Loop until integer input is in required range fails to work with non-digit character inputs

Question

I'm having a problem with what should be incredibly simple code. I want to take in an integer between 1 and 3 with error checking. It works fine for checking for numbers that are too large or too small, but when a alpha/number combination is entered, it gets stuck in an infinite loop. Suggestions?

#include <iostream>
using namespace std;

int main(int argc, char *argv[]){
    int input;

    cout << "\nPlease enter a number from 1 to 3:" << endl;
    cout << "-> ";
    cin >> input;

    while(input< 1 || input> 3){
        cout << "\n---------------------------------------" << endl;
        cout << "\n[!] The number you entered was invalid." << endl;
        cout << "\nPlease re-enter a number from 1 to 3" << endl;
        cout << "-> ";
        cin >> input;
    }

    cout << "You chose " << input << endl;
}

Read string and stackoverflow.com/questions/2844817/… and stackoverflow.com/questions/7021725/… — Mateusz
– Mateusz, Commented Nov 3, 2012 at 18:21

Loki Astari · Accepted Answer · 2017-09-12 02:15:18Z

8

The problem is that:

cin >> input;

Will cause the bad bit to be set when you try and read a non numeric value. After that happens any attempt to use the operator>> is silently ignored.

So the way to correct for this is to test if the stream is in a good state and if not then reset the state flags and try and read again. But note that the bad input (that caused the problem) is still on the input so you need to make sure you throw it away as well.

if (cin >> input)
{
    // It worked (input is now in a good state)
}
else
{
    // input is in a bad state.
    // So first clear the state.
    cin.clear();

    // Now you must get rid of the bad input.
    // Personally I would just ignore the rest of the line
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    // now that you have reset the stream you can go back and try and read again.
}

To prevent it getting stuck (which is caused by the bad bit being set) read into a string then use a string stream to parse user input. I also prefer this method (for user interactive input) as it allows for easier combination of different styles of reading (ie combining operator>> and std::getline() as you can use these on the stringstream).

#include <iostream>
#include <sstream>
#include <string>
// using namespace std;
// Try to stop using this.
// For anything other than a toy program it becomes a problem.

int main(int argc, char *argv[])
{
    int          input;

    std::string  line;
    while(std::getline(std::cin, line))   // read a line at a time for parsing.
    {
        std::stringstream linestream(line);
        if (!(linestream >> input))
        {
             // input was not a number
             // Error message and try again
             continue;
        }
        if ((input < 1) || (input > 3))
        {
             // Error out of range
             // Message and try again
             continue;
        }
        char errorTest;
        if (linestream >> errorTest)
        {
             // There was extra stuff on the same line.
             // ie sobody typed 2x<enter>
             // Error Message;
             continue;
        }

        // it worked perfectly.
        // The value is now in input.
        // So break out of the loop. 
        break;
    }
}

edited Sep 12, 2017 at 2:15

answered Nov 3, 2012 at 18:28

Loki Astari

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12 Comments

Loki Astari Over a year ago

@tuergeist: Fixed. But you could have fixed it for me.

Loki Astari Over a year ago

@emileb: I would disagree. Read: Why is “using namespace std;” considered bad practice? The using construct is a problem in most situations and leads to the introduction of bugs when you maintain the code that are hard to spot and diagnose.

Emile Bergeron Over a year ago

@LokiAstari I'm aware of the possible problems that this may lead to, but these problems are limited to the inside of a cpp source file if you keep the "using" stuff outside of any header file. Still, this may lead to problems, but in the code of the OP, it would be useless to be fully explicit.

Emile Bergeron Over a year ago

@LokiAstari another thing is, with e.g. using Foo::A; and then, you add using Bar::A, it's easy to spot that you're making two class with the same name accessible in the same scope. Most of the time, you'll have your using lines at the same place in your cpp, making the possible errors easy to fix. Never ever use using namespace A; and using namespace B;, with two namespaces fully accessible like this, your asking for trouble. I'm giving you the point still, better to avoid if you don't know the risk.

Loki Astari Over a year ago

@MikaelMello: Thanks. Fixed.

|

pokey909 · Accepted Answer · 2012-11-03 18:35:58Z

2

#include <iostream>
#include <string>

using namespace std;

int validatedInput(int min = 1, int max = 3)
{
    while(true)
    {
        cout << "Enter a number: ";
        string s;
        getline(cin,s);
        char *endp = 0;
        int ret = strtol(s.c_str(),&endp,10);
        if(endp!=s.c_str() && !*endp && ret >= min && ret <= max)
            return ret;
        cout << "Invalid input. Allowed range: " << min << "-" << max <<endl;
    }
}

int main(int argc, char *argv[])
{
    int val = validatedInput();
    cout << "You entered " << val <<endl;
    return 0;
}

answered Nov 3, 2012 at 18:35

pokey909

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3 Comments

tuergeist Over a year ago

you forgot to #include <stdlib.h> for strtol

pokey909 Over a year ago

hmm runs for me even without inclusion. but i think that might be the least of his problems ;-)

tuergeist Over a year ago

strtol - newer compiler would tell you this.

Community · Accepted Answer · 2017-05-23 11:54:16Z

1

Most of these answers include unnecessary complexity.

Input validation is a perfect time to use a do-while

do{

   cout << "\nPlease enter a number from 1 to 3:" << endl;
   cout << "-> ";

   if(!cin){
      cout << "Invalid input"
      cin.clear()
      cin.ignore(numeric_limits<streamsize>::max(), '\n');
   }

}while(!(cin >> input))

Use numeric_limits<streamsize>::max() to completely clear the buffer after a failed cin.
Use cin.clear() to reset the fail flag on cin so !cin wont always evaluate false.

cin.fail() is fine. However some would consider !cin more natural.

from my previous post https://stackoverflow.com/a/43421325/5890809

edited May 23, 2017 at 11:54

CommunityBot

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answered Apr 15, 2017 at 1:53

Shan L

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Comments

RDismyname · Accepted Answer · 2012-11-03 18:28:22Z

0

You declared input as int but when you write an alphanumeric character to input it will try to implicitly convert it into integer. But you error checking does not account for this.

Ur problem can be easily solved by changing your while loop. instead of checking this how about you check

while(input!=1 || input!=2 || input!=3)

answered Nov 3, 2012 at 18:28

RDismyname

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2 Comments

Loki Astari Over a year ago

Unfortunately not. The stream is in a bad state. You need to reset the state flags before retrying the read.

user0 Over a year ago

What if we have to check if input is between 1 and 12000, or just validate input to see that the user enters an integer and not alphanumeric gibberish?

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