I'm trying to get a random order of numbers 1:16 using srand and rand. The code works as expected but only up to the 12th iteration.Why?

Question

#include "stdafx.h"
#include <iostream>
#include <string>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <algorithm>
#include <iterator>


int _tmain(int argc, _TCHAR* argv[])
{
    using namespace std;   

    int ord[] = {
             0,0,0,0,
             0,0,0,0,
             0,0,0,0
            };

    srand(time(NULL));

    int i = 0;

    do{
        int r = rand() % 16 + 1;
        bool exista =( find(begin(ord), end(ord), r) != end(ord));
        if (!exista){
            ord[i] = r;
            cout << ord[i] << endl;
            i++;
            cin.get();
        }

    } while (find(begin(ord), end(ord), 0) != end(ord));
    return 0;
}

After 12 numbers ,the program exits and I cannot understand why ,it should be after 16 iterations .

Answer 1

The reason is that you have only 12 zeros in ord and when they're exhausted (in your case filled with unique values from 1 to 16 the loop stops (because there are no more zeros to find.

changing the declaration of ord to

int ord[] = {
         0,0,0,0,
         0,0,0,0,
         0,0,0,0,
         0,0,0,0
        };

or

int ord[16] = {0};

By the way if you only want to shuffle those numbers randomly I'd suggest to use random_shuffle from <algorithm> .

Answer 2

You defined an array size of 12 and you are filling the array ord[] with the randomized integers r . Even though you set the random num generator to run for 16 iterations, your array will be filled on the twelfth iteration. Just increase your array size.