C++ User input validation not working properly

Question

I have this code that asks for a number input and stores it in a variable. I'm trying to do validation on the input but it's behaving weirdly.

#include <iostream>
using namespace std;

float coursework_mark;
float exam_mark;
float module_mark;

int main() {

    //COURSEWORK INPUT WITH VALIDATION
    cout << "Please enter your coursework mark: ";
    while(!(cin >> coursework_mark)){
        cin.clear();
        cin.ignore(1000, '\n');
        cout << "Invalid data type, please enter valid coursework mark: ";
    }
    while (coursework_mark < 0 || coursework_mark > 100) {
        cout << "Out of range, please enter valid coursework mark: ";
        cin >> coursework_mark;
    }

    //EXAM MARK INPUT WITH VALIDATION
    cout << "Please enter your exam mark: ";
    while(!(cin >> exam_mark)) {
        cin.clear();
        cin.ignore(1000, '\n');
        cout << "Invalid data type, please enter valid exam mark: ";
    }
    while (exam_mark < 0 || exam_mark > 100) {
        cout << "Out of range, please enter valid exam mark: ";
        cin >> exam_mark;
    }

    //Coursework capping
    if (coursework_mark > exam_mark * 2) { coursework_mark = exam_mark * 2;}

    //Calculate Module mark
    module_mark = (coursework_mark* 0.4) + (exam_mark* 0.6);

    //Output results
    cout << coursework_mark << endl;
    cout << "Your module mark is " << module_mark << endl;
    if (module_mark >= 50) {
        cout << "Congratulations you have passed!" << endl;
    } else if (module_mark < 50) {
        cout << "Unfortunately, you have not passed" << endl;
    }


}

If user inputs '45kuefggf' the number 45 gets stored as the coursework mark and the code hits the line cout << "Out of range, please enter valid exam mark: ";. I have no idea why it's doing this, how do I make it so that it check if the user input a valid data type?

Please reduce the code to only the part causing the problem, see stackoverflow.com/help/minimal-reproducible-example . BTW you should sperate IO from calculation, use functions as modularization. Also the last else if could just be an else. — Superlokkus
– Superlokkus, Commented Oct 8, 2019 at 10:47

darune · Accepted Answer · 2019-10-08 11:01:02Z

1

Instead of

while(!(cin >> coursework_mark)){

you should use std::getline

std::getline(std::cin, coursework_mark);

https://en.cppreference.com/w/cpp/string/basic_string/getline

edited Oct 8, 2019 at 11:01

answered Oct 8, 2019 at 10:52

darune

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Pouria · Accepted Answer · 2019-10-08 14:41:42Z

0

bool has_only_digits(string s){
return s.find_first_not_of( "0123456789" ) == string::npos;}

This method is the simplest way I found to check if a string contains a number. So if it returns true the string has only digits else it contains more than just digits.

If the statement is false you can then clear the cin like you have done above.

answered Oct 8, 2019 at 14:41

Pouria

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