How to make a loop continuously keep asking for user input until a given character is entered that stops the program?

Question

I have this program:

int main(void){
    int x, number, factorial;

// The objective of this program is to compute the factorial
// for a user inputted number, stopping once "-1" is entered.

    printf("Please enter a positive number for factorial calculation (-1 to end) :");
    scanf("%d", &number);
        
    for (x = 1; x <= number; x++){
        factorial *= x;
        if (x == -1){
            break;
        }
    }   
    printf("factorial for %d is %d", number, factorial);
    
    
}

that is supposed to output like this:

Please enter a positive number for factorial calculation (-1 to end) :4
factorial for 4 is 24
Please enter a positive number for factorial calculation (-1 to end) :6
factorial for 6 is 720
Please enter a positive number for factorial calculation (-1 to end) :8
factorial for 8 is 40320
Please enter number for factorial calculation (-1 to end) :-1

but I keep getting this (on two different runs):

Please enter a positive number for factorial calculation (-1 to end) :4
factorial for 4 is 24

Please enter a positive number for factorial calculation (-1 to end) :-1
factorial for -1 is 1

How could I make it so that it continues to ask for more numbers until I type in -1? Also, why does typing -1 here give me the factorial for it instead of stopping the loop?

Answer 1

You could wrap the factorial calculation inside a loop like this:

    int number;
    int factorial = 1;
    while(true) {
        printf("Please enter a positive number for factorial calculation (-1 to end) :");
        scanf("%d", &number);
        if (number == -1)
            break; // or just return as you need
        for (x = 1; x <= number; x++){
            factorial *= x;
        }   
        printf("factorial for %d is %d", number, factorial);
        factorial = 1;
    }

In this case you would receive the number as input and just begin checking what the user have input.

In your code you were checking inside the for loop for x variable to be different than -1 but that condition would never be met as you were assigning 1 to x just when you begin the loop. In that case you should check for number but it would not be all correct.

Answer 2

Use a While loop and check the value after user input to see if you need to break out of the loop:

int main(void){
    int x, number, factorial;

// The objective of this program is to compute the factorial
// for a user inputted number, stopping once "-1" is entered.
 
    while (true){
        printf("Please enter a positive number for factorial calculation (-1 to end) :");
        scanf("%d", &number);
        
        if (number == -1){
            break;
        }
        for (x = 1; x <= number; x++){
            factorial *= x;                
        }   
        printf("factorial for %d is %d", number, factorial);
    }
    
}