Solve Exercise 4-6: Variable support

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diff --git a/ch4/4-06_var-support.c b/ch4/4-06_var-support.c
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+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+#include <math.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-6: Add commands for handling variables. (It's easy to provide
+ * twenty-six variables with single-letter names.) Add a variable for the most
+ * recently printed value.
+ *
+ * Answer: I'm not sure if I found the "easy" way to do a bunch of variables.
+ * The knowledge of ASCII I gained earlier in the book let me know that 'a'
+ * minus 'a' == 0, and thus I used an array of doubles to do my bidding, using
+ * the appropriate math. Add a few extra things to the '\n' command case, and
+ * it pretty much wrote itself.
+ *
+ * One glaring limitation is the fact that you *must* put the variable letter
+ * directly after the '=' command or it won't assign it to anything. I could
+ * add another loop to account for this but I really see no need, since you
+ * should know which variable you're assigning to. It differentiates it from
+ * inline variables, too.
+ */
+
+#define MAXOP    100
+#define NUMBER   '0'
+#define MAXVAL   100
+#define BUFSIZE  100
+
+int getop(char []);
+void push(double);
+double pop(void);
+int getch(void);
+void ungetch(int);
+void stack_top(void);
+double dupe_top(void);
+void swap_top_two(void);
+void clear_stack(void);
+double fetch_var(char);
+void store_var(char, double);
+void store_last(double);
+double fetch_last(void);
+
+int sp = 0; // Next free stack position
+double val[MAXVAL]; // Value stack
+char buf[BUFSIZE]; // buffer for ungetch
+int bufp = 0; // next free position in buf
+double vars[27];
+
+/* Reverse Polish calculator:
+ *
+ * Binary operations (+-*\%)
+ *     operand operand operator
+ *
+ * Example: 6 minus 2 in Reverse Polish Notation is "6 2 -"
+ */
+int main() {
+	int type;
+	double op2;
+	char s[MAXOP];
+	char ltr;
+
+	while ((type = getop(s)) != EOF) {
+		if (isalpha(type) && islower(type)) {
+			push(fetch_var(type));
+			continue;
+		}
+		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: Cannot divide by zero.\n");
+				}
+				break;
+			/* Yay for modulus! */
+			case '%':
+				op2 = pop();
+				if (op2 != 0.0) {
+					push((int)pop() % (int)op2);
+				} else {
+					printf("Error: Cannot modulo by zero.\n");
+				}
+				break;
+			/* Top of stack */
+			case '?':
+				stack_top();
+				break;
+			/* Dupe the top of the stack */
+			case '#':
+				dupe_top();
+				break;
+			/* Swap the top two */
+			case '~':
+				swap_top_two();
+				break;
+			/* Clear the stack */
+			case '!':
+				clear_stack();
+				break;
+			/* sin() support */
+			case '(':
+				op2 = sin(pop());
+				push(op2);
+				break;
+			/* exp() support */
+			case '{':
+				op2 = exp(pop());
+				push(op2);
+				break;
+			/* pow() support */
+			case '^':
+				op2 = pop();
+				push(pow(pop(), op2));
+				break;
+			/* 'lastprint' support */
+			case '@':
+				push(fetch_last());
+				break;
+			/* setting variables */
+			case '=':
+				ltr = getchar();
+				if (isalpha(ltr) && islower(ltr)) {
+					op2 = pop();
+					store_var(ltr, op2);
+					push(op2);
+				}
+				break;
+			/* Final output */
+			case '\n':
+				op2 = pop();
+				printf("\t%.8g\n", op2);
+				/* Extra Credit: Lets output every non-zero variable! */
+				for (ltr = 'a'; ltr <= 'z'; ltr++) {
+					if (fetch_var(ltr) != 0) {
+						printf("\t%c: %.8g\n", ltr, fetch_var(ltr));
+					}
+				}
+				store_last(op2);
+				break;
+			default:
+				printf("Error: Unknown command %s\n", s);
+				break;
+		}
+	}
+	return 0;
+}
+
+void push(double f) {
+	if (sp < MAXVAL) {
+		val[sp++] = f;
+	} else {
+		printf("Error: Stack full. Cannot push %g\n", f);
+	}
+}
+
+double pop(void) {
+	if (sp > 0) {
+		return val[--sp];
+	} else {
+		printf("Error: Stack empty.\n");
+		return 0.0;
+	}
+}
+
+int getop(char s[]) {
+	int i = 0;
+	int c, next;
+
+	while ((s[0] = c = getch()) == ' ' || c == '\t') {
+	}
+	s[1] = '\0';
+	if (s[i] >= 'a' && s[i] <= 'z') {
+		return s[i];
+	}
+	/* The final check is for negative numbers. */
+	if (!isdigit(c) && c != '.' && c != '-') {
+		return c;
+	}
+	/* The second half of this if-statement accounts for negatives */
+	if (c == '-') {
+		next = getch();
+		if (!isdigit(next) && next != '.') {
+			return c;
+		} else {
+			c = next;
+		}
+	} else {
+		c = getch();
+	}
+
+	while (isdigit(s[++i] = c)) {
+		c = getch();
+	}
+	if (c == '.') {
+		while (isdigit(s[++i] = c = getch())) {
+		}
+	}
+	s[i] = '\0';
+	if (c != EOF) {
+		ungetch(c);
+	}
+	return NUMBER;
+}
+
+int getch(void) {
+	return (bufp > 0) ? buf[--bufp] : getchar();
+}
+
+void ungetch(int c) {
+	if (bufp >= BUFSIZE) {
+		printf("ungetch: Too many characters.\n");
+	} else {
+		buf[bufp++] = c;
+	}
+}
+
+void stack_top(void) {
+	if (sp > 0) {
+		printf("Top of stack is %8g\n", val[sp - 1]);
+	} else {
+		printf("Error: Stack empty.\n");
+	}
+}
+
+double dupe_top(void) {
+	double temp = pop();
+	push(temp);
+	push(temp);
+}
+
+void swap_top_two(void) {
+	double tmp1, tmp2;
+	tmp1 = pop();
+	tmp2 = pop();
+	push(tmp1);
+	push(tmp2);
+}
+
+void clear_stack(void) {
+	sp = 0;
+}
+
+double fetch_var(char c) {
+	return vars[c - 'a'];
+}
+
+void store_var(char c, double f) {
+	vars[c - 'a'] = f;
+}
+
+void store_last(double f) {
+	vars[26] = f;
+}
+
+double fetch_last(void) {
+	return vars[26];
+}