C_PRG_LANG_EX_4.2

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <limits.h>
#include <ctype.h>

double my_atof(char *str)
{
    double res = 0;

    while(!isdigit(*str) && *str != 'e' && *str != 'E' && *str != '-')
        str++;

    if (*str == 'e' || *str == 'E')
        return 0;

    int sign_numb = 1;
    if (*str == '-')
    {
        sign_numb = -1;
        str++;
    }

    if (*str == 'e' || *str == 'E')
        return 0;

    while (isdigit(*str))
        res = res * 10 + (*str++ - '0');

    if (!*str)
        return res * sign_numb;

    int power = 0;
    if (*str == '.')
    {
        str++;
        while (isdigit(*str))
        {
            res = res * 10 + (*str++ - '0');
            power++;
        }
        for (int i = 0; i < power; i++)
        res /= 10;
    }

    if (!*str)
        return res * sign_numb;

    int exp = 0;
    if (*str == 'e' || *str == 'E')
    {
        str++;
        int sign_exp = 0;
        if (*str == '-')
        {
            sign_exp++;
            str++;
        }

        while (isdigit(*str))
            exp = exp * 10 + (*str++ - '0');

        if (sign_exp)
            for (int i = 0; i < exp; i++)
                res /= 10;
        else
            for (int i = 0; i < exp; i++)
                res *= 10;

    }

    return res * sign_numb;
}

int main(void)
{
    double res = 0;
    res = my_atof("123");
    printf("%s %lf\n", "123", res);

    res = my_atof("123.456");
    printf("%s %lf\n", "123.456", res);

    res = my_atof("-123");
    printf("%s %lf\n", "-123", res);

    res = my_atof("-123.456");
    printf("%s %lf\n", "-123.456", res);

    res = my_atof("123.456E4");
    printf("%s %lf\n", "123.456E4", res);

    res = my_atof("123.456E-4");
    printf("%s %lf\n", "123.456E-4", res);

    res = my_atof("-123.456E4");
    printf("%s %lf\n", "-123.456E4", res);

    res = my_atof("-123.456E-4");
    printf("%s %.8lf\n", "-123.456E-4", res);

    return 0;
}