Why does my loop request usable character twice?

Question

So I have a very simple code to ask the user if they agree or not (i'm just starting out). Currently I have the loop set up to allow the user to answer with 'y' or 'n', and to tell them "only y/n please" if they enter a different character. So far, everything works almost perfectly, however, upon entering a character other than 'y' or 'n', the correction message displays twice. any advice?

code:

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    char answer = get_char("Do You Agree? y/n \n"); // prompt user to answer question

    do{
        scanf("%c", &answer);

        if (answer == 'y')
        {
            printf("Great!\n");
            break;
        }
        else if(answer == 'n')
        {
            printf("That's unfortunate\n");
            break;
        }
        else
        {
            printf("only y/n please.\n"); // prompt user for acceptable character
        }
    }while (answer != 'y' || answer != 'n');
}

And here is what it returns:

Do You Agree? y/n 
a
a
only y/n please.
only y/n please.
a
only y/n please.
only y/n please.
a
only y/n please.
only y/n please.
s
only y/n please.
only y/n please.
y
Great!

Answer 1

You don't need to use scanf() , you already asked for the input with get_char() . Just move that inside the loop in place of the scanf() .

You don't need to check answer in the while() condition, since you break out of the loop when it's y or n .

int main(void)
{
    char answer;

    while (1) {
        answer = get_char("Do You Agree? y/n \n"); // prompt user to answer question

        if (answer == 'y')
        {
            printf("Great!\n");
            break;
        }
        else if(answer == 'n')
        {
            printf("That's unfortunate\n");
            break;
        }
        else
        {
            printf("only y/n please.\n"); // prompt user for acceptable character
        }
    }
}

Answer 2

The CS50 function get_char will extract a whole line of input from the input stream and then return the first character of that input., assuming that there was only a single character on the line. Otherwise, it will reprompt the user for input.

The function call

scanf("%c", &answer);

on the other hand will only extract a single character from the input stream, not a whole line, without performing any input validation. The character that scanf extracts can also be a '\n' newline character, which marks the end of the line.

The sequence of events that are occurring is the following:

1. You call get_char , which extracts 'a' and '\n' from the input stream, and returns 'a' .
2. You call scanf , which extracts 'a' from the input stream, but leaves '\n' on the input stream.
3. Your program detects that the user did not enter 'y' or 'n' , so it prints "only y/n please.\n" .
4. You call scanf again, which simply extracts the '\n' that was left over by the previous call to scanf , so scanf does not obtain any new input from the user.
5. Your program incorrectly thinks that the user entered '\n' instead of 'y' or 'n' , so it prints another error message.

In order to solve the problem, I recommend that you only use get_char and don't mix it with scanf . In contrast to scanf , the function get_char will never return a '\n' character. Therefore, the function get_char is much easier to use.

Since you want to call get_char more than once if necessary, you should move that function call inside the loop, so that it replaces the call to scanf .

Answer 3

After it was pointed out that get_char() is safer and more suitable than scanf() , I've revised the code of my comment...

char c;
while( ( c = get_char( "Do You Agree? y/n " ) ) != 'y' && c != 'n' )
    printf("only y/n please.\n");

if ( c == 'y' )
    printf( "Great!\n" );
else
    printf( "That's unfortunate\n" );