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

Answer 2

That's a pretty obvious thing to happen. An easy solution would be to make an array to hold the options already displayed. Use a while loop to add unique options to the array.You could check whether there is any repetition in the array using another function. Then, display capitals[r] along with three other options from the array.
bool noRepeat(int arr[], int o){
for(int i=0; i<3; i++){
if(arr[i] == o)
return false;
}
return true;
}
int main(){
...
//picking correct answer and determining layout
int m = rand()%4, n=0, y, options[3];
if (m == 0) {
while(n<3){
y = rand()%50;
if(noRepeat(options, y) && capitals[y]!=capitals[r])
options[n++] = y;
}
//display according to layout
cout << "\nA: " << capitals[r] << endl;
cout << "B: " << capitals[options[0]] << endl;
cout << "C: " << capitals[options[1]] << endl;
cout << "D: " << capitals[options[2]] << endl;
}
//do the same for the rest
...
}

Answer 3

Just like #Isaiah said you can use a while loop to test for the index generated to not be same your correct answer.
Something like :
int index = rand() % 50;
while(index == r)
{
index = rand() % 50;
}
cout << "B: " << capitals[index] << endl;
NOTE: this still can produce repeats of "incorrect answers", but i guess you get the point to avoid the repeats produced by rand. And obviously this is code is just to correct the repeats of corrects answer, you should be using rand at all as mentioned in comments by others.

Answer 4

Your problem is to pick three random capitals without repeats. So here's some pseudo code.
Put all capitals, except the true answer into a vector
Generate a random index into this vector
Use the capital at that index, and erase the capital from the vector
Repeat Steps 2 and 3 for the second and third random capitals. Note the random index should allow for the reduced vector size on each iteration.