我不能在 else if 语句中调用 function

Question

#include <iostream>
#include <vector>

void Invalid_input(int& num1, int& num2) 
{
    while (!cin)
    {
        cout << "You entered an incorrect value!" << '\n';
        cin.clear();
        cin.ignore(100, '\n');
        cout << "Please input again : ";
        cin >> num1 >> num2;
    }
}

int main() {
    while (1) { 
        int num_of_tower = 0, num_of_disk = 0;

        cout << "Please select the number of towers and disks" << '\n'
            << "tower and disk : ";
        cin >> num_of_tower >> num_of_disk;

        if (!cin) Invalid_input(num_of_tower, num_of_disk);
        
        else if (num_of_tower < 2)
            Invalid_input(num_of_tower, num_of_disk);
    }
}  

This is just part of the whole code. I can't call the function, Invalid_input , in the else if statement (for example, after an input of 1 and 3).

Excluding this function call, all operations are performed. How should I fix it?

Answer 1

Your Invalid_input function is being called … but it's not doing anything, This is because, for an input of 1 for num_of_tower and a valid input for num_of_disk , there will not be an error on the cin stream, so the while loop in your function won't run (because the while (!cin) test will be false ).

What you should do is to change that while() {...} loop into a do {... } while(); loop, which will always run at least once, even if cin is not in an error state:

void Invalid_input(int& num1, int& num2)
{
    do {
        cout << "You entered an incorrect value!" << '\n';
        cin.clear();
        cin.ignore(100, '\n');
        cout << "Please input again : ";
        cin >> num1 >> num2;
     } while (!cin);
}

You might also like to add a further check in that function, to ensure that the first input given is at least 2 (as in the main function):

//...
        cin >> num1 >> num2;
     } while (!cin || num1 < 2);
}