Solve Exercise 4-4: Basic stack manipulation

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diff --git a/ch4/4-04_top-swap.c b/ch4/4-04_top-swap.c
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+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-4: Add commands to print the top element of the stack without
+ * popping, to duplicate it, and to swap the top two elements. Add a
+ * command to clear the stack.
+ *
+ * Answer: Achieving this is pretty simple. Make your functions, then assign
+ * them to "commands", which are basically any special characters that aren't
+ * being used in the switch statement. Pretty basic manipulation of arrays.
+ *
+ * Note that any expression that has subtraction at the end needs either a
+ * space or an additional newline input after it in order to calculate. This
+ * is a limitation of the getchar() based approach.
+ */
+
+#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);
+
+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
+
+/* 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];
+
+	while ((type = getop(s)) != EOF) {
+		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;
+			case '\n':
+				printf("\t%.8g\n", pop());
+				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';
+	/* 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;
+}