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simonica
simonica

Reputation: 117

Exclude elements from a list from a file in python

Be the following list of elements:

listELEMENTS = ['aaa', 'bbb', 'ccc']

and be the following txt file (elements.txt) with lists of elements that should be kept in the list:

elements.txt
aaa
ccc
ddd

the idea is to remove the elements from the listELEMENTS that are not in the elements elements.txt file, so the final result after deletion will be this:

listELEMENTS = ['aaa', 'ccc']

the code I made was this, but it is eliminating all elements. Does anyone know where the error is?

with open("elements.txt") as f:
    for line in f:
       (key) = line.split()
           for i in listELEMENTS :
               if i not in key:
                   listELEMENTS.remove(i)

Upvotes: 0

Views: 824

Answers (3)

arthursribeiro
arthursribeiro

Reputation: 336

You're doing two iterations on your code, one for each line in the file and inside that you're doing one for each element in the list, so, getting your example we would have:

  • Iteration 01:
    list_elements = ['aaa', 'bbb', 'ccc']
    line = 'aaa'
    key = ['aaa']

    Inside this iteration another one is going to happen, so:

    - Iteration 01:
        i = 'aaa'
        i is in key, no element removed.

    - Iteration 02:
        i = 'bbb'
        i is NOT in key, 'bbb' gets removed.
        Finished iteration because an element was removed from the list
  • Iteration 02
    list_elements = ['aaa', 'ccc']
    line = 'ccc'
    key = ['ccc']

    Inside this iteration another one is going to happen, so:

    - Iteration 01:
        i = 'aaa'
        i is NOT in key, 'aaa' gets removed INCORRECTLY.
        Iterator finished.

I suggest you to change the way you're removing the data from your list, instead of editing a list you're iterating over it, create a new one with the data that you're actually going to be using:

listELEMENTS = ['aaa', 'bbb', 'ccc']
used_elements = []
with open("elements.txt") as f:
    file_content = f.read().splitlines()
    for i in listELEMENTS:
        if i in file_content:
            used_elements.append(i)

print(used_elements)

Upvotes: 1

Gabio
Gabio

Reputation: 9504

The problem in your code is that you are checking for each element of listELEMENTS if it exists in a specific element of the file instead of all file's elements (if i not in key).

Change your code to:

listELEMENTS = ['aaa', 'bbb', 'ccc']

with open("elements.txt") as f:
    file_elements = f.read().splitlines() # read all elements of the file into a list
    for i in listELEMENTS:
        if i not in file_elements:
        # check that i not exists in the full list of elements 
            listELEMENTS.remove(i)
print(listELEMENTS) # ['aaa', 'ccc']

Or, you can do it in a bit shorter way using list comprehension:

with open("elements.txt") as f:
    file_elements = f.read().splitlines()
    listELEMENTS = [i for i in listELEMENTS if i in file_elements]
print(listELEMENTS) # ['aaa', 'ccc']

Upvotes: 1

Beny Gj
Beny Gj

Reputation: 615

hi you can use intersection:

a =  ['aaa', 'bbb', 'ccc'] 

b =  ['aaa', 'ccc', 'ddd']


list(set(a).intersection(b))
['ccc', 'aaa']

so you can do something like that:

listELEMENTS = ['aaa', 'bbb', 'ccc'] 

with open("elements.txt") as f: 
    file_elements = f.read().splitlines() 
    #print(file_elements) 
    listELEMENTS =list(set(listELEMENTS).intersection(file_elements)) 
print(listELEMENTS)                                                   

['ccc', 'aaa']

Upvotes: 1

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