what I should do? I picked up the code relative to the library online but it won't work. I'm using DevC++ 5.11.

1. default_random_engine is not a member of 'std'
2. uniform_real_distribution is not a member of 'std'
3. 'generator' was not declared in this scope
4. 'distribution' was not declared in this scope

Also, how do I count time in C++?

    #include <iostream>
    #include <random> 

    int main() {        

    int i, j, n, nmax;   
    std::default_random_engine generator;   
    std::uniform_real_distribution<double> distribution(0.0,1.0);                                   
    nmax = 3000;    

    for (n=200; n<nmax; i+=200) {       
       double x[n], y[n], A[n][n];      
       for (i=0; i<n; i++) {            
          x[i] = distribution(generator);           
          y[i] = 0;             
          for (j=0; j<n; j++) {
             A[i][j] = distribution(generator);             
          }         
       }        
       // start counting time 
       for (i=0; i<n; i++) {            
          for (j=0; j<n; j++) {
             y[i] = A[i,j] * x[j];          
          }         
       }        
       // stop counting time        
       // total_time = stop - start         
       // std::cout << total_time   
    } 
    return 0;    
    }

Answer 1

Bugs in code:

1. default_random_engine and uniform_real_distribution are not C++98 standards so all four errors are related with this. You should install C++11 compiler on your machine. Here if you play with standards in options section you will understand what I mean.

2. Change A[i,j] to A[i][j].

3. You should change i+=200 to n+=200 in the very first for loop.

4. You can create 2D vector this way:
vector<vector<int>> vec(row_num, vector<int>(col_num));

And as of calculation of execution time you could use <time.h>. Here is working code using std::vector:

#include <iostream>
#include <random> 
#include <time.h>
#include <vector>

int main()
{
    int nmax;
    std::default_random_engine generator;
    std::uniform_real_distribution<double> distribution(0.0,1.0);
    nmax = 3000;

    for (int n=200; n < nmax; n+=200) {
        std::vector<double> x(n);
        std::vector<double> y(n);
        std::vector< std::vector<double> > A(n, std::vector<double>(n));

        for (int i=0; i<n; i++) {
            x[i] = distribution(generator);
            y[i] = 0;

            for (int j=0; j<n; j++) {
                A[i][j] = distribution(generator);
            }
        }

        // start counting time
        clock_t start = clock();
        for (int i=0; i<n; i++) {
            for (int j=0; j<n; j++) {
                y[i] = A[i][j] * x[j];
            }
        }
        // stop counting time
        clock_t stop = clock();
        // total_time = stop - start
        double elapsed = (double)(stop - start) * 1000.0 / CLOCKS_PER_SEC;
        std::cout << "Elapsed time: " << elapsed << std::endl;
    }
    return 0;
}