字符串转浮点数

自己实现的字符串转换浮点数的代码

模电期中考试无聊写的- -

先记录一下， 以后有时间在优化。

精度之类的等等问题也暂时没有考虑

// =====================================================================================
//
//       Filename:  my_atoi.c
//
//    Description:  
//
//        Version:  1.0
//        Created:  2014年11月14日 19时19分48秒
//       Revision:  none
//       Compiler:  g++
//
//         Author:  wangweihao (wangweihao), 578867817@qq.com
//        Company:  none
//
// =====================================================================================

/*
 * 实现字符转转浮点函数
 * 浮点数可能有普通浮点数和科学计数法的浮点数
 * 情况比较多
 * 举个例子
 * 3.14  0.314  314   3.14e10(等价于3.14e+10)    3.14e-10   
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>

double my_atoi(char *s)
{
    int i, length, newlength;
    int flag = 0, flag2 = 0;           //标记'.' , 和'e'
    int flagIndex = 0, flagIndex2 = 0;
    char symbol;
    double sum = 0;                    //计算最后的总和
    double sum1 = 0, sum2 = 0, sum3 = 0;
    double ten1 = 1, ten2 = 0.1, ten3 = 1;

    length = strlen(s);

    for(i = 0; i < length; ++i)
    {
        if(isdigit(s[i]))
            ;
        else if(ispunct(s[i]))
        {
            if(s[i] == '.')
            {
                flagIndex = i;
                ++flag;
            }
        }
        else if(isalpha(s[i]))
        {
            if(s[i] == 'e')
            {
                ++flag2;
                flagIndex2 = i;
            }
            if(i < length)
            {
                symbol = s[i+1];   //存储符号
            }
        }
    }
    
    if(flag > 1 || flag2 > 1)
    {
        printf("error input\n");
        exit(-1);
    }
    //printf("flag = %d, flagIndex = %d, flag2 = %d, flagIndex2 = %d\nsymbol = %c\n", flag, flagIndex, flag2, flagIndex2, symbol);

    if(flag2 != 0)
        length = flagIndex2;

    if(flag == 0 && flag2 == 0)
    {
        for(i = length-1; i >= 0; --i)
        {
            sum += (s[i]-48)*ten1;
            ten1 *= 10;
        }
        return sum;
    }
    for(i = flagIndex-1; i >= 0; --i)
    {
        sum1 += (s[i]-48)*ten1;
        ten1 *= 10;
    }
    for(i = flagIndex+1; i < length; ++i)
    {
        sum2 += (s[i]-48)*ten2;
        ten2 /= 10;
    }
    length = strlen(s);
    for(i = length-1; isdigit(s[i]); --i)
    {
        sum3 += (s[i]-48)*ten3;
        ten3 *= 10;
    }
    if(flag2 != 0)
    {
        if(symbol == '-')
        {
            sum3 = -sum3;
            sum = (sum1 + sum2)* (pow(10, sum3));
            return sum;
        }
        else
            sum = (sum1 + sum2)* (pow(10, sum3));
        return sum;
    }
    
    sum = sum1 + sum2;

    return sum;
}

int main(int argc, char *argv[])
{
    char *p1 = "3.14";
    char *p2 = "0.314";
    char *p3 = "314";
    char *p4 = "3.14e3";
    char *p5 = "3.14e+3";
    char *p6 = "3.14e-3";
    char a[100];

    printf("p1:%lf, p2:%lf, p3:%lf, p4:%lf, p5:%lf, p6:%lf\n", my_atoi(p1), my_atoi(p2), my_atoi(p3), my_atoi(p4), my_atoi(p5), my_atoi(p6));

    scanf("%s", a);
    printf("my_atio(%s): %lf\n", a, my_atoi(a));
    return 0;