/*
 * program to get integer from input, maybe more nicely than scanf
 *
 * version using loops
 */
#include <stdio.h>

int getint(int *result_p);
char first_non_blank();
int collect_digits(char first, int *result_p);
int char_to_digit(char c);

int main(void) {
	int result;
	printf("enter an integer\n");
	if (getint(&result) == 0) {
		printf("%d\n", result);
	}
	else {
		printf("error\n");
	}
	return 0;
}

/*
 * get integer from stdin
 * returns 0 on success, something else on error
 */
int getint(int *result_p) {

	/* skip over leading blanks */
	char first = first_non_blank();

	int sign = 1;

	/* negative */
	if (first == '-') {
		sign = -1;
		first = getchar();
	}

	/* first nonblank is not a digit */
	else if (first < '0' || first > '9') {
		return 1;
	}

	/* first nonblank is a digit */
	int status = collect_digits(first, result_p);
	if (status == 0) {
		*result_p *= sign;
	}
	return status;
}

/* 
 * get first nonblank from stdin 
 */
char first_non_blank() {
	/* simple way 
	char ch = getchar();
	while (ch == ' ') {
		ch = getchar();
	}
	*/
	/* shorter way */
	char ch;
	while ((ch = getchar()) == ' ') { }
	return ch;
}

/*
 * collect digits and turn into integer
 * "first" is a digit
 * returns 0 on success, something else on error
 */
int collect_digits(char first, int *result_p) {
	int work = char_to_digit(first);

	/* get digits while more to get */

	/* straightforward way */
	char next = getchar();
	while ((next >= '0') && (next <= '9')) {
		work = 10*work + char_to_digit(next);
		next = getchar();
	}
	/* shorter but less readable(?) way */
	/*
	char next;
	while ((next = getchar()) && (next >= '0') && (next <= '9')) {
		work = 10*work + char_to_digit(next);
	}
	*/
	
	/* at exit from the loop, "next" has a non-digit */
	if ((next == ' ') || (next == '\n')) {
		*result_p = work;
		return 0;
	}
	else {
		return 1;
	}
}

int char_to_digit(char c) {
	return c-'0';
}