Code compiled in Xcode gives an incorrect(?) input vs several other compilers with same inputs

Question

Test inputs and outputs from Xcode (incorrect) and other (web based compilers) are as following:

Input 1: 3 30 0.90 40

Xcode Output 1: Acceptable success rate: 0.90
Average attempted sales: 30.00
Average completed sales: 19.33
Observed success rate: 0.64
Observed success rate not in acceptable range.
Program ended with exit code: 0

Other Compilers' Output 1(correct): Acceptable success rate: 0.90
Average attempted sales: 30.00
Average completed sales: 19.67
Observed success rate: 0.66
Observed success rate not in acceptable range.

Input 2: 10 100 0.5 1

Xcode Output 2: Acceptable success rate: 0.50
Average attempted sales: 100.00
Average completed sales: 29.40
Observed success rate: 0.29
Observed success rate not in acceptable range.
Program ended with exit code: 0

Other Compilers' Output 1(correct): Acceptable success rate: 0.50
Average attempted sales: 100.00
Average completed sales: 28.80
Observed success rate: 0.29
Observed success rate not in acceptable range.

#include <iostream>
#include  <iomanip>
#include <stdlib.h>
using namespace std;

int main()
{
    int iterations, seed;
    double successRate;

    float completedSales = 0, numAttempted;
    int  i, t;
    int seatChoice;

    //Add other variables necessary for your program

    //cout << "Please enter number of iterations to be evaluated" <<endl;
    cin >> iterations; //number of iterations to be evaluated

    //cout << "Please enter number of attempted sales" <<endl;
    cin >> numAttempted; //number of attempted sales

    //cout << "Please enter expected success rate" <<endl;
    cin >> successRate;

    //cout << "Please enter seed for random number generation" <<endl;
    cin >> seed; //seed for random number generation


    //Validate input
    if(iterations <= 0 || numAttempted < 0 || seed < 0 ) {
        cout << "Error. Invalid parameters.";

    }
    else if(successRate < 0 || successRate > 1) {
        cout << "Error. Invalid parameters.";
    }
    else {
        //Set seed and start iterations
        srand(seed);

        //Every iteration of the simulation starts by setting all seats to empty

        for(i = 0; i < iterations; ++i) {

            int availableSeats[30] = {0};

            for(t = 0; t < numAttempted; ++t) {
                seatChoice = (rand() % 30);

                if(availableSeats[seatChoice] == 0) {
                    availableSeats[seatChoice] = 1.0;
                    completedSales++;
                }
            }
        }
        float averageSales = (float) completedSales/iterations;
        float observedSuccessRate = (float) averageSales/numAttempted;

        //Complete final output:
        cout << setprecision(2) <<fixed;
        cout << "Acceptable success rate: "  <<successRate <<endl;
        cout << "Average attempted sales: " <<(double)numAttempted <<endl;
        cout << "Average completed sales: " <<averageSales <<endl;
        cout << "Observed success rate: " <<(float)observedSuccessRate <<endl;

        //Complete if statement to test success rate
        if(observedSuccessRate < successRate) {
            cout << "Observed success rate not in acceptable range." << endl;
        }
        else {
            cout << "Observed success rate is acceptable." << endl;
        }
    }
    return 0;
}

Answer 1

Upon re-reading your code, is the "incorrect" behavior simply the different outcome given the same seed? This could be due to a different implementation of srand and rand between C++ compilers (and standard libraries).

Xcode uses Clang, which defaults to libc++, while your other compilers might use GCC, which uses lib std c++. The Microsoft Visual Studio compiler has its own implementation as well. In general, you cannot rely on these implementations being equal.

Your options are to either create your own Pseudo-random number generator (PRNG) or use the built-in Mersenne twister generator std::mt19937 . That should yield the same results across platforms.