Exercise 4.03

Page 79

Given the basic framework, it's straightforward to extend the calculator. Add the modulus ( % ) operator and provisions for negative numbers.

#include <stdio.h>
#include <stdlib.h>

#define MAXOP 100
#define NUMBER '0'

int getop(char[]);
void push(double);
double pop(void);

int main()
{
	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((int)pop() % (int)op2);
			} else {
				printf("error: zero divisor\n");
			}
			break;
		case '\n':
			printf("\t%.8g\n", pop());
			break;
		default:
			printf("error: unknown comand %s\n", s);
			break;
		}
	}
	return 0;
}

#define MAXVAL 100

int sp = 0;
double val[MAXVAL];

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

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

#include <ctype.h>

int getch(void);
void ungetch(int);

int getop(char s[])
{
	int i, c;

	/* Get first char, return if non-numeric */
	while ((s[0] = c = getch()) == ' ' || c == '\t')
		;
	s[1] = '\0';

	if (!isdigit(c) && c != '.' && c != '-' && c != '+') {
		return c;
	}

	/* Handle negative or positive sign */
	i = 0;
	if (c == '-' || c == '+') {
		if (!isdigit(s[++i] = c = getch())) {
			ungetch(s[i--]);
			s[1] = '\0';
			return s[0];
		}
	}

	/* Fetch numeric string */
	if (isdigit(c)) {
		while (isdigit(s[++i] = c = getch()))
			;
	}
	if (c == '.') {
		while (isdigit(s[++i] = c = getch()))
			;
	}
	s[i] = '\0';

	/* Unget the last unread char */
	if (c != EOF) {
		ungetch(c);
	}

	/* Return number indicator */
	return NUMBER;
}

#define BUFSIZE 100
char buf[BUFSIZE];
int bufp = 0;

int getch(void)
{
	return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c)
{
	if (bufp >= BUFSIZE) {
		printf("ungetch: too many characters\n");
	} else {
		buf[bufp++] = c;
	}
}