Random Permutation in C++

rand_perm.c++

// random_shuffle example
#include <iostream>     // std::cout
#include <algorithm>    // std::random_shuffle
#include <vector>       // std::vector
#include <ctime>        // std::time
#include <cstdlib>      // std::rand, std::srand
using namespace std;

const int SIZE = 5;

// random generator function:
int generateSeed (int i) {
    return rand() % i;
}

int * randPerm() {
    static int arr[SIZE];
    
    // srand - produce a seed value (randomness)
    // https://stackoverflow.com/a/4736527/9049053
    srand ( unsigned ( time(0) ) );
    
    // create a vector
    vector<int> myvector;
    
    // push some value (0-4) into the vector
    for (int i=0; i<SIZE; i++) {
        myvector.push_back(i);
    }
    
    // shuffle all elements inside the vector
    random_shuffle(myvector.begin(), myvector.end());
    
    // if we run this program more than 1 time, we will find that every time the vector will be same
    // so we pass in a random number to make sure it is really random
    // try to comment out this line so see what I mean
    random_shuffle(myvector.begin(), myvector.end(), generateSeed);
    
    // convert vector to an array (we want to return an array)
    copy(myvector.begin(), myvector.end(), arr);
    
    return arr;
}

int main () {
    int *permArray = randPerm();
    for (int i=0; i<SIZE; i++) {
        cout << permArray[i] << ' ';
    }
    cout << "\n";
    
    int *permArray2 = randPerm();
    for (int i=0; i<SIZE; i++) {
        cout << permArray2[i] << ' ';
    }
    cout << "\n";
    
    int *permArray3 = randPerm();
    for (int i=0; i<SIZE; i++) {
        cout << permArray3[i] << ' ';
    }
    cout << "\n";
    
    return 0;
}