How to detect that a console input is not a number in C++?

Question

This is the code...

#include <iostream>     
#include <iomanip>      
#include <cmath>    
#include <math.h>       

using namespace std;    

void CorrectPercent(double Percent) {

unsigned short int Error = 0;
while (Error==0){

    cout << "\nEnter the annual interest rate (%):\t\t";
    cin >> Percent;
do {
        if (Percent <= 0) {
            cout << endl << "Please enter a valid annual interest rate (%): ";
            cin >> Percent;
        } else if (isnan(Percent) == true) {
            cout << endl << "Please enter a valid annual interest rate (%): ";
            cin >> Percent;
        }
        else {
            Error=1;
        }
    } while (Error == 0);
    //Error++;
}//while (Error == 0);
}


int main()
{
double Percent;
char Retry = 'Y';

do
    {
        cout << setprecision(2) << fixed;
        system("cls");
        CorrectPercent(Percent);
    } while (Retry == 'Y' || Retry == 'y');
return 0;
}

The CorrectPercent function is supposed to keep running until a valid numeric value is entered. So, the question is, how can I detect that the input is numeric? Just for additional info, I'm working on Visual Studio 2015.

Code-Apprentice · Answer 1 · 2017-02-03T23:40:13.303

-3

Since Percent is declared as a double, cin >> Percent will fail whenever the user enters non-numerical data. You can detect this with cin.fail():

std::cin >> Percent;
while (std::cin.fail()) {
    std::cin.clear();
    std::cin.ignore(std::max<std::streamsize>());
    std::cout << "Please enter a valid number." << std::endl;
    std::cin >> Percent;
}

edited Feb 03 '17 at 23:40

answered Feb 03 '17 at 23:16

Code-Apprentice

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That's an infinite loop if it's ever entered. – Baum mit Augen Feb 03 '17 at 23:21
@BaummitAugen Oops...forgot the reset...that better? – Code-Apprentice Feb 03 '17 at 23:23
Still infinite loop, bad input is still in the stream. – Baum mit Augen Feb 03 '17 at 23:25
@BaummitAugen okay, thanks for reminding me. – Code-Apprentice Feb 03 '17 at 23:28
Pretty sure there's no `reset`. Perhaps you meant `clear`. – chris Feb 03 '17 at 23:35
There is no `std::cin::reset`, and `clear` does not clear the buffer. [Here's one way](http://melpon.org/wandbox/permlink/BuyY06SWAH5KhLu5), to get this done finally. XD – Baum mit Augen Feb 03 '17 at 23:36
@chris He already has a `clear`, he meant `ignore`. What's wrong with our established users today? =D – Baum mit Augen Feb 03 '17 at 23:37
@BaummitAugen Yah, the details are a bit fuzzy. Been a while since I did much C++. – Code-Apprentice Feb 03 '17 at 23:37
@BaummitAugen I don't really consider myself established in the C++ tag...otherwise I would have known where to find the dupe for this question. I'm much more active in other tags most of the time. – Code-Apprentice Feb 03 '17 at 23:39
Still doesn't even compile... Wanna just put this answer out of its misery? Like, copy-paste my snippet or just delete it? – Baum mit Augen Feb 03 '17 at 23:39

How to detect that a console input is not a number in C++?

1 Answers1