How to correctly delete elements in lists in Python

1. Problem raising

When operating lists in Python, netizens often traverse the list elements directly and then delete them with the remove function, so there will be code like this:

items = ['A', 'B', 'C', 'D', 'E']
 
for item in items:
    if item == 'B':
        ('B')
    else:
        print(item)

The consequences of this operation are:

After deleting the element 'B', the for loop will skip the next element in the list because the Python for loop will continue to move to the next index after deleting the current item, which points to the next element after deleting. The result is ['A','D','E']. After deleting B, the list index changes, causing the pointer to skip C. That is to say:

Initial items = ['A', 'B', 'C', 'D', 'E'].

When traversing to 'B', delete it and the list becomes ['A', 'C', 'D', 'E']. The loop continues, skipping 'C' because 'C' is now in its original 'B' position.

2. Solve the problem

So, how to solve this problem? The first is to create a temporary list, the second is to create a copy of the list, and the third is to use a while loop.

1. Create a temporary list

items = ['A', 'B', 'C', 'D', 'E']
new_items = []
 
for item in items:
    if item == 'B':
        continue
    else:
        print(item)
        new_items.append(item)

2. Create a copy of the list

items = ['A', 'B', 'C', 'D', 'E']
 
for item in items[:]:  # Create a copy using items[:]    if item == 'B':
        ('B')
    else:
        print(item)

Here, items[:] creates a copy and the loop is not affected by the deleted elements. This way can avoid the problem of skipping elements.

3. Use while loop

items = ['A', 'B', 'C', 'D', 'E']
index = 0
 
while index < len(items):
    if items[index] == 'B':
        ('B')
    else:
        print(items[index])
        index += 1  # Add index only if element is not deleted

We use an index variable to manually track the current index.

If the current element is 'B', it is removed from the list without adding index, so the next loop will check again for the new element at the current index.

If the current element is not 'B', print and increase index by 1.

This way ensures that no elements are skipped when deleting elements and that all items are properly traversed.

3. After-study summary

1. Python is a logically rigorous programming language. Its lists are a very important data structure. When learning, you must understand the principles of list traversal and master the correct method of traversal of lists.

2. Generally speaking, do not use remove to directly delete the original list element to avoid index confusion.

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