pop_back() causes "Vector subscript out of range" - c++

Question

I'm making a program just to practice vectors and this has me stumped. The point of the program was to make a tournament to help people choose between restaurants.
Here is the code that causes an error:
#include <iostream>
#include <vector>
#include <string>
const int ITEMS_PER_MATCHUP = 2;
using namespace std;
int PromptChoice(string first, string second, string helpMessage = "") {
string input;
while (true) {
cout << "Which restaurant do you prefer, " << first << "(1) or " << second << "(2)? ";
cin >> input; //no need to protect as it is going into a string variable
if (input == "first" || input == "1" || input == "left" || input == "First" || input == "Left") {
return 0;
}
else if (input == "second" || input == "2" || input == "right" || input == "Second" || input == "Right") {
return 1;
}
// if the response was not recognized
else cout << endl << helpMessage << endl << "Please enter 1 or 2" << endl << "Please try again." << endl << endl;
}
}
int main() {
vector<string> list = { "Burger Place", "Italian Place", "Soup And Salad Place", "BBQ Place" };
vector<int> stillInTheRunning;
for (int index = 0; index < list.size(); index++) { //populates an array full of values 0 to the size of the array
stillInTheRunning.push_back(index);
}
int matchupsThisRound = stillInTheRunning.size();
for (int i = 0; i < matchupsThisRound; i += ITEMS_PER_MATCHUP) { //ITEMS_PER_MATCHEP == 2
int choice = PromptChoice(list[stillInTheRunning[i]], list[stillInTheRunning[i + 1]]);
//stillInTheRunning.erase(stillInTheRunning.begin() + i + choice);
stillInTheRunning.pop_back();
}
for (int i = 0; i < stillInTheRunning.size(); i++) { //print the vector
cout << endl << stillInTheRunning[i] << endl;
}
system("pause"); //I know I know don't use system. just for debuging?
}
If I understand the error it usually happens when you try to visit a vector index that is out of range. Something bigger than vector.size() - 1 but in this case it happened when I was trying to use vector.erase()
Thinking perhaps I just mugged the for loop I tried switching to pop_back() because I thought you can't mess that one up. But I still got the error.
Playing around with it I tried commenting out a few things.
Like if I comment out the prompting function:
for (int i = 0; i < matchupsThisRound; i += ITEMS_PER_MATCHUP) { //ITEMS_PER_MATCHEP == 2
//int choice = PromptChoice(list[stillInTheRunning[i]], list[stillInTheRunning[i + 1]]);
//stillInTheRunning.erase(stillInTheRunning.begin() + i + choice);
stillInTheRunning.pop_back();
}
No error
And if I comment out the pop_back():
for (int i = 0; i < matchupsThisRound; i += ITEMS_PER_MATCHUP) { //ITEMS_PER_MATCHEP == 2
int choice = PromptChoice(list[stillInTheRunning[i]], list[stillInTheRunning[i + 1]]);
//stillInTheRunning.erase(stillInTheRunning.begin() + i + choice);
//stillInTheRunning.pop_back();
}
also no error.
What is causing the problem???

Answer 1

I found it!
its here
PromptChoice(list[stillInTheRunning[i]], list[stillInTheRunning[i + 1]])
on the second run of the for loop i = 2;
after poping off one item off the back the vector's size was now 3 instead of 4 so stillInTheRunning[i + 1] was trying to get the item in index 3 which now does not exist.

Answer 2

The problem is when you pop_back, the size of stillInTheRunning decreases, but matchupsThisRound stays the same, so eventually the loop reaches the point where i+1 is out of the stillInTheRunning's range.
Also it's a bit weird that you're increasing i by 2 every cycle, but pop_back only once.