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 #include <ctype.h> /* for tolower() */
12 
13 #define MAXOP 100 /* max size of operand or operator */
14 #define NUMBER '0' /* signal that a number was found */
15 #define IDENTIFIER '1' /* signal that an identifier was found */
16 #define VARIABLE '2' /* signal that a variable was found */
17 #define ASSIGNMENT '3' /* signal that a variable was found */
18 #define LETTERS 26 /* number of letters in alphabet */
19 
20 int getop(char []);
21 void push(double);
22 double pop(void);
23 double printtop(void);
24 int duplicate(void);
25 int swap(void);
26 void clear(void);
27 int assign(int, double);
28 double eval(int index);
29 
30 /* reverse Polish calculator */
31 int main() {
32 	int type;
33 	double op2, val;
34 	char s[MAXOP];
35 	int index;
36 	while ((type = getop(s)) != EOF) {
37 		switch (type) {
38 		case NUMBER:
39 			push(atof(s));
40 			break;
41 		case IDENTIFIER:
42 			if (strcmp(s, "sin") == 0)
43 				push(sin(pop()));
44 			else if (strcmp(s, "cos") == 0)
45 				push(cos(pop()));
46 			else if (strcmp(s, "exp") == 0)
47 				push(exp(pop()));
48 			break;
49 		case VARIABLE:
50 			index = tolower(s[0]) - 'a';
51 			if (0 <= index && index < LETTERS) {
52 				push(eval(index));
53 			} else {
54 				printf("error: variable must be single letter\n");
55 			}
56 			break;
57 		case ASSIGNMENT:
58 			index = tolower(s[0]) - 'a';
59 		        val = atof(s+2);
60 			if (!assign(index, val)) {
61 				printf("error: assignment failed, variable needs to be single letter\n");
62 			}
63 			break;
64 		case '+':
65 			push(pop() + pop());
66 			break;
67 		case '*':
68 			push(pop() * pop());
69 			break;
70 		case '-':
71 			op2 = pop();
72 			push(pop() - op2);
73 			break;
74 		case '/':
75 			op2 = pop();
76 			if (op2 != 0.0)
77 				push(pop() / op2);
78 			else
79 				printf("error: zero divisor\n");
80 			break;
81 		case '%':
82 			op2 = pop();
83 			if (op2 != 0.0)
84 				push((int) pop() % (int) op2);
85 			else
86 				printf("error: zero divisor\n");
87 			break;
88 		case '?':
89 			printf("\t%.8g\n", printtop());
90 			break;
91 		case '#':
92 			if (duplicate() == 0)
93 				printf("error: can't duplicate\n");
94 			break;
95 		case '~':
96 			if (swap() == 0)
97 				printf("error: swap failed, stack needs two numbers\n");
98 			break;
99 		case '!':
100 			clear();
101 			break;
102 		case '\n':
103 			printf("\t%.8g\n", pop());
104 			break;
105 		default:
106 			printf("error: unknown command %s\n", s);
107 			break;
108 		}
109 	}
110 }
111 
112 #define MAXVAL 100 /* maximum depth of val stack */
113 
114 int sp = 0; /* next free stack position */
115 double val[MAXVAL]; /* value stack */
116 double var[LETTERS]; /* variable stack */
117 
118 /* push: push f onto value stack */
119 void push (double f) {
120 	if (sp < MAXVAL)
121 		val[sp++] = f;
122 	else
123 		printf ("error: stack full, can't push %g\n", f);
124 }
125 
126 /* pop: pop and return top value from stack */
127 double pop(void) {
128 	if (sp > 0)
129 		return val[--sp];
130 	else {
131 		printf("error: stack empty\n");
132 		return 0.0;
133 	}
134 }
135 
136 /* printtop: print the top value of the stack without popping it */
137 double printtop(void) {
138 	if (sp > 0)
139 		return val[sp-1];
140 	else {
141 		printf("error: stack empty\n");
142 		return 0.0;
143 	}
144 }
145 
146 /* duplicate: duplicate the top value of the stack
147  * return 1 if successful, 0 on failure */
148 int duplicate(void) {
149 	if (sp <= 0) {
150 		return 0;
151 	} else if (sp >= MAXVAL) {
152 		return 0;
153 	} else {
154 		val[sp] = val[sp-1];
155 		sp++;
156 		return 1;
157 	}
158 }
159 
160 /* swap: swap the top two values of the stack
161  * return 1 if successful, 0 on failure */
162 int swap(void) {
163 	if (sp > 1) {
164 		int temp = val[sp-1];
165 		val[sp-1] = val[sp-2];
166 		val[sp-2] = temp;
167 		return 1;
168 	}
169 	return 0;
170 }
171 
172 /* clear: clear the stack */
173 void clear(void) {
174 	sp = 0;
175 }
176 
177 /* assign: assign a double to a variable, letters represented by int from 0-25
178  * return 1 on success, 0 on failure */
179 int assign(int index, double value) {
180 	if (0 <= index && index < LETTERS) {
181 		var[index] = value;
182 		return 1;
183 	}
184 	return 0;
185 }
186 
187 /* eval: evaluate variable, letters represented by int from 0-25 */
188 double eval(int index) {
189 	if (0 <= index && index < LETTERS) {
190 		return var[index];
191 	}
192 	return 0.0;
193 }
194 
195 #include <ctype.h>
196 
197 int getch(void);
198 void ungetch(int);
199 
200 /* getop: get next operator or numeric operand */
201 int getop(char s[]) {
202 	int i, c;
203 	while ((s[0] = c = getch()) == ' ' || c == '\t')
204 		;
205 	s[1] = '\0';
206 	i = 0;
207 	if (isalpha(c)) {
208 		/*
209 		while (isalpha(s[++i] = c = getch()))
210 			;
211 		*/
212 		while ((s[++i] = c = getch()) != ' ' && c != '\t' && c != '\n')
213 			;
214 		s[i] = '\0';
215 		if (c != EOF)
216 			ungetch(c);
217 		if (s[1] == '=') {
218 			return ASSIGNMENT;
219 		} else if (s[1] == '\0') {
220 			return VARIABLE;
221 		} else {
222 			return IDENTIFIER;
223 		}
224 	}
225 	/* negative number or minus */
226 	if (c == '-') {
227 		s[++i] = c = getch();
228 		if (!isdigit(c) && c != '.') {
229 			ungetch(c);
230 			return '-';
231 		}
232 	}
233 	if (!isdigit(c) && c != '.')
234 		return c; /* not a number */
235 	if (isdigit(c)) /* collect integer part */
236 		while (isdigit(s[++i] = c = getch()))
237 			;
238 	if (c == '.') /* collect fraction part */
239 		while (isdigit(s[++i] = c = getch()))
240 			;
241 	s[i] = '\0';
242 	if (c != EOF)
243 		ungetch(c);
244 	return NUMBER;
245 }
246 
247 #define BUFSIZE 100
248 
249 char buf[BUFSIZE]; /* buffer for ungetch */
250 int bufp = 0;
251 
252 /* get a (possibly pushed back) character */
253 int getch(void) {
254 	return (bufp > 0) ? buf[--bufp] : getchar();
255 }
256 
257 /* push character back on input */
258 void ungetch(int c) {
259 	if (bufp >= BUFSIZE)
260 		printf("ungetch: too many characters\n");
261 	else
262 		buf[bufp++] = c;
263 }