1 /*
2  * Exercise 4-3. Given the basic framework, it's straightforward to extend the
3  * calculator. Add the modulus (%) operator and provisions for negative
4  * numbers.
5  */
6 
7 #include <stdio.h>
8 #include <stdlib.h> /* for atof() */
9 
10 #define MAXOP 100 /* max size of operand or operator */
11 #define NUMBER '0' /* signal that a number was found */
12 
13 int getop(char []);
14 void push(double);
15 double pop(void);
16 
17 /* reverse Polish calculator */
18 int main() {
19 	int type;
20 	double op2;
21 	char s[MAXOP];
22 
23 	while ((type = getop(s)) != EOF) {
24 		switch (type) {
25 		case NUMBER:
26 			push(atof(s));
27 			break;
28 		case '+':
29 			push(pop() + pop());
30 			break;
31 		case '*':
32 			push(pop() * pop());
33 			break;
34 		case '-':
35 			op2 = pop();
36 			push(pop() - op2);
37 			break;
38 		case '/':
39 			op2 = pop();
40 			if (op2 != 0.0)
41 				push(pop() / op2);
42 			else
43 				printf("error: zero divisor\n");
44 			break;
45 		case '%':
46 			op2 = pop();
47 			if (op2 != 0.0)
48 				push((int) pop() % (int) op2);
49 			else
50 				printf("error: zero divisor\n");
51 			break;
52 		case '\n':
53 			printf("\t%.8g\n", pop());
54 			break;
55 		default:
56 			printf("error: unknown command %s\n", s);
57 			break;
58 		}
59 	}
60 }
61 
62 #define MAXVAL 100 /* maximum depth of val stack */
63 
64 int sp = 0; /* next free stack position */
65 double val[MAXVAL]; /* value stack */
66 
67 /* push: push f onto value stack */
68 void push (double f) {
69 	if (sp < MAXVAL)
70 		val[sp++] = f;
71 	else
72 		printf ("error: stack full, can't push %g\n", f);
73 }
74 
75 /* pop: pop and return top value from stack */
76 double pop(void) {
77 	if (sp > 0)
78 		return val[--sp];
79 	else {
80 		printf("error: stack empty\n");
81 		return 0.0;
82 	}
83 }
84 
85 #include <ctype.h>
86 
87 int getch(void);
88 void ungetch(int);
89 
90 /* getop: get next operator or numeric operand */
91 int getop(char s[]) {
92 	int i, c;
93 	while ((s[0] = c = getch()) == ' ' || c == '\t')
94 		;
95 	s[1] = '\0';
96 	i = 0;
97 	/* negative number or minus */
98 	if (c == '-') {
99 		s[++i] = c = getch();
100 		if (!isdigit(c) && c != '.') {
101 			ungetch(c);
102 			return '-';
103 		}
104 	}
105 	if (!isdigit(c) && c != '.')
106 		return c; /* not a number */
107 	if (isdigit(c)) /* collect integer part */
108 		while (isdigit(s[++i] = c = getch()))
109 			;
110 	if (c == '.') /* collect fraction part */
111 		while (isdigit(s[++i] = c = getch()))
112 			;
113 	s[i] = '\0';
114 	if (c != EOF)
115 		ungetch(c);
116 	return NUMBER;
117 }
118 
119 #define BUFSIZE 100
120 
121 char buf[BUFSIZE]; /* buffer for ungetch */
122 int bufp = 0;
123 
124 /* get a (possibly pushed back) character */
125 int getch(void) {
126 	return (bufp > 0) ? buf[--bufp] : getchar();
127 }
128 
129 /* push character back on input */
130 void ungetch(int c) {
131 	if (bufp >= BUFSIZE)
132 		printf("ungetch: too many characters\n");
133 	else
134 		buf[bufp++] = c;
135 }