tcpl-ex-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.
 */

#include <stdio.h>
#include <ctype.h>
#define EPSILON 0.000001

int float_equal(double arg0, double arg1);
double atof(char s[]);

int main(void)
{
	char s[] = "123.45e-6";
	double r = 123.45e-6;
	double a = -1;
	a = atof(s);
	printf("atof(%s) = %.10f\n", s, a);
	if (float_equal(a, r))
		printf("assert success.\n");
	else
		printf("assert fail.\n");
	return 0;
}

int float_equal(double arg0, double arg1)
{
	return fabs(arg0 - arg1) < EPSILON;
}

double atof(char s[])
{
	double val, power; 
    int i, sign, exp; 
 
    for (i = 0; isspace(s[i]); i++)  /* skip white space */ 
        ; 
    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; 
    } 
    val = sign * val / power;

	if (s[i] == 'e' || s[i] == 'E')
	{
		sign = (s[++i] == '-') ? -1 : 1;
		if (s[i] == '+' || s[i] == '-')
			i++;
		for (exp = 0; isdigit(s[i]); i++)
			exp = 10 * exp + (s[i] - '0');
		if (sign == 1)
			while (exp-- > 0)
				val *= 10;
		else
			while (exp-- > 0)
				val /= 10;
	}

	return val;

}