/* 
 * Program to get an integer from stdin without using scanf.
 *
 * (reading one character at a time and using recursion)
 */
#include <stdio.h>
#include <stdlib.h>

int getint(void);

int main(void) {
	printf("enter a non-negative number\n");
	int n = getint();
	int next_ch = getchar();
	if ((next_ch != ' ') && (next_ch != '\n')) {
		printf("not an integer\n");
		return 1;
	}
	printf("value is %d\n", n);
	return 0;
}

/* 
 * for non-main functions, I just define them before first use rather
 * than having separate declarations and definitions
 */

/* 
 * convert character to digit, returning -1 if character does not 
 * represent a digit
 */
int char2digit (char c) {
	if ((c < '0') || (c > '9'))
		return -1;
	else
		return c - '0';

}

/* 
 * return integer that is 10 * previous_digits + rest of input 
 */
int getint_r(int previous_digits) {
	int input = getchar();
	int next = char2digit(input);
	if (next == -1) {
		ungetc(input, stdin);
		return previous_digits;
	} 
	else {
		int temp = 10 * previous_digits + next;
		return getint_r(temp);
	}
}

/* 
 * get an integer char by char from stdin, stopping when next char
 * is not a digit
 */
int getint(void) {
	int input = getchar();
	int first = char2digit(input);
	if (first == -1) {
		/* 
		 * FIXME?  this is ugly -- maybe think of some better way to 
		 * deal with errors?
		 */
		printf("not a number\n");
		exit(1);
	}
	return getint_r(first);
}