while loop strange behaviour in C

Question

I am experiencing a strange behaviour of while loop in the following program.

#include <stdio.h>

int main ()
{

    char confirm;

    printf("Do you want to proceed? (Y/N):\n");
    scanf(" %c", &confirm);
    while (confirm != 'Y' && confirm != 'y' && confirm != 'N' && confirm != 'n')
    {
         printf("Please enter a valid answer (Y/N):\n");
         scanf(" %c", &confirm);
    }
 return 0;
}

Basically, I want the program only to accept 'Y', 'N', 'y' or 'n' characters. The above code works but when I enter a string as input, it prints "Please enter a valid answer (Y/N):" as many times as the characters of the string. Something like below:

Do you want to proceed? (Y/N):
a
Please enter a valid answer (Y/N):
adsfaf
Please enter a valid answer (Y/N):
Please enter a valid answer (Y/N):
Please enter a valid answer (Y/N):
Please enter a valid answer (Y/N):
Please enter a valid answer (Y/N):
Please enter a valid answer (Y/N):

How can I get rid of this. Can somebody help?

Answer 1

scanf() takes the input that matches the format string, returning the number of characters consumed. Any character that doesn't match the format causes it to stop scanning and leaves the invalid character still in the buffer.

To let the strange not happen, the possible ways to fix this issues:

1. Use fgets() to take the string and compare - recommended.
2. Use fflush(stdin) - not recommended.

Aside: Use int confirm rather than using char for it. The reason is:

A char is required to accept all values between 0 and 127 (included). So in common environments it occupies exactly one byte (8 bits). It is unspecified by the standard whether it is signed (-128 - 127) or unsigned (0 - 255).

An int is required to be at least a 16 bits signed word, and to accept all values between -32767 and 32767. That means that an int can accept all values from a char, be the latter signed or unsigned.

Note: You may use char confirm when you specifically needs to store characters.

Reference: Difference between char and int when declaring character.

Example 1 (recommended):

#include <stdio.h>
#define MAX_LENGTH 100

int main(void) {
    char confirm[MAX_LENGTH];

    printf("Enter a valid command (Y/N): ");
    fgets(confirm, MAX_LENGTH, stdin); // using fgets() here

    while (confirm[0] != 'Y' && confirm[0] != 'y' \
           && confirm[0] != 'N' && confirm[0] != 'n') {

        printf("Please enter a valid answer (Y/N): ");
        fgets(confirm, MAX_LENGTH, stdin); // using fgets() here
    }

    return 0;
}

Expected sample output should be:

$ gcc -o prog prog.c; ./prog
Enter a valid command (Y/N): asdf
Please enter a valid answer (Y/N): asdlfasdf asdf a
Please enter a valid answer (Y/N): 234
Please enter a valid answer (Y/N): n

Example 2:

Do something like:

while (...)
{
     printf("Please enter a valid answer (Y/N):\n");
     fflush(stdin);
     scanf(" %c", &confirm);
}

Then you may expect this output:

Do you want to proceed? (Y/N):
d
Please enter a valid answer (Y/N):
asdf
Please enter a valid answer (Y/N):
asdfasd
Please enter a valid answer (Y/N):
n

Answer 2

I tried the following. Don't know whether it is a good approach, but it works.

#include <stdio.h>

int main ()
{
    char confirm[256];

    printf("Do you want to proceed? (Y/N):\n");
    scanf("%256s", confirm);
    while (confirm[0] != 'Y' && confirm[0] != 'y' && confirm[0] != 'N' && confirm[0] != 'n')
    {
         printf("Please enter a valid answer (Y/N):\n");
         scanf("%256s", confirm);
    }

printf("%s\n", confirm);
}

give the output

Do you want to proceed? (Y/N):
somestring
Please enter a valid answer (Y/N):
234234
Please enter a valid answer (Y/N):
YES
YES