c++ - My while statement is getting infinite loop? -

#include "stdafx.h" #include <iostream> #include <string>  #include <stdio.h> #include <stdlib.h> #include <time.h>    using namespace std;  //this program let user input assignment score , see letter grade int main() {      int score;      cout << "input score: ";      //to make while loop     int x = 1;      while (x == 1) {          cin >> score;          if (score >= 90){             cout << "\na";             break;         }          else if (score >= 80) {             cout << "\nb";             break;         }          else if (score >= 70) {             cout << "\nc";             break;         }          else if (score >= 60) {             cout << "\nd";             break;         }          else if (score >= 0) {             cout << "\nf";             break;         }          else             cout << "\ninvalid input";     } } 

i'm trying write program let user input score assignment , display resulting letter grade. if user input not valid score, prints "invalid input" , should ask user input again. however, when run program , type in invalid value, goes infinite loop of printing "invalid input". why this? in advance.

when user enters invalid input, cin >> score fails , leaves error flag set on stream.

subsequent read operations don't until clear flag std::basic_ios::clear().

furthermore, since read failed, score has unspecified value (as did not initialise it), , apparently on test runs unspecified value happens not match of continues, never hit break.

instead of just:

std::cin >> score; 

try this:

if (!(cin >> score)) {    // if reading int failed, come here    cout << "invalid value! try again" << endl;     // clear error flag    cin.clear();     // restart loop    continue; } 

you may need ask stream eat newlines in input buffer. if "invalid value!" message twice, on how that.