4.11.c

/* Exercise 4-11. Modify getop so that it doesn't need to use ungetch.
 * Hint: use an internal static variable.
 */
#include <stdio.h>
#include <stdlib.h> /* for atof() */
#include <ctype.h> /* for isdigit() */
#define MAXOP 100 /* max size of operand or operator */
#define NUMBER '0' /* signal that a number was found */

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

/* 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 '\n':
			printf("\t%.8g\n", pop());
			break;
		default:
			printf("error: unknown command %s\n", s);
			break;
		}
	}
	return 0;
}

/* push: push f onto vlaue stack */
#define MAXVAL 100 /* max depth of val stack */
int sp = 0; /* next free stack position */
double val[MAXVAL]; /* 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 character or numeric operand */
int
getop(char s[])
{
	int i, c;
	static int b = EOF;

	/* if buffer is not EOF, whitespace, or a number, return it as an op */
	if (b != EOF && b != ' ' && b != '\t' && !isdigit(b) && b != '.') {
		c = b;
		b = EOF;
		return c;
	}

	i = 0;
	/* if buffer is EOF or whitespace, then continue as normal */
	if (b == EOF || b == ' ' || b == '\t')
		while ((s[0] = c = getchar()) == ' ' || c == '\t') ; /* skip blanks */
	else /* buffer could be part of an operand, put it in the operand string */
		s[i] = c = b;
	s[++i] = '\0';
	b = EOF;

	if (! isdigit(c) && c != '.')
		return c;	/* not a number */
	if (isdigit(c))	/* collect integer part of number */
		while (isdigit(s[++i] = c = getchar())) ;
	if (c == '.')	/* collect fractional part of number */
		while (isdigit(s[++i] = c = getchar())) ;
	s[i] = '\0';
	b = c; /* if we get here, we've read a number and gone one character too far */
	return NUMBER;
}

#define BUFSIZE 100 /* buffer for getch() and ungetch() */
char buf[BUFSIZE]; /* buffer for ungetch */
int bufp = 0; /* next free position in buf */
int
getch(void) /* get a (possibly pushed-back) character */
{
	return (bufp > 0) ? buf[--bufp] : getchar();
}

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