（Python 3.3）无法弄清楚为什么值达到5时函数会创建无限循环

Question

I'm working on a program that simulates a vending machine. For some this section of my code creates an infinite loop, I cant quite figure out why. I ran it through python tutor to see where my problem was, it seems that when my variable price_remaining = 5 it will sometimes not go through the elif statement. I'm not entirely sure what would trigger it one way or another. I'm going to copy and paste what I entered in to python tutor in hopes that someone can explain based on that. EDIT: I just checked through python tutor some more, it appears to only occur when I have a value for price_remaining ending with a 5. For example 2.05,0.05, 1.15, etc.

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

When I add the line

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

at the start of your while loop, the code produces

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 ..]

Because of how floating point numbers work (see here for a reminder), you can't always perfectly represent each possible number, hence all the 9s.

In any case, once price_remaining < 5 , even if it's only less by a tiny amount, your code doesn't have any if branch which triggers.