From da60299c2c4ae40eadb76c4a12aa471ede286d98 Mon Sep 17 00:00:00 2001
From: zlg <zlg@zlg.space>
Date: Wed, 8 May 2013 00:59:24 -0500
Subject: Solve Exercise 4-2: atof() enhanced

---
 ch4/4-02_atof2.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 ch4/4-02_atof2.c

diff --git a/ch4/4-02_atof2.c b/ch4/4-02_atof2.c
new file mode 100644
index 0000000..69c42fc
--- /dev/null
+++ b/ch4/4-02_atof2.c
@@ -0,0 +1,81 @@
+#include <stdio.h>
+#include <ctype.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-2: Extend atof to handle scientific notation of the form
+ *
+ * 123.45e-6
+ *
+ * where a floating-point number may be followed by 'e' or 'E' and an
+ * optionally signed exponent.
+ *
+ * Answer: It's relatively simple to extend atof. You need a variable to count
+ * the number of places the decimal needs to be moved, and in which direction.
+ * With a little clever manipulation of the `power` variable, you don't even
+ * need to touch the math that's used just before atof() returns. :)
+ */
+
+double atof(char s[]) {
+	double val, power;
+	int i, sign, esign, exp;
+
+	/* Skip whitespace */
+	for (i = 0; isspace(s[i]); i++) {
+	}
+
+	sign = (s[i] == '-') ? -1 : 1;
+
+	if (s[i] == '+' || s[i] == '-') {
+		i++;
+	}
+	for (val = 0.0; isdigit(s[i]); i++) {
+		val = 10.0 * val + (s[i] - '0');
+	}
+	if (s[i] == '.') {
+		i++;
+	}
+	for (power = 1.0; isdigit(s[i]); i++) {
+		val = 10.0 * val + (s[i] - '0');
+		power *= 10.0;
+	}
+	/* Start the exponent checking... */
+	if (s[i] == 'e' || s[i] == 'E') {
+		i++;
+		esign = (s[i] == '-') ? 1 : 0;
+		if (s[i] == '+' || s[i] == '-') {
+			i++;
+		}
+		/* This allows us to count how many times we need to mess with
+		 * the power variable */
+		exp = 0;
+		while (isdigit(s[i])) {
+			exp *= 10;
+			exp += (s[i] - '0');
+			i++;
+		}
+		/* And do our business, which is a bit tricky */
+		while (exp > 0) {
+			/* Because `power` is being divided with later, we need to reverse
+			 * operands to ensure it shrinks and expands as needed. */
+			if (esign == 1) {
+				// A negative exponent means we need to divide by more
+				power *= 10.0;
+			} else {
+				// And a positive exponent means we need to divide by less
+				power /= 10.0;
+			}
+			exp--;
+		}
+	}
+	/* The math stays the same */
+	return sign * val / power;
+}
+
+int main(void) {
+	// Declare the function so there's no confusion over what it returns.
+	double atof(char s[]);
+	/* Should print "0.0000012345678900" */
+	printf("%20.16f\n", atof("   123.456789e-8     "));
+	return 0;
+}
-- 
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