Thread: Trouble with a simple calculator

Hello all,

I have very recently started basic C-Programming and have decided to teach myself by trying to make a very basic calculator.

The only issue I'm having is with validating that the input is in the form (Number) (operator) (Number).

My operator validation works fine but if I type an invalid character for either of the numbers, I get an infinite loop of prinft("").

If anyone could try and see what the problem is I would be much obliged.

Thanks,

Sam.

Code:

#include <stdio.h>
#include <stdlib.h>


int main(void)
{
    /* Define variables and assign inital values for validation*/
    
    int        invalid_operator = 0, invalid_number = 0;
    char    operator;
    float    n1, n2, result, valid_input;
    
    /* Ask user for input and validate*/
    
    printf("Please enter two numbers and your operation in the format:\n (Number 1) (operator) (Number2)\n");
    if(3!=scanf("%f %c %f", &n1, &operator, &n2))
        invalid_number = 1;
        
    /* If input is not valid, start while loop - ask user to input values until correct format is obtained*/    
        
    while(invalid_number == 1)
    {
        printf("One or more of your inputs are invalid. Please enter your two numbers along with a valid operator (+, -, * or /) in the format:\n (Number 1) (operator) (Number2)\n");
        
        if(3!=scanf("%f %c %f", &n1, &operator, &n2))
            invalid_number = 1;
        else
            invalid_number = 0;
    }
            
    /* Once input is validated, attempt to perform operation requested*/


    if(operator == '+')
        result = n1 + n2;
    else if(operator == '-')
        result = n1 - n2;
    else if(operator == '*')
        result = n1 * n2;
    else if(operator == '/')
        result = n1 / n2;
    else
        invalid_operator = 1;
    
    /* If operator is not defined, start while loop - ask user to input values until valid operator is obtained*/
    
    while(invalid_operator == 1)
    {
        printf("Your operator is invalid. Please enter your two numbers along with a valid operator (+, -, * or /) in the format:\n (Number 1) (operator) (Number2)\n");
        scanf("%f %c %f", &n1, &operator, &n2);
        
        valid_input = (operator == '+') || (operator == '-') || (operator == '*') || (operator == '/');
            
        if(!valid_input)
            invalid_operator = 1;
        else
            invalid_operator = 0;
    }
    
    if(operator == '+')
        result = n1 + n2;
    else if(operator == '-')
        result = n1 - n2;
    else if(operator == '*')
        result = n1 * n2;
    else if(operator == '/')
        result = n1 / n2;
    
    /* Print result*/
    
    printf("The result of %f %c %f is: %f\n", n1, operator, n2, result);
    
    exit (EXIT_SUCCESS);
}

Originally Posted by Impulse

... so the scanf is always returning 0 for an invalid input, so why doesn't it return 0 and then repeat the while loop and ask for the user to input values again?

Because it doesn't remove the character that originated the error from the input stream.
Next time through the loop it reads the same character again, and, of course, gives the same error as before.

Imagine you've asked for an integer (with "%d") but typed "boo".
As soon as scanf() sees the 'b', it stops, puts the 'b' back in the input stream and returns an error.
The next time a scanf() happens, the first thing it sees is that same 'b'.

The safest way to get user input is with fgets() followed by a parsing of the input.

OH I see! Thank you very much. I understand what you mean.

I have heard fgets is much safer, I shall try this.

Thank you to the both of you for your replies.