CODEWITHSUNDEEP
c++cin
Omkant
Omkant

Reputation: 9204

Why am I getting infinite loop?

int main()
{
  unsigned short wC;

  while(1)
  {
    cout << "Enter the Value"<<endl;
    cin >> wC;
    if(wC < 0 || wC > 3)
    {
      cout << "Wrong value, Please Enter again" << endl;
    }
    else break;
  }// end of while

  cout << "The value is : " << wC << endl;
}//end of main

In the above code, when I give value in range of short 0xffff it work fine. And comes out of loop and print the value only when user has given value for wC in between 0 to 3 otherwise prompt message to Enter again and wait for user's input.

But if value entered for wC is more than 0xffff then it goes to Infinite Loop.

I think it is due to value still exist in input cin buffer or what? Please help and provide some solution(hint), so that it should work.

NOTE : User is free to give any integer value. the code has to filter it out. Using g++ compiler on Ubuntu/Linux... and sizeof(unsigned short int) = 2 bytes

Upvotes: 1

Views: 231

Answers (1)

Sebastian Redl
Sebastian Redl

Reputation: 71909

If input fails, either because what you entered isn't an integer at all or because it overflows or underflows the target type, cin goes into an error state (cin.good() returns false) and any further read operation is a no-op. Put calls to cin.ignore (to clear out remaining input) and cin.clear (to reset error flags) into the loop.

Even then, you can still run into an issue if the user enters EOF (Ctrl+D on Unix, Ctrl+Z on Windows). You need your loop to understand that part too and break out.

#include <iostream>
#include <limits>
using namespace std;
int main() {
  unsigned short value;
  cout << "Enter the value (0-3):" << endl;
  while(true) {
    cin >> value;
    if (!cin || value < 0 || value > 3) {
      if (cin.eof()) {
        cout << "End of input, terminating." << endl;
        return 0;
      }
      cout << "Bad input, try again (0-3):" << endl;
      cin.clear();
      cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }
  }
  cout << "The value is: " << value << endl;
}

Of course that is one hell of a lot of code just to input one number. You can practice coming up with good interfaces by trying to write a function that deals with this stuff for you.

Upvotes: 2

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