1 /*
2  * Exercise 4-4. Add commands to print the top element of the stack without
3  * popping, to duplicate it, and to swap the top two elements. Add a command to
4  * clear the stack.
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 double printtop(void);
17 int duplicate(void);
18 int swap(void);
19 void clear(void);
20 
21 /* reverse Polish calculator */
22 int main() {
23 	int type;
24 	double op2;
25 	char s[MAXOP];
26 
27 	while ((type = getop(s)) != EOF) {
28 		switch (type) {
29 		case NUMBER:
30 			push(atof(s));
31 			break;
32 		case '+':
33 			push(pop() + pop());
34 			break;
35 		case '*':
36 			push(pop() * pop());
37 			break;
38 		case '-':
39 			op2 = pop();
40 			push(pop() - op2);
41 			break;
42 		case '/':
43 			op2 = pop();
44 			if (op2 != 0.0)
45 				push(pop() / op2);
46 			else
47 				printf("error: zero divisor\n");
48 			break;
49 		case '%':
50 			op2 = pop();
51 			if (op2 != 0.0)
52 				push((int) pop() % (int) op2);
53 			else
54 				printf("error: zero divisor\n");
55 			break;
56 		case '?':
57 			printf("\t%.8g\n", printtop());
58 			break;
59 		case '#':
60 			if (duplicate() == 0)
61 				printf("error: can't duplicate\n");
62 			break;
63 		case '~':
64 			if (swap() == 0)
65 				printf("error: swap failed, stack needs two numbers\n");
66 			break;
67 		case '!':
68 			clear();
69 			break;
70 		case '\n':
71 			printf("\t%.8g\n", pop());
72 			break;
73 		default:
74 			printf("error: unknown command %s\n", s);
75 			break;
76 		}
77 	}
78 }
79 
80 #define MAXVAL 100 /* maximum depth of val stack */
81 
82 int sp = 0; /* next free stack position */
83 double val[MAXVAL]; /* value stack */
84 
85 /* push: push f onto value stack */
86 void push (double f) {
87 	if (sp < MAXVAL)
88 		val[sp++] = f;
89 	else
90 		printf ("error: stack full, can't push %g\n", f);
91 }
92 
93 /* pop: pop and return top value from stack */
94 double pop(void) {
95 	if (sp > 0)
96 		return val[--sp];
97 	else {
98 		printf("error: stack empty\n");
99 		return 0.0;
100 	}
101 }
102 
103 /* printtop: print the top value of the stack without popping it */
104 double printtop(void) {
105 	if (sp > 0)
106 		return val[sp-1];
107 	else {
108 		printf("error: stack empty\n");
109 		return 0.0;
110 	}
111 }
112 
113 /* duplicate: duplicate the top value of the stack
114  * return 1 if successful, 0 on failure */
115 int duplicate(void) {
116 	if (sp <= 0) {
117 		return 0;
118 	} else if (sp >= MAXVAL) {
119 		return 0;
120 	} else {
121 		val[sp] = val[sp-1];
122 		sp++;
123 		return 1;
124 	}
125 }
126 
127 /* swap: swap the top two values of the stack
128  * return 1 if successful, 0 on failure */
129 int swap(void) {
130 	if (sp > 1) {
131 		int temp = val[sp-1];
132 		val[sp-1] = val[sp-2];
133 		val[sp-2] = temp;
134 		return 1;
135 	}
136 	return 0;
137 }
138 
139 /* clear: clear the stack */
140 void clear(void) {
141 	sp = 0;
142 }
143 
144 #include <ctype.h>
145 
146 int getch(void);
147 void ungetch(int);
148 
149 /* getop: get next operator or numeric operand */
150 int getop(char s[]) {
151 	int i, c;
152 	while ((s[0] = c = getch()) == ' ' || c == '\t')
153 		;
154 	s[1] = '\0';
155 	i = 0;
156 	/* negative number or minus */
157 	if (c == '-') {
158 		s[++i] = c = getch();
159 		if (!isdigit(c) && c != '.') {
160 			ungetch(c);
161 			return '-';
162 		}
163 	}
164 	if (!isdigit(c) && c != '.')
165 		return c; /* not a number */
166 	if (isdigit(c)) /* collect integer part */
167 		while (isdigit(s[++i] = c = getch()))
168 			;
169 	if (c == '.') /* collect fraction part */
170 		while (isdigit(s[++i] = c = getch()))
171 			;
172 	s[i] = '\0';
173 	if (c != EOF)
174 		ungetch(c);
175 	return NUMBER;
176 }
177 
178 #define BUFSIZE 100
179 
180 char buf[BUFSIZE]; /* buffer for ungetch */
181 int bufp = 0;
182 
183 /* get a (possibly pushed back) character */
184 int getch(void) {
185 	return (bufp > 0) ? buf[--bufp] : getchar();
186 }
187 
188 /* push character back on input */
189 void ungetch(int c) {
190 	if (bufp >= BUFSIZE)
191 		printf("ungetch: too many characters\n");
192 	else
193 		buf[bufp++] = c;
194 }