atof的实现

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

/* atof: convert string s to double */
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 *= 10.0;
    }
    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;        /* positive exponent */
        } else {
            while (exp-- > 0)
                val /= 10;        /* negative exponent */
        }
    }

    return val;
}

int main()
{
    char x[] = "-123.45e-6";
    double val;

    val = atof(x);

    printf("%g\n", val);

    return 0;
}