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

/* 
 * compute n! recursively
 */
int fact(int n)
{
		/* error check */
		if (n < 0)
			return(-1);
		/* base case */
		if (n == 0)
			return(1);
		/* recursion */
		return(n * fact(n-1));
}
/*
 * convert string to int with error checking
 * no leading signs or magnitude checking
 */
int cvttoint(char *s)
{
		int n = 0;			/* integer being read */
		/* skip leading white space */
		while(isspace(*s))
		s++;
		/* if it's not a digit, it's not an integer */
		if (!isdigit(*s))
			return(-1);
		/* read in the integer */
		while(isdigit(*s))
			n = n * 10 + *s++ - '0';
		/* if it's ended by a NUL, it's an integer */
		return(*s ? -1 : n);
}
int main(int argc, char *argv[])
{
		int i;		/* counter in a for loop */
		int n;		/* number read in */
		int rv = 0;		/* exit status code */
		/*
		 * do each arg separately
		 */
		for(i = 1; i < argc; i++)
			if ((n = cvttoint(argv[i])) != -1)
				printf("%d! = %d\n", n, fact(n));
			else{
				/* error handler*/
				rv++;
				printf("%s: invalid number\n", argv[i]);
			}
		/*
		 * bye!
		 */
		return(rv);
}