C: scanf input single character and validation

Question

I've encountered a problem when validating a single-char scanf input in C and I cannot find an existing solution that works...

The scenario is: a method is taking a single letter 'char' type input and then validating this input, if the criteria is not met, then pops an error message and re-enter, otherwise return this character value.

my code is:

char GetStuff(void)
{
    char c;
    scanf("%c", &c);
    while(c != 'A' || c != 'P')
    {
          printf("invalid input, enter again (A for AM or P for PM): ");
          scanf ("%c", &dtChar);
    }
    return c;
}

however, i got the infinite loop of error message no matter what input I type in. I read some other posts and guess it's the problem that %c specifier does no automatically get rid of the newline when I hit enter, and so far I have tried:

1. putting a white space before/after %c like:

   scanf(" %c", &c);
   

2. write a separate method or include in this GetStuff method to clean the newline like:

   void cleanBuffer(){
     int n;
     while((n = getchar()) != EOF && n != '\n' );
   }
   

Can anyone help me with this problem please? Thank you in advance.

Answer 1

#include <stdio.h>

char GetStuff(void) {
    char c;
    scanf("%c", &c);
    getchar();
    while ((c != 'A') && (c != 'a') && (c != 'P') && (c != 'p')) {
        printf("invalid input, enter again (A for AM or P for PM): ");
        scanf ("%c", &c);
        getchar();
    }
    return c;
}

int main(void) {
    printf("Calling GetStuff()...\n");
    char x = GetStuff();
    printf("User entered %c\n", x);
    return 0;
}

You are using while (c != 'A' || c != 'P') as your loop conditional, but this will always return true. What you meant to use is the && "and" operator, instead of the || "or" operator.

Also, call getchar() after your scanf statements, to capture the newline. This should work the way you want it to.