The C Programming Language, 2nd Edition, by Kernighan and Ritchie
Exercise 1.06 on page 17

Verify that the expression getchar() != EOF is 0 or 1.

Warning: the answers on these pages are crowdsourced, open to anybody to provide/edit after free sign-up, have not all been vetted, might not be correct, let alone optimal, and should not be assumed to be.

Solution by Richard Heathfield

/* This program prompts for input, and then captures a character
 * from the keyboard. If EOF is signalled (typically through a
 * control-D or control-Z character, though not necessarily),
 * the program prints 0. Otherwise, it prints 1.
 *
 * If your input stream is buffered (and it probably is), then
 * you will need to press the ENTER key before the program will
 * respond.
 */

#include <stdio.h>

int main(void)
{
  printf("Press a key. ENTER would be nice :-)\n\n");
  printf("The expression getchar() != EOF evaluates to %d\n", getchar() != EOF);
  return 0;
}