Integer Input Validation in C++

Question

I have a question about input validation in C++. Here is my code I am having trouble with:

#include <iostream>
using namespace std;

int main()
{
    int in;

    while(true)
    {
        if(cin >> in)
        {
            if(in < 0 || in > 2)
            {
                cout << "Invalid input.  Number needs to be 0, 1, or 2.";
                cin.clear();
                while(cin.get() != '\n');
                cin.ignore();
            }
            else
            {
                cout << "Output: " << in;
                break;
            }
        }
        else
        {
            cout << "Invalid input. Please enter a number.";
            cin.clear();
            while(cin.get() != '\n');
            cin.ignore();
        }
    }
}

This code works fine unless two invalid entries are made in a row with the second input of the form '12hfhd'. Then it accepts this as input and I can't figure out why. I have searched on SO and have found a bunch of questions regarding input validation but can't seem to find any about their code accepting certain input.

if you want it to be between 0 and 2 it should be if( in < 0 || in > 2 ) — Fabio
– Fabio, Commented Jun 29, 2014 at 20:37
you mean, when it outputs that the output should be 0, 1 or 2 when I write something it will end? — Creris
– Creris, Commented Jun 29, 2014 at 20:38
The definition is that it reads a number and stops at the first character it doesn't understand. Only if the number of characters read is zero, it is considered failure, otherwise the rest remain buffered for the next read operation. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Jun 29, 2014 at 21:00

Ulrich Eckhardt · Accepted Answer · 2014-06-29 21:40:04Z

5

The main problem is that, when requesting an int from std::cin using the >> operator, a sequence of numeric chars from the beginning of the input will be converted. Examples:

2 will convert to 2
75$ will convert to 75
12asdfgh will convert to 12
hello,world will convert to 0, because the first char is already not a number

The best thing to do is to use some char operations:

int getNumber() {
  char input;
  std::cin >> input;
  if(input > '2' || input < '0') { // yes, chars act like ASCII numbers
    // handle this problem
    return -1; //error
  } else {
    return int(input - '0'); // input's code - '0''s code = number
  }
}

edited Jun 29, 2014 at 21:40

Ulrich Eckhardt

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answered Jun 29, 2014 at 20:48

user3130569

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4 Comments

Ulrich Eckhardt Over a year ago

"hello,world" will not convert to zero but set the failbit.

Moha the almighty camel Over a year ago

I thin you mean if(input > '2' || input < '0')

homegrown Over a year ago

Thank you for the answer and the explanation of what is going on behind the scenes.

Ulrich Eckhardt Over a year ago

The code above doesn't even check if input succeeded. If it didn't, the char can contain random garbage. Any junk (e.g. "2asdfgh" from "12asdfgh") will remain in the stream's buffer and probably cause confusion in subsequent operations. Also, please don't start using error codes but just throw a runtime_error, which can hold a free-form string.

Ulrich Eckhardt · Accepted Answer · 2014-06-29 20:58:48Z

I'd use the following approach when dealing with user input:

string l;
if(!getline(cin, l))
    /* handle error that no input could be read at all */
try
{
    int const res = boost::lexical_cast<int>(l);
    /* numerically validate input here */
    return res;
}
catch(exception const&)
{
    /* handle parsing error here */
}

In words, read a line and then parse and validate it using Boost's lexical_cast() function template. Note that the first error, that getline() fails, happens if someone reads input from a file (e.g. via shell redirect), but this can also be achieved with certain keypresses, depending on the shell. This state can't be recovered from, so prompting for a different answer will result in an endless loop.

KjMag · Accepted Answer · 2014-06-29 21:04:42Z

0

If you look at the documentation of the >> extraction operator, for example here:

http://www.cplusplus.com/reference/istream/istream/operator%3E%3E/

you will notice the following quote:

(1) arithmetic types Extracts and parses characters sequentially from the stream to interpret them as the representation of a value of the proper type, which is stored as the value of val.

This essentially means that the program will try to treat all the data you pass as formatted in the format specified by the rvalue, in your case int. Simpler: in your case it will try to make ints of what you're passing into the stream and make the data as 'integerish' as it gets.

answered Jun 29, 2014 at 21:04

KjMag

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