Perfect solution for Python traversal delete the null-value element in the dictionary error problem

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Exam = {' Math ': ' Up ', ' eng ': ' + ', ' CHN ': ' + ', ' phy ': ', ' Chem ': '}

Use the following traversal methods to delete:

1. For e in exam:
2. If exam[e] = = ":
3. Del Exam[e]

The result is the following error, How to resolve:

Traceback (most recent): File "untitled.py", line 3, in <module> for  E in Exam:RuntimeError:dictionar Y changed size during iteration

Correct practice:

1. S = {"1": A, "2": B, "3": C, "4":d, "5": E}
2. S_key = List (S.keys ())
3. For k_s in S_key:

4. #比如我要删除第四个元素

5.del s["4"]

Just in the For loop, which is equivalent to the operation of the linked list, it automatically calls the next method!

The dictionary iterator will traverse its keys, and in this process, you cannot change the dictionary! Cannot delete, add data

To first record the index of the element to be deleted, and then delete it after traversing, Exam.keys () returns a separate list

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