atof(char s[])函数的实现

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Q：编写一个函数atof(s)，其功能是将数字字符串s转换成与其等价的双精度浮点数，

并能处理以科学计数法表示的数据，如123.45e-5

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

double atof(char s[])
{
    int i;
    int sign;
    int expn;
    double val;
    double power;
    sign = 1;
    power = 1.0;
    expn = 0;
    for(i = 0; isspace(s[i]); ++i)
        ;
    if(s[i] == '-')
        sign = -1;
    if(s[i] == '+' || s[i] == '-')
        ++i;
    for(val = 0.0; isdigit(s[i]); ++i)
        val = val * 10.0 + (s[i] - '0');
    if(s[i] == '.')
        ++i;
    for(; isdigit(s[i]); ++i)
    {
        val = val * 10.0 + (s[i] - '0');
        power = 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(expn = 0; isdigit(s[i]); ++i)
            expn = expn * 10.0 + (s[i] - '0');
        if(sign == 1)
            while(expn-- > 0)
                val = val * 10.0;
        else
            while(expn-- > 0)
                val = val / 10.0;
    }
    return val;
}
int main()
{
    char str[1005];
    gets(str);
    printf("%.8f\n", atof(str));
    return 0;
}