How to restrict to a one letter input? - c

Question

I realised that if the input is a word starting with 'y' or 'n', it will escape the loop. How can I restrict the loop such that it will continue looping unless the input is a single character?
do
{
printf("Do you want to try again? (Y/N): ");
fflush(stdin);
scanf("%c", &repeat);
repeat = toupper(repeat);
if (repeat != 'Y' && repeat != 'N')
printf("Invalid answer. Please enter 'Y' or 'N'.\n\n");
} while (repeat != 'N' && repeat != 'Y');

Answer 1

like this:
#include <stdio.h>
#include <ctype.h>
int main(void){
char repeat[3] = {0};//3 : one character + one character + NUL
do{
printf("Do you want to try again? (Y/N): ");fflush(stdout);
if(EOF==scanf("%2s", repeat)){ *repeat = 'N'; break; }
*repeat = toupper(*repeat);
if (repeat[1] || *repeat != 'Y' && *repeat != 'N'){//repeat[1] != '\0'..
printf("Invalid answer. Please enter 'Y' or 'N'.\n\n");
scanf("%*[^\n]");scanf("%*c");//clear upto newline
*repeat = 0;
}
} while (*repeat != 'N' && *repeat != 'Y');
puts("Bye!");//try agein or see ya, bye
return 0;
}

Answer 2

First fflush(stdin); does not make sense except in Microsoft's world.
Then, the scanf family function returns a value which is the number of input token successfully decoded and that return value should always be controlled. And %c should be used with caution because it can return a blank character (space or newline) remaining in buffer while %s only return printable characters. With those remarks you code could become:
repeat = '\0';
do
{
char dummy[2], inp[2];
printf("Do you want to try again? (Y/N): ");
// fflush(stdin);
if (1 == scanf("%1s%1s", inp,dummy) repeat = toupper(inp[0]);
if (repeat != 'Y' && repeat != 'N')
printf("Invalid answer. Please enter 'Y' or 'N'.\n\n");
} while (repeat != 'N' && repeat != 'Y');

Answer 3

Alternatively to using scanf() one can use fgets() to read a line and then do the parsing one self:
#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
int main(void)
{
char repeat = '\0';
do
{
int input_valid = 0; /* Be pessimistic. */
char line[3] = {0};
puts("Do you want to try again? (Y/N):");
do /* On-time loop, to break out on parsing error. */
{
if (NULL == fgets(line, sizeof line, stdin))
{
break; /* Either fgets() failed or EOF was read. Start over ... */
}
if (line[1] != '\0' && line[1] != '\n')
{
break; /* There was more then one character read. Start over ... */
}
line[0] = toupper(line[0]);
if (line[0] != 'Y' && line[0] != 'N')
{
break; /* Something else but Y or N was read. Start over ... */
}
input_valid = 1;
} while (0);
if (input_valid == 0)
{
int c;
do /* Flush rest of input. if any. */
{
c = getc(stdin);
} while (EOF != c && '\n' != c);
fprintf(stderr, "Invalid answer. Please enter 'Y' or 'N'.\n\n");
}
else
{
repeat = line[0];
}
} while ('\0' == repeat);
printf("The user entered: '%c'\n", repeat); /* Will only print either Y or N. */
return EXIT_SUCCESS;
}