1 /*
2  * Exercise 4-6. Add commands for handling variables. (It's easy to provide
3  * twenty-six variables with single-letter names.) Add a variable for the most
4  * recently printed value.
5  */
6 
7 #include <stdio.h>
8 #include <stdlib.h> /* for atof() */
9 #include <math.h> /* for math functions */
10 #include <string.h> /* for strcmp() */
11 
12 #define MAXOP 100 /* max size of operand or operator */
13 #define NUMBER '0' /* signal that a number was found */
14 #define IDENTIFIER '1' /* signal that an identifier was found */
15 #define VARIABLE '2' /* signal that a variable was found */
16 #define ASSIGNMENT '3' /* signal that a variable was found */
17 #define LETTERS 26 /* number of letters in alphabet */
18 
19 int getop(char []);
20 void push(double);
21 double pop(void);
22 double printtop(void);
23 int duplicate(void);
24 int swap(void);
25 void clear(void);
26 int assign(char, double);
27 double interpolate(char);
28 
29 /* reverse Polish calculator */
30 int main() {
31 	int type;
32 	double op2, val;
33 	char s[MAXOP];
34 	int index;
35 	while ((type = getop(s)) != EOF) {
36 		switch (type) {
37 		case NUMBER:
38 			push(atof(s));
39 			break;
40 		case IDENTIFIER:
41 			if (strcmp(s, "sin") == 0)
42 				push(sin(pop()));
43 			else if (strcmp(s, "cos") == 0)
44 				push(cos(pop()));
45 			else if (strcmp(s, "exp") == 0)
46 				push(exp(pop()));
47 			break;
48 		case VARIABLE:
49 			push(interpolate(s[0]));
50 			break;
51 		case ASSIGNMENT:
52 			assign(s[0], atof(s+2));
53 			break;
54 		case '+':
55 			push(pop() + pop());
56 			break;
57 		case '*':
58 			push(pop() * pop());
59 			break;
60 		case '-':
61 			op2 = pop();
62 			push(pop() - op2);
63 			break;
64 		case '/':
65 			op2 = pop();
66 			if (op2 != 0.0)
67 				push(pop() / op2);
68 			else
69 				printf("error: zero divisor\n");
70 			break;
71 		case '%':
72 			op2 = pop();
73 			if (op2 != 0.0)
74 				push((int) pop() % (int) op2);
75 			else
76 				printf("error: zero divisor\n");
77 			break;
78 		case '?':
79 			printf("\t%.8g\n", printtop());
80 			break;
81 		case '#':
82 			if (duplicate() == 0)
83 				printf("error: can't duplicate\n");
84 			break;
85 		case '~':
86 			if (swap() == 0)
87 				printf("error: swap failed, stack needs two numbers\n");
88 			break;
89 		case '!':
90 			clear();
91 			break;
92 		case '\n':
93 			printf("\t%.8g\n", pop());
94 			break;
95 		default:
96 			printf("error: unknown command %s\n", s);
97 			break;
98 		}
99 	}
100 }
101 
102 #include <ctype.h> /* for tolower() */
103 #define MAXVAL 100 /* maximum depth of val stack */
104 
105 int sp = 0; /* next free stack position */
106 double val[MAXVAL]; /* value stack */
107 double var[LETTERS]; /* variable stack */
108 
109 /* push: push f onto value stack */
110 void push (double f) {
111 	if (sp < MAXVAL)
112 		val[sp++] = f;
113 	else
114 		printf ("error: stack full, can't push %g\n", f);
115 }
116 
117 /* pop: pop and return top value from stack */
118 double pop(void) {
119 	if (sp > 0)
120 		return val[--sp];
121 	else {
122 		printf("error: stack empty\n");
123 		return 0.0;
124 	}
125 }
126 
127 /* printtop: print the top value of the stack without popping it */
128 double printtop(void) {
129 	if (sp > 0)
130 		return val[sp-1];
131 	else {
132 		printf("error: stack empty\n");
133 		return 0.0;
134 	}
135 }
136 
137 /* duplicate: duplicate the top value of the stack
138  * return 1 if successful, 0 on failure */
139 int duplicate(void) {
140 	if (sp <= 0) {
141 		return 0;
142 	} else if (sp >= MAXVAL) {
143 		return 0;
144 	} else {
145 		val[sp] = val[sp-1];
146 		sp++;
147 		return 1;
148 	}
149 }
150 
151 /* swap: swap the top two values of the stack
152  * return 1 if successful, 0 on failure */
153 int swap(void) {
154 	if (sp > 1) {
155 		int temp = val[sp-1];
156 		val[sp-1] = val[sp-2];
157 		val[sp-2] = temp;
158 		return 1;
159 	}
160 	return 0;
161 }
162 
163 /* clear: clear the stack */
164 void clear(void) {
165 	sp = 0;
166 }
167 
168 /* assign: assign a double to a variable
169  * return 1 on success, 0 on failure */
170 int assign(char variable, double value) {
171 	int index = tolower(variable) - 'a';
172 	if (0 <= index && index < LETTERS) {
173 		var[index] = value;
174 		return 1;
175 	}
176 	printf("error: assignment failed, variable needs to be single letter\n");
177 	return 0;
178 }
179 
180 /* interpolate: interpolate variable */
181 double interpolate(char variable) {
182 	int index = tolower(variable) - 'a';
183 	if (0 <= index && index < LETTERS) {
184 		return var[index];
185 	}
186 	printf("error: variable must be single letter\n");
187 	return 0.0;
188 }
189 
190 #include <ctype.h>
191 
192 int getch(void);
193 void ungetch(int);
194 
195 /* getop: get next operator or numeric operand */
196 int getop(char s[]) {
197 	int i, c;
198 	while ((s[0] = c = getch()) == ' ' || c == '\t')
199 		;
200 	s[1] = '\0';
201 	i = 0;
202 	if (isalpha(c)) {
203 		while ((s[++i] = c = getch()) != ' ' && c != '\t' && c != '\n')
204 			;
205 		s[i] = '\0';
206 		if (c != EOF)
207 			ungetch(c);
208 		if (s[1] == '=') {
209 			return ASSIGNMENT;
210 		} else if (s[1] == '\0') {
211 			return VARIABLE;
212 		} else {
213 			return IDENTIFIER;
214 		}
215 	}
216 	/* negative number or minus */
217 	if (c == '-') {
218 		s[++i] = c = getch();
219 		if (!isdigit(c) && c != '.') {
220 			ungetch(c);
221 			return '-';
222 		}
223 	}
224 	if (!isdigit(c) && c != '.')
225 		return c; /* not a number */
226 	if (isdigit(c)) /* collect integer part */
227 		while (isdigit(s[++i] = c = getch()))
228 			;
229 	if (c == '.') /* collect fraction part */
230 		while (isdigit(s[++i] = c = getch()))
231 			;
232 	s[i] = '\0';
233 	if (c != EOF)
234 		ungetch(c);
235 	return NUMBER;
236 }
237 
238 #define BUFSIZE 100
239 
240 char buf[BUFSIZE]; /* buffer for ungetch */
241 int bufp = 0;
242 
243 /* get a (possibly pushed back) character */
244 int getch(void) {
245 	return (bufp > 0) ? buf[--bufp] : getchar();
246 }
247 
248 /* push character back on input */
249 void ungetch(int c) {
250 	if (bufp >= BUFSIZE)
251 		printf("ungetch: too many characters\n");
252 	else
253 		buf[bufp++] = c;
254 }