Exercise 4.09

Page 79

Our getch and ungetch do not handle a pushed-back EOF correctly. Decide what their properties ought to be if an EOF is pushed back, then implement your design.

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

#define LOWVAR 'a'
#define HIGHVAR 'z'
#define MAXOP 100
#define NUMBER '0'

int getop(char[]);
void push(double);
double pop(void);
double read(void);
void clear(void);
void assign(char, double);
double fetch(char);

/* To assign a variable provide a lowercase letter after the relevant number 
 * `2 a` will assign a = 2 
 * To access a variable, prefix with @ 
 * `2 a @a @a +` will assign a = 2 then will add `a + a` = `2 + 2` = 4 */

int main()
{
	int type;
	double op, 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 '#': // print top element
			printf("\t%.8g\n", read());
			break;
		case '&': // duplicate top element
			push(read());
			break;
		case '~': // swap top two elements
			op = pop();
			op2 = pop();
			push(op);
			push(op2);
			break;
		case '!': // clear the stack
			clear();
			break;
		case '$':
			push(sin(pop()));
			break;
		case '?':
			push(exp(pop()));
			break;
		case '^':
			op2 = pop();
			push(pow(op2, pop()));
			break;
		case '@':
			printf("fetch error: @ must be followed by a variable name!\n");
			break;
		case '\n':
			printf("\t%.8g\n", pop());
			break;
		default:
			assign(type, pop());
			break;
		}
	}
	return 0;
}

#define MAXVAL 100

int sp = 0;
double val[MAXVAL];

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

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

double read(void)
{
	if (sp > 0) {
		return val[sp - 1];
	} else {
		printf("read error: stack empty\n");
		return 0.0;
	}
}

void clear(void)
{
	sp = 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')
		;

	if (c == '@') {
		c = getch();
		if (c < LOWVAR || c > HIGHVAR) {
			return '@';
		} else {
			sprintf(s, "%g", fetch(c));
			return NUMBER;
		}
	}

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

	i = 0;
	/* Handle negative or positive sign */
	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
int 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;
	}
}

double vars[HIGHVAR - LOWVAR];

void assign(char c, double d)
{
	vars[c - LOWVAR] = d;
}

double fetch(char c)
{
	return vars[c - LOWVAR];
}