From 59921a2bceb7df1728436f260281235a3fb8471c Mon Sep 17 00:00:00 2001
From: zlg <zlg@zlg.space>
Date: Fri, 21 Jun 2013 02:09:25 -0500
Subject: Solve Exercise 4-6: Variable support
---
ch4/4-06_var-support.c | 272 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 272 insertions(+)
create mode 100644 ch4/4-06_var-support.c
diff --git a/ch4/4-06_var-support.c b/ch4/4-06_var-support.c
new file mode 100644
index 0000000..f57aa92
--- /dev/null
+++ b/ch4/4-06_var-support.c
@@ -0,0 +1,272 @@
+#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];
+}
--
cgit v1.2.3-54-g00ecf