How to prevent repetition of randomly generated options in C++?

Question

Here is what I have done so far, I did my best to label each bit of the code. It should also be noted that this was written in XCode, so it's running on a Mac.

/*
Ayush Sharma
4 November 2016
*/

#include <iostream>
#include <ctime>

using namespace std;

int main (){

//clearing the screen
system("clear");

//seeding the random
srand((unsigned int)time(NULL));

//variables & arrays
char answer;
int r, m, correct = 0;
string capitals[50] =
    {"Montgomery", "Juneau", "Phoenix", "Little Rock", "Sacramento", "Denver", "Hartford", "Dover", "Tallahassee", "Atlanta", "Honolulu", "Boise", "Springfield", "Indianapolis", "Des Moines", "Topeka", "Frankfort", "Baton Rouge", "Augusta", "Annapolis", "Boston", "Lansing", "St. Paul", "Jackson", "Jefferson City", "Helena", "Lincoln", "Carson City", "Concord", "Trenton", "Santa Fe", "Albany", "Raleigh", "Bismarck", "Columbus", "Oklahoma City", "Salem", "Harrisburg", "Providence", "Columbia", "Pierre", "Nashville", "Austin", "Salt Lake City", "Montpelier", "Richmond", "Olympia", "Charleston", "Madison", "Cheyenne"};
string states[50] = {"Alabama","Alaska","Arizona","Arkansas","California","Colorado","Connecticut","Delaware","Florida","Georgia","Hawaii","Idaho","Illinois","Indiana","Iowa","Kansas","Kentucky","Louisiana","Maine","Maryland","Massachusetts","Michigan","Minnesota","Mississippi","Missouri","Montana","Nebraska","Nevada","New Hampshire","New Jersey","New Mexico","New York","North Carolina","North Dakota","Ohio","Oklahoma","Oregon","Pennsylvania","Rhode Island","South Carolina","South Dakota","Tennessee","Texas","Utah","Vermont","Virginia","Washington","West Virginia","Wisconsin","Wyoming"};

//title
cout << "***************************************************************\n";
cout << "*                                                             *\n";
cout << "*                 United States Capitals Quiz                 *\n";
cout << "*                                                             *\n";
cout << "***************************************************************\n\n";



for (int i = 0; i < 15; i++){
    //Picking A Random State
    r = rand() % 50;
    //Checking if State is a Repeat
    if (states[r] != "-1") {
        cout << "What is the capital of " << states[r] << "? ";
        //Picking Correct Answer Choice and Respective Layout
        m = rand() % 4;
        if (m == 0) {
            cout << "\nA: " << capitals[r] << endl;
            cout << "B: " << capitals[rand()%50] << endl;
            cout << "C: " << capitals[rand()%50] << endl;
            cout << "D: " << capitals[rand()%50] << endl;
        }
        if (m == 1) {
            cout << "\nA: " << capitals[rand()%50] << endl;
            cout << "B: " << capitals[r] << endl;
            cout << "C: " << capitals[rand()%50] << endl;
            cout << "D: " << capitals[rand()%50] << endl;
        }
        if (m == 2) {
            cout << "\nA: " << capitals[rand()%50] << endl;
            cout << "B: " << capitals[rand()%50] << endl;
            cout << "C: " << capitals[r] << endl;
            cout << "D: " << capitals[rand()%50] << endl;
        }
        if (m == 3) {
            cout << "\nA: " << capitals[rand()%50] << endl;
            cout << "B: " << capitals[rand()%50] << endl;
            cout << "C: " << capitals[rand()%50] << endl;
            cout << "D: " << capitals[r] << endl;
        }
        //Recieving Answer
        cout << "Answer: ";
        cin >> answer;

        //Converting Letter to Number
        if (answer == 'A' || answer == 'a') answer = 0; if (answer == 'B' || answer == 'b') answer = 1;
        if (answer == 'C' || answer == 'c') answer = 2; if (answer == 'D' || answer == 'd') answer = 3;

        //Comparing Answer to Correct Answer
        if (m == answer) {
            cout << "Correct!" << endl << endl;
            correct++;
        }else{
            cout << "Incorrect! The correct answer was " << capitals[r] << "! \n\n";
        }

        //Removing State from Array
        states[r] = "-1";

    }else{
        //If State was a Repeat, generate another State
        i--;
    }
}

//Printing Results
cout << "Number Correct: " << correct << "/15 or " << ((correct/15.00)*100) << "%!\n";

return 0;
}

The code works, almost. The problem is that answers are sometimes being repeated, such as in the scenario: What is the capital of Wisconsin? A. Madison B. Frankfort C. Jackson D. Madison Only A or D is the "correct" answer despite both having the same text (altough I'd rather make it impossible for the answers to repeat). I also would like to know if there is a more efficient way to create the layout of multiple choice questions. Thanks in advance!

-Ayush

Answer 1

Given that there are 50 values you want to draw from at random, without repetition, simply create an array or vector containing those values, shuffle it, and then access the elements of the shuffled array in order.

In C++11, this is easy using algorithms std::iota() and std::random_shuffle() from <algorithm>.

  int value[50];
  std::iota(std::begin(value), std::end(value), 0);   //  populate array with values 0 to 49

  std::random_shuffle(std::begin(value), std::end(value));

Then in your outer loop, instead of r = rand()%50 use r=value[i].

std::begin() and std::end() are in standard header <iterator>.

The same idea can be used before C++11, but the method is a little different (C++11 didn't support std::begin(), std::end() or std::iota(), but equivalents are easy enough to implement).

Instead of value being an array, I'd create it as an std::vector<int>, also with 50 elements. I've illustrated above using an array, since you seem to be defaulting to using an array.