#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#include <math.h>

#include <time.h>

#include <ctype.h>

double atof_num(char s[])

{

    double val,power = 1.0;

    int i,sign,j,val_lat = 0;

    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' || s[i] == 'E')

    {

        i++;

        if(s[i] == '-')

        {

            i++;

            while(isdigit(s[i]))

            {

                val_lat = 10 * val_lat + (s[i] - '0');

                i++;

            }

            for(j = 0;j < val_lat;j++ )

                power *= 10.0;

            return val * sign / power;

        }

        else if(s[i] == '+')

        {

            i++;

            while(isdigit(s[i]))

            {

                val_lat = 10 * val_lat + (s[i] - '0');

                i++;

            }

            for(j = 0;j < val_lat;j++ )

                power /= 10.0;

            return val * sign / power;

        }

        else

        {

            while(isdigit(s[i]))

            {

                val_lat = 10 * val_lat + (s[i] - '0');

                i++;

            }

            for(j = 0;j < val_lat;j++ )

                power /= 10.0;

            return val * sign / power;

        }

    }

    return val * sign / power;

}

int main()

{

    char s[100],c;

    int i = 0;;

    double result;

    while((c = getchar()) != '\n')

        s[i++] = c;

    s[i] = '\0';

    result = atof_num(s);

    printf("%f\n",result);

	return 0;

}

1、isspace、isdigit函数在ctype.h中；

2、else后面不能加判断语句；

3、s[i++]此类要小心用！