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?

Answer 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

Answer 2

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.