C - While 循环条件具有 OR 与 Char：未终止

Question

When using the logical OR operator in a while loop for Char values, the program does not run correctly.

The code is the following:

#include <stdio.h>

void main() {
    char choice = 'y';

    while( choice != 'n' || choice != 'N') {

        int num;
        printf("\nEnter any number: ");
        scanf("%d", &num);

        int i = num - 1;

        while(i >= 1) {
            num *= i;
            i--;
        }

        printf("Factorial is: %d\n", num);
        printf("Do you want to continue? \n");
        printf("Enter n to quit or y to continue\n");
        scanf(" %c", &choice);
    }
}

Ouput:

Enter any number: 3
Factorial is: 6
Do you want to continue? 
Enter n to quit or y to continue
n

Enter any number: 3
Factorial is: 6
Do you want to continue? 
Enter n to quit or y to continue
N

Enter any number: 

If I change the while statement, for example, to:

while( choice != 'n' ) {

The program runs fine. It is the logical OR that is creating an error.

Why is this issue occurring? And how do I make the program run with the OR logical operator?

Answer 1

The logical OR operator || evaluates to true if either expression is true.

So if choice is N then choice != 'n' is true, and if choice is n then choice != 'N' is true. If choice is neither of those, then both are true. So the full condition will always be true.

You want to instead use the logical AND operator && . This evaluates to true only if both expressions are true.

while( choice != 'n' && choice != 'N') {

Answer 2

If you put n , choice != 'N' is true and it continues.

If you put N , choice != 'n' is true and it continues.

There should be && instead of || .