exercise07.c

// C Primer Plus
// Chapter 9 Exercise 7:

// Write a program that reads characters from the standard input to
// end-of-file. For each character, have the program report whether it is a
// letter. If it is a letter, also report its numerical location in the
// alphabet. For example, c and C would both be letter 3. Incorporate a
// function that takes a character as an argument and returns the numerical
// location if the character is a letter and that returns –1 otherwise.

#include <stdio.h>
#include <ctype.h>

int letter_location(char ch);

int main(void)
{
	char ch;
	int location;

	while ((ch = getchar()) != EOF)
	{
		if ((location = letter_location(ch)) == -1)
			printf("%c is not a letter\n", ch);
		else
			printf("%c is a letter: location = %d\n", ch, location);
	} 

	return 0;
}

int letter_location(char ch)
{
	if (isalpha(ch))
	{
		ch = tolower(ch);
		return (ch - 'a' + 1);
	}
	else
		return -1;
}