From 71f25e917429698f119dad2939d9cc585183e332 Mon Sep 17 00:00:00 2001
From: Steven Schronk <steven@schronk.com>
Date: Tue, 15 Dec 2009 20:46:09 -0600
Subject: [PATCH] Completed tutorial for 4-2. Converts string of chars into
 long double.

---
 ex_4-2.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 58 insertions(+), 12 deletions(-)

diff --git a/ex_4-2.c b/ex_4-2.c
index ff8d1fc..a42325b 100644
--- a/ex_4-2.c
+++ b/ex_4-2.c
@@ -1,42 +1,88 @@
 #include <stdio.h>
 #include <ctype.h>
+
 /*
 
-Add support for scientific notation.
+Converts string of char into a double.
+Supports scientific notation, signed exponents.
 
-123.45e-6
-123.45E6
--123.45e56
+Warning: very large or small numbers exibit rounding errors.
 
 */
 
 double atof(char s[])
 {
-	double val, power;
-	int i, sign;
-	for(i=0; isspace(s[i]); i++) /* skip spaces */
+	long double val, power, expower;
+	int i, sign, exsign;
+	for(i=0; isspace(s[i]); i++) // skip spaces
 		;
 
 	sign = (s[i] == '-') ? -1 : 1; // mark sign for number
-	if(s[i] == '+' || s[i] == '-') // skip over signs
+	while(!isdigit(s[i])) // skip over signs and non numbers, dollar signs
 		i++;
-	for(val = 0.0; isdigit(s[i]); i++)
+
+	val = 0.0;
+	while(isdigit(s[i])) // calc val of nums before decimal
+	{
 		val = 10.0 * val + (s[i] - '0');
+		i++;
+	}
+
 	if(s[i] == '.') // skip over decimals
 		i++;
 
-	for(power = 1.0; isdigit(s[i]); i++)
+	power = 1.0;
+	while(isdigit(s[i])) // calc val of nums after decimal
 	{
 		val = 10.0 * val + (s[i] - '0');
 		power *= 10.0;
+		i++;
 	}
-	return sign * val / power;
+
+	val = sign * val / power;
+
+	// check for exponent
+	if(s[i] == 'e' || s[i] == 'E')
+	{
+		++i; // move past e or E
+		exsign = (s[i] == '-') ? -1 : 1; // mark sign for number
+		while(!isdigit(s[i])) // skip over sign(s)
+			i++;
+
+		expower = 0.0;	
+		while(isdigit(s[i]))
+		{
+			expower = 10.0 * expower + (s[i] - '0');
+			++i;
+		}
+	}
+
+	/* cannot get pow() to work in math.h ... improvising :) */
+	if(exsign == 1)
+	{
+		while(expower > 0) { val = val * 10; --expower; }
+	} else {
+		while(expower > 0) { val = val / 10; --expower; }
+	}
+	return val;
 }
 
 int main()
 {
-	char s[] = "-10928473.25";
+	char s[] = "-123.456e-7";
 	double ans = atof(s);
 	printf("%f\n", ans);
+
+	char r[] = "123.45e-6";
+	ans = atof(r);
+	printf("%f\n", ans);
+
+	char t[] = "123.45E6";
+	ans = atof(t);
+	printf("%f\n", ans);
+
+	char u[] = "-123.45e56";
+	ans = atof(u);
+	printf("%f\n", ans);
 	return 0;
 }
-- 
2.11.4.GIT