Exercise 4.02

Page 73

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.

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

double atof(char s[]);

int main()
{
	printf("%f", atof("123.45e-3"));
}

double atof(char s[])
{
	double val, power, exponent;
	int i, sign, expsign;

	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;
	}

	if (s[i] == 'e') {
		i++;
	}

	expsign = (s[i] == '-') ? -1 : 1;

	if (s[i] == '+' || s[i] == '-') {
		i++;
	}

	for (exponent = 0.0; isdigit(s[i]); i++) {
		exponent = exponent * 10.0 + (s[i] - '0');
	}

	for (int j = 0; j < exponent; j++) {
		power = expsign > 0 ? power / 10 : power * 10;
	}

	return sign * val / power;
}