Problem with string input validation (C++)

Question

I ran into a problem while trying to validate my user input. Here's my code:

#include <iostream>
#include <string>

using namespace std;

int main() 
{
  string choice;


  cout << "Please type something: ";
  cin >> choice;
  cin.ignore();

  while (choice != "1" && choice != "2" && choice != "3" && choice != "4" && choice != "5")
  {
    cout << "Incorrect choice! Please input a valid number: >_";
    cin >> choice;
    cin.ignore();
  } 

  return 0;
}

When I input "wronginput", that input fell in the while loop and displayed

Incorrect choice! Please input a valid number: >_

Which is good. However, when I try "wrong input", I get a "weird" version of it:

Incorrect choice! Please input a valid number: >_ Incorrect choice! Please input a valid number: >_

My guess is that the space in between is the culprit. Is there any way for me to fix this? Thank you.

Answer 1

When you enter "wrong input" for input, the line

cin >> choice;

reads and stores "wrong" in choice .

The line

cin.ignore();

ignores only one character. Hence, you stull have "input" and the subsequent newline character in the stream. Next time you read into choice , you get "input" in choice .

That explains the behavior of your program.

In order to fix it, make sure to ignore the rest of the line instead of just one character. Use

cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

Add

#include <limits>

to be able to use std::numeric_limits .

Answer 2

That's because std::cin >> choice is a formatted input that will read everything to the next whitespace .

When you type in "wronginput", it reads it all, but if you type in "wrong input", only the "wrong" part will be read. Your while loop will fire, perform std::cin >> choice inside its body once again and that will read "input".

This behaviour causes your program to output the message twice in a row.

How to fix it? Instead of reading a single token , up to the next whitespace, consider reading an entire line .

---

How to achieve that? Simply instead of using

std::cin >> choice;

use a std::getline function:

std::getline(std::cin, choice);

both outside and inside your while loop.