How to check if the input is a valid integer without any other chars?

user2699298 Published at Dev

user2699298

#include <iostream>
#include <limits>

using namespace std;

int main()
{
    int x;
    cout << "5 + 4 = ";
    while(!(cin >> x)){
        cout << "Error, please try again." << endl;
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }
    if (x == (5 + 4)){
        cout << "Correct!" << endl;
    }
    else{
        cout << "Wrong!" << endl;
    }
    return 0;
}

How can I check if the user inputs a valid integer? In this program I wrote above, if the user inputs 9, it should be correct, however, if the user inputs 9a for example, it should return an error, but it doesn't for some reason. How can I correct it?

How I did it using cin.peek()

#include <iostream>
#include <limits>
#include <stdio.h>

using namespace std;

int main()
{
    int x;
    bool ok;
    cout << "5 + 4 = ";

    cin >> x;

    while(!ok){
  cin >> x;

  if(!cin.fail() && (cin.peek() == EOF || cin.peek() == '\n')){
  ok = true;
  }
  else{
  cout << "Error, please try again." << endl;
  cin.clear();
  cin.ignore(numeric_limits<streamsize>::max(), '\n');
  }
    }

    if (x == (5 + 4)){
  cout << "Correct!" << endl;
    }
    else{
  cout << "Wrong!" << endl;
    }

    return 0;
}

Angew is no longer proud of SO

You could read a string, extract an integer from it and then make sure there's nothing left:

std::string line;
std::cin >> line;
std::istringstream s(line);
int x;
if (!(s >> x)) {
  // Error, not a number
}
char c;
if (s >> c) {
  // Error, there was something past the number
}

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