How can I call my function to make this program work to calculate grades?

Question

#include <iostream>

using namespace std;
int grade_calculation (int grade_input){
    if ( grade_input >= 100 && grade_input <= 90){
        cout << 'A';
    } else if (grade_input >= 89 && grade_input <= 80){
        cout << 'B';
    } else if (grade_input >= 79 && grade_input <= 70){
        cout << 'C';
    } else if (grade_input >= 69 && grade_input <= 60){
        cout << 'D';
    } else if (grade_input >= 59 && grade_input <= 0){
        cout << 'F';
    } else {
        cout << "NOT VALID.";
    }
}

int main()
{
    int score = 100;
    int grade_input;
    cout << "Enter a grade: ";
    cin >> grade_input;
    while (grade_input != score){
        cout << "Enter a new grade: ";
        cin >> grade_input;
        grade_calculation(grade_input);
        
    } cout << grade_input;
    return 0;
}

How can I properly use the while loop to ask the user for a letter and then output the corresponding grade? Is there another way to call the function besides using "grade_calculation (grade_input)"?

Answer 1

if ( grade_input >= 100 && grade_input <= 90)

This means: if the grade is above 100 and the grade is below 90 . There is no grade that satisfy this condition. You probably want:

if ( grade_input <= 100 && grade_input >= 90)

which is: if grade is below or equal to 100 and grade is equal or above 90 .

And then, change all other conditions in a similar way.