From f42ae1643002dac0ededc9aae37ba28c047d89f4 Mon Sep 17 00:00:00 2001
From: zlg <zlg@zlg.space>
Date: Tue, 18 Jun 2013 04:45:27 -0500
Subject: Solve Exercise 4-3: Add modulus to RPN calculator

---
 ch4/4-03_calc-modulus.c | 147 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 147 insertions(+)
 create mode 100644 ch4/4-03_calc-modulus.c

diff --git a/ch4/4-03_calc-modulus.c b/ch4/4-03_calc-modulus.c
new file mode 100644
index 0000000..05d5c1b
--- /dev/null
+++ b/ch4/4-03_calc-modulus.c
@@ -0,0 +1,147 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-3: Given the basic framework, it's straightforward to extend the
+ * calculator. Add the modulus (%) operator and provisions for negative
+ * numbers.
+ *
+ * Answer: Supporting modulus is indeed rather easy. The modulus operator
+ * divides the first operand by the second and returns the remainder, and C
+ * supports this out of the box, so there's no need to implement much math.
+ * Note that the modulus operator does integer division to get its remainder.
+ * Doing a "proper" modulus operator would require more work, and floats can
+ * be tricky. I might implement this sometime. For now, I typecasted.
+ *
+ * For negative numbers, you just need to make sure you allow the dash symbol
+ * in two key places.
+ */
+
+#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);
+
+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;
+			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, c;
+
+	while ((s[0] = c = getch()) == ' ' || c == '\t') {
+	}
+	s[1] = '\0';
+	/* The final check is for negative numbers. */
+	if (!isdigit(c) && c != '.' && c != '-') {
+		return c;
+	}
+	i = 0;
+	/* The second half of this if-statement accounts for negatives */
+	if (isdigit(c) || (i == 0 && c == '-')) {
+		while (isdigit(s[++i] = 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;
+	}
+}
-- 
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