列表「怪癖」

Question

我嘗試了一些事情列出了交互式解釋，我注意到了這一點：

>>> list = range(1, 11) 
>>> for i in list: 
...  list.remove(i) 
... 
>>> list 
[2, 4, 6, 8, 10] 

任何人都可以解釋爲什麼它留下偶數？這讓我很困惑......非常感謝。

Answer 1

modify a list這是不安全的，你正在迭代。

Answer 2

我的猜測是，for循環等實現如下：

list = range(1, 11) 

i = 0 
while i < len(list): 
    list.remove(list[i]) 
    i += 1 

print(list) 

每次元素被刪除，「下一個」元素滑入其位置，但是i得到反正遞增，跳過2種元素。

但是，是的，ObscureRobot是正確的，這是不是真的安全（這可能是未定義的行爲）。

Answer 3

如果你想從後到前修改的列表，同時遍歷它，工作方式：

lst = range(1, 11) 
for i in reversed(lst): 
    lst.remove(i) 

Answer 4

我覺得這是最簡單的用Python解釋：

>>> for iteration, i in enumerate(lst): 
...  print 'Begin iteration', iteration, 'where lst =', str(lst), 'and the value at index', iteration, 'is', lst[iteration] 
...  lst.remove(i) 
...  print 'End iteration', iteration, 'where lst =', str(lst), 'with', i, 'removed\n' 
... 
Begin iteration 0 where lst = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] and the value at index 0 is 1 
End iteration 0 where lst = [2, 3, 4, 5, 6, 7, 8, 9, 10] with 1 removed 

Begin iteration 1 where lst = [2, 3, 4, 5, 6, 7, 8, 9, 10] and the value at index 1 is 3 
End iteration 1 where lst = [2, 4, 5, 6, 7, 8, 9, 10] with 3 removed 

Begin iteration 2 where lst = [2, 4, 5, 6, 7, 8, 9, 10] and the value at index 2 is 5 
End iteration 2 where lst = [2, 4, 6, 7, 8, 9, 10] with 5 removed 

Begin iteration 3 where lst = [2, 4, 6, 7, 8, 9, 10] and the value at index 3 is 7 
End iteration 3 where lst = [2, 4, 6, 8, 9, 10] with 7 removed 

Begin iteration 4 where lst = [2, 4, 6, 8, 9, 10] and the value at index 4 is 9 
End iteration 4 where lst = [2, 4, 6, 8, 10] with 9 removed 

請注意，這是一個壞想法：（a）在遍歷它時修改list;（b）調用list「列表」。