ansi_c_pg067_ch04_ex02.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>

/* atof:  convert string s to double */
double
atof(char s[])
{
	double val, power, exp, expv;
	int    i, sign, expsgn;
	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;
	}

	if (s[i] == 'e' || s[i] == 'E') {
		i++;
		expsgn = 1;
		if (s[i] == '-')
			expsgn = -1;
		if (s[i] == '-' || s[i] == '+')
			i++;
	}
	for (exp = 0.0; isdigit(s[i]); i++)
		exp = 10.0 * exp + (s[i] - '0');

	expv = 1.0;
	while (exp--) {
		expv *= 10.0;
	}

	if (expsgn == -1)
		expv = 1 / expv;

	return (sign * val / power * expv);
}


int
main(int argc, char **argv) {
	double d;
	d = atof("1.332E2");
	printf("d = %lf\n", d);
	return (0);
}