一个关于 for 和 while 语句的问题

sandwich_orders = ["pastrami","vegetable","chicken","pastrami","pastrami"]
finished_sandwiches = []
#while sandwich_orders:
for sandwich_order in sandwich_orders:
print("I made your " + sandwich_order + " sandwich")
sandwich_orders.remove(sandwich_order)

输出是：

I made your pastrami sandwich
I made your chicken sandwich
I made your pastrami sandwich

如果不注释掉 while 语句，就可以全部输出：

I made your pastrami sandwich
I made your chicken sandwich
I made your pastrami sandwich
I made your vegetable sandwich
I made your pastrami sandwich

请问为什么只用 for 语句会出现输出不完整的情况？添加了 while 语句后为什么可以避免出现问题？

sandwich

pastrami

made

your

4 条回复 • 2019-11-10 19:30:17 +08:00

ZeldaPeach

2019-11-05 23:33:01 +08:00

list 的 for ... in 迭代，是内部生成一个计数器，从 0 开始，每次加 1，将这个数字作为索引取该 list 的元素，直到产生 StopIteration 终止
内部计数器从 0 开始
list 为 ["pastrami","vegetable","chicken","pastrami","pastrami"]
给出 list[0] 用作迭代，此时为 "pastrami"
删掉"pastrami"，list 变为 ["vegetable","chicken","pastrami","pastrami"]
内部计数器加 1 变为 1
给出 list[1] 用作迭代，此时为 "chicken"，那个 "vegetable" 的索引已经变为 0 了，所以被跳过了
删掉 "chicken"，list 变为 ["vegetable","pastrami","pastrami"]
内部计数器加 1 变为 2
给出 list[2] 用作迭代，此时为 "pastrami"
删掉 "pastrami"，list 变为 ["vegetable","pastrami"]
内部计数器加 1 变为 3
给出 list[3] 用作迭代，超过现在 list 的长度，触发 StopIteration，迭代终止

此时如果 while list，因为非空，所以再将 ["vegetable","pastrami"] 进行 for 迭代
list[0] -> "vegetable"，删掉此元素，list 变为 ["pastrami"]
list[1] -> StopIteration，迭代终止

再 while list，还有一个元素，所以再将 ["pastrami"] 进行 for 迭代
list[0] -> "pastrami"，删掉此元素，list 变为 []
list[1] -> StopIteration，迭代终止

再 while list，空列表，不再继续，while 结束

ZeldaPeach

2019-11-05 23:39:49 +08:00

官方文档
docs.python.org/3/reference/compound_stmts.html#the-for-statement
这一小节最后的 note 把这个使用 internal counter 的迭代机制讲了

XIVN1987

2019-11-06 09:13:14 +08:00

不能在遍历 sandwich_orders 的同时对 sandwich_orders 进行增减 item 吧

eggs

2019-11-10 19:30:17 +08:00 via iPhone

不行