c++ checking whether input is a number

Question

I am using netbean ide for c++ study I would like to force the user to pick only number between 1 to 3

int displayMenu()
{
    while(true)
    {  
        cout<<"Please choose one of following options"<<endl;
        cout<<"1. deposit"<<endl;
        cout<<"2. withdraw"<<endl;
        cout<<"3. exit"<<endl;
        int input;
        cin >>input;

        if(input>=1 && input<=3 && cin)
        {
            return input;
            break;
        }
        else
        {
            cout<<"You have only 3 options. Please chooses 1,2 or 3"<<endl;
        }
    }
}

It works fine if the input is int number If input is less then 1 or greater than 3, this function re-ask user to input number btw 1 and 3.

However, if the input is character such as 'f', it does an infinite loop. This function know that 'f' is wrong input..

I did my own research in the Internet. !cin and cin.fail() do not work.

Can you help me?

You need to add the isdigit check – Kiloreux Sep 08 '15 at 21:12 — Kiloreux, Sep 08 '15 at 21:12

Maksim Solovjov · Answer 1 · 2015-09-08T21:25:13.100

0

When you try to read an integer but pass something else, the reading fails and the stream becomes invalid. Whatever caused the error remains in the stream. This leads to infinite loop.

To fix that, clear the error flags and ignore the rest of the line in your else clause:

    else
    {
        cout<<"You have only 3 options. Please chooses 1,2 or 3"<<endl;
        cin.clear(); // remove error flags
        // skip until the end of the line
        // #include <limits> for std::numeric_limits
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

edited Sep 08 '15 at 21:25

answered Sep 08 '15 at 21:14

Maksim Solovjov

3,147
18
28

Better use `std::numeric_limits::max()` instead of that magic number. – Emil Laine Sep 08 '15 at 21:22

psliwa · Answer 2 · 2015-09-08T23:22:34.907

0

You can modify it like this:

int displayMenu()
{
    while(true)
    {  
        cout<<"Please choose one of following options"<<endl;
        cout<<"1. deposit"<<endl;
        cout<<"2. withdraw"<<endl;
        cout<<"3. exit"<<endl;
        char input = cin.get();  //read a character

        cin.ignore(numeric_limits<streamsize>::max(), '\n');  //skip the rest of characters in cin buffer so that even if a user puts "123" only the first one is taken into account

        if(input>='1' && input<='3' && cin)  //check whether this is '1', '2' or '3'
        {
            return input;
            break;
        }
        else
        {
            cout<<"You have only 3 options. Please chooses 1,2 or 3"<<endl;
        }
    }
}

edited Sep 08 '15 at 23:22

answered Sep 08 '15 at 21:14

psliwa

1,094
5
9

2

256 is not sufficient amount of characters skipped. – Creris Sep 08 '15 at 21:26
True enough. You lose almost nothing going completely gonzo with `cin.ignore(numeric_limits::max(), '\n');` Just remember to `#include ` – user4581301 Sep 08 '15 at 22:20
@user4581301 Thank you, the answer has been gonzoed ;) – psliwa Sep 08 '15 at 23:23

c++ checking whether input is a number

2 Answers2