(Python 3.3)无法弄清楚当值达到5时函数创建无限循环的原因

Question

我正在研究一个模拟自动售货机的程序.对于一些我的代码的这一部分创建一个无限循环,我无法弄清楚为什么.我通过python导师运行它来查看我的问题所在,似乎当我的变量price_remaining = 5时,它有时不会通过elif语句.我不完全确定会以某种方式触发它.我打算将我输入的内容复制并粘贴到python导师中,希望有人能够根据这一点进行解释.编辑:我刚刚通过python导师检查了一些,它似乎只发生在我有一个以5结尾的price_remaining的值时.例如2.05,0.05,1.15等.

price_remaining = 2.55
price_remaining = price_remaining * 100
q_stock = 25
q_returned = -0
d_stock = 25
d_returned = 0
n_stock = 25
n_returned = 0
while price_remaining > 0:
    if price_remaining >=25 and q_stock > 0:
        price_remaining = price_remaining - 25
        q_stock = q_stock - 1
        q_returned = q_returned + 1
    elif price_remaining >=10 and d_stock > 0:
        price_remaining = price_remaining - 10
        d_stock = d_stock - 1
        d_returned = d_returned + 1
    elif price_remaining >=5 and n_stock > 0:
        price_remaining = price_remaining -5
        n_stock = n_stock - 1
        n_returned = n_returned + 1

print( q_returned)
print( d_returned)
print( n_returned

)

Answer 1

当我添加该行

print(repr(price_remaining), repr(n_stock))

在while循环开始时,代码生成

254.99999999999997 25
229.99999999999997 25
204.99999999999997 25
179.99999999999997 25
154.99999999999997 25
129.99999999999997 25
104.99999999999997 25
79.99999999999997 25
54.99999999999997 25
29.99999999999997 25
4.999999999999972 25
4.999999999999972 25
4.999999999999972 25
[.. forever ..]

由于浮点数的工作原理(请参阅此处提醒),您无法始终完美地表示每个可能的数字,因此所有9都是如此.

在任何情况下,一旦price_remaining < 5,即使它只是少量,你的代码也没有任何if触发的分支.