CODEWITHSUNDEEP
c++while-loopintswitch-statement
alecam
alecam

Reputation: 197

Switch-statement inside a while-loop which loop infinitely

Here is my code:

int main()
{
    int nothing;
    string name;
    int classnum;
    bool classchosen;
    string classname;

    cout << "Welcome adventurer, your journey is about to begin.\n\n";
    cout << "Firstly, what's your name? ";
    cin >> name;
    classchosen = false;
    while (classchosen == false)
    {
        cout << "\n\nNow, " << name << ", choose your class entering its number.\n\n";
        cout << "1- Warrior\n" << "2- Mage\n" << "3- Paladin\n" << "4- Monk\n\n";
        cout << "Class number: ";
        cin >> classnum;
        switch(classnum){
            case 1:
                    classname = "Warrior";
                    classchosen = true;
                    break;
            case 2:
                    classname = "Mage";
                    classchosen = true;
                    break;
            case 3:
                    classname = "Paladin";
                    classchosen = true;
                    break;
            case 4:
                    classname = "Monk";
                    classchosen = true;
                    break;
            default:
                    cout << "\nWrong choice, you have to enter a number between 1 and 4.\n" << endl;
                    break;
        }
    }
    cout << "\nSo you are a " << classname << " ? Well, tell me something more about you...\n";
    cin >> nothing;
    return 0;
}

Now, when I run it and input a string (for example "fjdfhdk") when it asks about the class number, the program loops infinitely instead of going in the default statement, writing again the question and letting me choose another class. Why?

Upvotes: 1

Views: 5808

Answers (2)

James Kanze
James Kanze

Reputation: 153977

Because you're reading into an int, and the read fails. This has two effects:

  1. your use of classnum afterwards is undefined behavior, and
  2. the stream has memorized the error condition, so you can check it later.

As long as the error condition is not cleared, all further operations on the stream are no-ops. The simplest changes in your program to make this work would be:

std::cin >> classnum;
if ( !std::cin ) {
    classnum = 0;
    std::cin.clear();
    std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );
}
switch ( classnum ) // ...

In case of an error, this sets classnum to a known value, clears the error state, and skips all input up to the next newline. (Otherwise, you'll just fail again, because the characters which triggered the error are still there.)

Consider, however, using a separate function to extract the int, and using getline, as per user814628's suggestion. The above is more to explain to you what is happening, and why your see the symptoms you see. user814628's suggestion is far better software engineering.

Upvotes: 1

dchhetri
dchhetri

Reputation: 7136

Try something like this:

#include <sstream>
#include <string>
using namespace std;

int getInt(const int defaultValue = -1){
  std::string input;
  cin >> input;
  stringstream stream(input);
  int result = defaultValue;
  if(stream >> result) return result;
  else return defaultValue;

}

//..in main
cout << "Class number: ";
int classNum = getInt();
switch(classNum){ .... }

The reason why it fails in your case is because cin is trying to read a bunch of chars into a int variable. You can either read it as a string and convert as necessary, or you can check the cin state explicitly when reading into a int variable by checking if any of the fail bits are set. The fail bits would be set if for example you try to read bunch of chars into an int.

Upvotes: 1

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