4-3.c

/* 
 * Exercise 4-3. Given the basic framework, it's straightforward to extend the
 * calculator. Add the modulus (%) operator and provisions for negative numbers
 *
 * By Faisal Saadatmand
 */

#include <stdio.h>
#include <stdlib.h>          /* for atof() */
#include <ctype.h>

#define MAXOP   100          /* max size of operand or operator */
#define NUMBER  '0'          /* signal that a number was found */
#define MAXVAL  100          /* maximum depth of val stack */
#define BUFSIZE 100

/* functions */
int    getop(char []);
void   push(double);
double pop(void);
int    getch(void);
void   ungetch(int);

/* globals */
int    sp;                   /* next free stack position */
double val[MAXVAL];          /* value stack */
char   buf[BUFSIZE];         /* buffer from ungetch */
int    bufp;                 /* next free position in buf */

/* push: push f onto value stack */
void push(double f)
{
	if (sp < MAXVAL)
		val[sp++] = f;
	else
		printf("error: stack full, can't push %g\n", f);
}

/* pop: pop and return top value from stack */
double pop(void)
{
	if (sp > 0)
		return val[--sp];
	else {
		printf("error: stack empty\n");
		return 0.0;
	}
}

/* getop: get next operator or numeric operand */
int getop(char s[])
{
	int i, c;

	while ((s[0] = c = getch()) == ' ' || c == '\t')
		;
	s[1] = '\0';

	i = 0;
	if (c == '-')                      /* check sign */
		if (!isdigit(s[++i] = c = getch())) {
			ungetch(c);                    
			c = s[0];                  /* not a sign */
		}

	if (!isdigit(c) && c != '.')
		return c;                      /* not a number */

	if (isdigit(c))
		while (isdigit(s[++i] = c = getch()))
			;
	if( c == '.')                      /* collect fraction part */
		while (isdigit(s[++i] = c = getch()))
			;
	s[i] = '\0';
	if (c != EOF)
		ungetch(c);
	return NUMBER;
}

/* getch: get a (possibly pushed back) character */
int getch(void)
{
	return (bufp > 0) ? buf[--bufp] : getchar();
}

/* ungetch: push character back on input */
void ungetch(int c)
{
	if (bufp >= BUFSIZE)
		printf("ungetch: too many characters\n");
	else
		buf[bufp++] = c;
}

/* reverse Polish Calculator */
int main(void)
{
	int type;
	double op2;
	char s[MAXOP];

	while ((type = getop(s)) != EOF) {
		switch (type) {
		case NUMBER:
			push(atof(s));
			break;
		case '+':
			push(pop() + pop());
			break;
		case '*':
			push(pop() * pop());
			break;
		case '-':
			op2 = pop();
			push(pop() - op2);
			break;
		case '/':
			op2 = pop();
			if (op2 != 0.0)
				push(pop() / op2);
			else
				printf("error: zero divisor\n");
			break;
		case '%':
			op2 = pop();
			if (op2 != 0.0)
				push((long) pop() % (long) op2);
			else
				printf("error: zero divisor\n");
			break;
		case '\n':
			printf("\t%.8g\n", pop());
			break;
		default:
			printf("error: unknown command %s\n", s);
			break;
		}
	}
	return 0;
}