Agfdhyk

I have a very simple question. The following code (with the purpose of creating a random matrix of ints) does not compile. Specifically the line

int m[nrow * ncol];

in the randomMatrix function causes the problem. It says the expression nrow*ncol has to be a constant value. I am new to C++ hence my attempts have been uncreative (e.g. putting 'const' in front of the arguments).

Can someone give me a hint?
Thank you.

#include "pch.h"

#include <iostream>

#include <ctime>



using namespace std;



int* randomMatrix(int nrow, int ncol) {     

    int m[nrow * ncol];

    int index;

    for (int col = 0; col < ncol; col++) {

        for (int row = 0; row < nrow; row++) {

            index = nrow * col + row;

            m[index] = rand() % 2;

        }

    }

    return m;

}





int main() {

    const unsigned int nrow = 10;

    const unsigned int ncol = 10;



    int m[nrow * ncol];

    m = randomMatrix(nrow, ncol);

    print(m, nrow, ncol);

    system("PAUSE");

}

You are getting an error because arrays lengths must be known at compile time. You are writing code that creates an array, but the length is only known at runtime... This means the compiler doesn't know how much space to allocate and you have to do it yourself. You could use malloc when you create the array - but I wouldn't recommend it.

The best bet is to use std::vector instead of an array.

The size of an array in C++ must be fixed at compile time. You can not have it depend on non-const parameters.

You should not use C-style arrays in C++ unless there is a good reason for it. The idiomatic way if the length is unknown at compile-time is to use a std::vector:

#include<vector> // <- At top



std::vector<int> m(nrow*ncol); // Instead of int m[nrow*ncol]

The return type of your function should then just be std::vector<int>.