roll_number = [47, 64, 69, 37, 76, 83, 95, 97]
sample_dict = {'Jhon':47, 'Emma':69, 'Kelly':76, 'Jason':97}
 
for a in roll_number:
    if a not in sample_dict.values():
        roll_number.remove(a)

print(roll_number)

My output is [47, 69, 76, 95, 97] instead of [47, 69, 76, 97]. Can someone please explain?

CodePudding user response：

The issue I see is that you're iterating over the array while removing items from it. That leads to unsafe removal.

You could use a set to 'freeze' the array and remove its values safely:

roll_number = [47, 64, 69, 37, 76, 83, 95, 97]
sample_dict = {'Jhon':47, 'Emma':69, 'Kelly':76, 'Jason':97}

roll_set = set(roll_number)
for a in roll_number:
    if a not in sample_dict.values():
        roll_set.remove(a)

print(roll_set)

It'll print {97, 69, 76, 47}. If you want a list you can use list(roll_set).

Althoug set is a standard python type, you could also use another temporary list and would have the same result.

CodePudding user response：

You are removing elements while iterating the list. (Bad Idea) Removing elements while iterating the list will affect the iterator.

As mentioned here, there are several approaches:

• use a list comprehension to create a new list containing only the elements you don't want to remove
• create a copy of the original list
• by assigning to the slice somelist[:], you can mutate the existing list to contain only the items you want
• use itertools
• iterate backwards

CodePudding user response：

I believe it happen because you remove a value from an array while you loop on the exist array, just copy the existing array and remove the values of the copy

import copy

roll_number = [47, 64, 69, 37, 76, 83, 95, 97]
sample_dict = {'Jhon': 47, 'Emma': 69, 'Kelly': 76, 'Jason': 97}

roll_number_b = copy.deepcopy(roll_number)
for a in roll_number:
    if a not in sample_dict.values():
        roll_number_b.remove(a)

print(roll_number_b)

#output: [47, 69, 76, 97]

CodePudding user response：

I would use a list comprehension:

[x for x in roll_number if x in sample_dict.values()]

[47, 69, 76, 97]

You can see the example in this question:

How to remove items from a list while iterating?

CodePudding user response：

It is because the result is being updated in the loop.

To understand the behavior of the code, consider this example:

>>> lst = [1, 2, 3]
>>> lst.pop(0)    
1
>>> lst.pop(1)
3

You can obtain the expected output with this code (using the while loop and only incrementing the loop counter when the element does not remove):

i = 0
while i < len(roll_number):
    if roll_number[i] not in sample_dict.values():
        roll_number.pop(i)
    else:
        i  = 1

Output

The output with the above code and with the given input is:

[47, 69, 76, 97]