4-2.c

/*
 * 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.
 *
 * By Faisal Saadatmand
 */

#include <stdio.h>
#include <ctype.h>

/* functions */
double atof(char []);

/* atof: convert string s to double */
double atof(char s[])
{
	int i, sign, expSign;
	double val, power, exponent;

	for (i = 0; isspace(s[i]); ++i)           /* skip whitespace */
		;

	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;
	}

	if (tolower(s[i]) == 'e') /* handle scientific notation */
		++i;

	expSign = (s[i] == '-') ? -1 : 1; /* record  exponent's sign */
	if (s[i] == '+' || s[i] == '-')
		++i;

	for (exponent = 0.0; isdigit(s[i]); i++)  /* extract the exponent */
		exponent = 10.0 * exponent + (s[i] - '0');

	while (exponent-- != 0)         /* adjust power according to exponent */
		power = (expSign > 0) ? power / 10: power * 10; 

	return sign * val / power;
}

int main(void)
{
	char number[]  = "123.45";
	char number2[] = "123.45e-6";
	char number3[] = "123.45e+6";
	char number4[] = "-123.45E+6";

	printf("%f\n", atof(number));
	printf("%f\n", atof(number2));
	printf("%f\n", atof(number3));
	printf("%f\n", atof(number4));

	return 0;
}