关于一个迭代的问题

zetro

新人第一帖，没有多少金币，就不散了(主要是没有找到在哪可以悬赏的)。望大神关注。
问题是这样的算一个关于利息和花钱的作业，每年的利息是是用一个list表示出来的
迭代公式是这样的:F[n]=F[n-1]*(1+0.01*growthRates[n])-expenses
但是我算出来的答案是这样的：[80000.00000000001, 54000.000000000015, 20000.000000000015, -1964.9999999999818, -38563.70919999998]
正确答案是：[80000.000000000015, 54000.000000000015, 24000.000000000015, -4799.9999999999854, -34847.999999999985]
检查出来的错误应该是出现在retireFound()部分，但是不明白是哪里出现了问题。求大神们指点一二
另外，问什么小数点后面会出现这么0.000000000015这种数据，感觉是因为计算机把浮点数转换为二进制的时候出现的问题，不知道这种解释对不对？还有个问题，怎么添加代码文字，还要有行标记的那种呢
以下是代码：

def retireFund(first,bito,n,expenses):
    a=1+0.01*bito
    print (bito)
    b=first
    i=n
    if i==0:
        return b
    else:
        return retireFund(first,bito,i-1,expenses)*a-expenses

def postRetirement(saving, growthRate,expenses):
    """
    - salary: the amount of money you make each year.
    - save: the percent of your salary to save in the investment account each
      year (an integer between 0 and 100).
    - growthRate: the annual percent increase in your investment account (an
      integer between 0 and 100).
    - years: the number of years to work.
    - return: a list whose values are the size of your retirement account at
      the end of each year.
    """
    F=[]
    n=0
    years=len(growthRate)
    first=saving*(1+0.01*growthRate[0])-expenses
    while n<years:
        bito=growthRate[n]      
        F.append(retireFund(first,bito,n,expenses))
        n=n+1
    return F

def testPostRetirement():
    savings     = 100000
    growthRates = [10, 5, 0, 5, 1]
    expenses    = 30000
    savingsRecord = postRetirement(savings, growthRates, expenses)
    print (savingsRecord)
    # Output should have values close to:
    # [80000.000000000015, 54000.000000000015, 24000.000000000015,
    # -4799.9999999999854, -34847.999999999985]
    # TODO: Add more test cases here.    # TODO: Add more test cases here.

lightninng

python里面没有double和float之分，默认的带小数点的出发和乘法都用float变量，而这个float变量的精度是17位，也就是说超过17位的数字是没有意义的，如果要处理高精度的处理，需要第三方模块的支持，可以搜索python高精度，应该会有相应的介绍文章，个人见解，楼主试一下吧

d5zhao

看一下