Question

I got this strange problem while writing code for an exercise.

First when I pick the first option, enter a faulty entry. It’s supposed to go to an else branch of my code but instead it gets stuck in there. I really don’t know why. This also happens when I enter a “game title” with a space in between.

Secondly the line I commented out at the delete branch:

iter = gameTitles.erase(iter);

… doesn’t work at all. What I’m trying to do is deleting an entry by typing it out and after that comparing it to an entry so it knows what to delete. That’s why I’m also using the iterator.

// Exercise 1
/*
Write a program using vectors and iterators that allows a user to maintain a list of
his or her favorite games. The program should allow the user to list all game titles,
add a game title, and remove a game title.
*/

#include <iostream>
#include <windows.h>
#include <string>
#include <vector>

using namespace std;

int main(){
    bool    bLoop   = true;
    int     nChoice;
    char    cChoice;
    string  sInput;

    vector<string>::const_iterator iter;
    vector<string> gameTitles;

    while(bLoop){
        // -Head
        cout << "///////////////////////////////////\n// My Favorite Games\n\n";
        cout << "1. Add title\n2. Delete title\n3. Clear list\n\n";

        // -List
        if(!gameTitles.empty()){
            for(iter = gameTitles.begin(); iter!=gameTitles.end(); ++iter){
                cout << "-" << *iter << endl;
            }
        }

        cout << "\n:: ";
        cin  >> nChoice;

        // 1. Add
        if(nChoice == 1){
            cout << "\nGame Title: ";
            cin >> sInput;
            gameTitles.push_back(sInput);
        }

        // 2. Delete
        else if(nChoice == 2) {
            cout << "Delete Title: ";
            cin >> sInput;

            for(iter = gameTitles.begin(); iter!=gameTitles.end(); ++iter){
                if(*iter == sInput){
                    cout << "erased";
                    //iter = gameTitles.erase(iter);
                }
            }
        }

        // 3. Clear
        else if(nChoice == 3){
            cout << "Are you sure? (y/n) ";
            cin >> cChoice;
            if(cChoice == 'y'){
                gameTitles.clear();
            }
        } else {
            cout << "\nInvalid Choice, Please try again.\n";
        }

        // -Clean
        system("PAUSE");
        system("cls");
    }
}

EDIT: Solved first issue. Used a normal iterator instead of a constant one

EDIT2: Solved second issue, here is my corrected code:

// Exercise 1
/*
Write a program using vectors and iterators that allows a user to maintain a list of
his or her favorite games. The program should allow the user to list all game titles,
add a game title, and remove a game title.
*/

#include <iostream>
#include <windows.h>
#include <string>
#include <vector>

using namespace std;

int main(){
    bool    bLoop   = true;
    int     nChoice;
    char    cChoice;
    string  sInput;

    vector<string>::iterator iter;
    vector<string> gameTitles;

    while(bLoop){
        // -Head
        cout << "///////////////////////////////////\n// My Favorite Games\n\n";
        cout << "1. Add title\n2. Delete title\n3. Clear list\n\n";

        // -List
        if(!gameTitles.empty()){
            for(iter = gameTitles.begin(); iter!=gameTitles.end(); ++iter){
                cout << "-" << *iter << endl;
            }
        }

        cout << "\n:: ";
        cin >> nChoice;

        if(cin.fail()){
            cin.clear();
            cin.ignore();
        }

        // 1. Add
        if(nChoice == 1){
            cout << "\nGame Title: ";
            cin >> sInput;
            gameTitles.push_back(sInput);
        }

        // 2. Delete
        else if(nChoice == 2) {
            cout << "Delete Title: ";
            cin >> sInput;

            for(iter = gameTitles.begin(); iter!=gameTitles.end(); ){
                if(*iter == sInput){
                    cout << "erased";
                    iter = gameTitles.erase(iter);
                } else {
                    ++iter;
                }
            }
        }

        // 3. Clear
        else if(nChoice == 3){
            cout << "Are you sure? (y/n) ";
            cin >> cChoice;
            if(cChoice == 'y'){
                gameTitles.clear();
            }
        } else {
            cout << "\nInvalid Choice, Please try again.\n";
        }

        // -Clean
        system("PAUSE");
        system("cls");
    }
}

Answer 1

There are actually two questions:

1. Why does my input get stuck if enter something faulty? You need to verify that you input was successful using something like if (std::cin >> nChoice) { /* actual processing */ }. Note that the value of nChoice won’t change when the input fails. If the input failed, you need to do some error recovery: The stream has gone into fail state (i.e., std::ios_base::failbit is set in the error flags) and won’t refuse to do any further input until it got clear()ed. This still leaves the offending character(s) in the input which you may want to ignore().

2. Why is the loop using erase() misbehaving? When you actually do erase() a value you don’t want to increment the iterator again at the end of the loop. If you do, it may very well move the iterator beyond the end resulting in undefined behavior. That is, the loop should look something like this:

   for(iter = gameTitles.begin(); iter!=gameTitles.end(); ) {
       if (*iter == sInput) {
            cout << "erased";
            iter = gameTitles.erase(iter);
       }
       else {
           ++iter;
       }
   }
   

   Of course, a short version of the same logic is this:

   gamesTitles.erase(std::remove(gamesTitles.begin(), games.Titles.end(),
                                 sInput),
                     games.Titles.end());