How to make user input numbers only in C++?

Question

So far, this is my code:

while(bet > remaining_money || bet < 100)
    {
        cout << "You may not bet lower than 100 or more than your current money. Characters are not accepted." << endl;
        cout << "Please bet again: ";
        cin >> bet;
    }

It works fine but I'm trying to figure out how to make it loop if the user inputs anything that isn't a number as well.

When I press a letter or say a symbol/sign, the code just breaks.

Possible duplicate of [How to handle wrong data type input](http://stackoverflow.com/questions/10349857/how-to-handle-wrong-data-type-input) — Fureeish, Apr 17 '17 at 10:43

score 2 · Answer 1 · answered Apr 17 '17 at 11:02

2

Using the function

isdigit()

This function returns true if the argument is a decimal digit (0–9)

Don't forget to

#include <cctype>

answered Apr 17 '17 at 11:02

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score 1 · Answer 2 · answered Apr 17 '17 at 10:32

I would use std::getline and std::string to read the whole line and then only break out of the loop when you can convert the entire line to a double.

#include <string>
#include <sstream>

int main()
{
    std::string line;
    double d;
    while (std::getline(std::cin, line))
    {
        std::stringstream ss(line);
        if (ss >> d)
        {
            if (ss.eof())
            {   // Success
                break;
            }
        }
        std::cout << "Error!" << std::endl;
    }
    std::cout << "Finally: " << d << std::endl;
}

That's probably fine, but a bit asymmetric in that it accepts/ignores whitespace before the number but not after. You could use `ss >> d >> std::skipws` (and `#include `) to allow whitespace at either end, if it mattered. — Tony Delroy, Apr 17 '17 at 10:42

score 1 · Answer 3 · edited Dec 05 '21 at 09:17

A good way of doing this is to take the input as a string. Now find the length of the string as:

int length = str.length();

Make sure to include string and cctype. Now, run a loop that checks the whole string and sees if there is a character that is not a digit.

bool isInt = true; 
for (int i = 0; i < length; i++) {
        if(!isdigit(str[i]))
        isInt = false; 
    }

If any character is not a digit, isInt will be false. Now, if your input(a string) is all digits, convert it back to an integer as:

int integerForm = stoi(str);

Store integerForm in your array.

How to make user input numbers only in C++?

3 Answers3