Problem with removing elements from list if the element exists in another list

Question

I have a list of lists which goes something like this...

data = [['09/10/2018', '11/10/2018', 'Subject A', 'Teacher Name A', 'Assignment text...'], ['09/10/2018', '10/10/2018', 'Subject B', 'Teacher Name B', 'Assignment text...']]

and I want to remove every element from that list in which the first three elements are in a .csv file.

Example CSV:

 09/10/2018, 11/10/2018, Subject A
 09/10/2018, 10/10/2018, Subject B
 24/09/2018, 01/10/2018, Subject E

This .csv may contain a row that does not match any of the lists in the data list.

My problem is that I am unable to successfully remove lists from the data list based on the .csv

Here is the relevant code:

data = pull_data() # Gets the list of lists
with open('Log.csv') as f: #Opens Log.csv
    prevData=[tuple(line) for line in csv.reader(f)] #Gets the data from the .csv
for sublist in data:
    for prevSub in prevdata:
        if(len(sublist) > 0 and len(prevSub) > 0):
            if sublist[0] == prevSub[0]:
                if sublist[1] == prevSub[1]:
                    if sublist[2] == prevSub[2]
                        data.pop(data.index(sublist)) # Should remove list from list of lists

jpp · Accepted Answer · 2018-10-10 01:26:42Z

1

Here's one way. The idea is to construct a set of tuples from the csv file. Then use a list comprehension to filter data. This ensure O(1) lookup complexity. Tuple conversion is necessary since tuple is hashable, while list is not.

from io import StringIO
import csv

data = [['09/10/2018', '11/10/2018', 'Subject A', 'Teacher Name A', 'Assignment text...'],
        ['09/10/2018', '10/10/2018', 'Subject B', 'Teacher Name B', 'Assignment text...']]

x = StringIO(""" 09/10/2018, 11/10/2018, Subject A
 09/10/2018, 10/10/2018, Subject B
 24/09/2018, 01/10/2018, Subject E""")

# replace x with open('Log.csv')
with x as fin:
    set_of_tuples = set(map(tuple, csv.reader(fin, skipinitialspace=True)))

# apply filter
res = [i for i in data if tuple(i[:3]) not in set_of_tuples]

answered Oct 10, 2018 at 1:26

jpp

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1 Comment

Sam Over a year ago

Thank you, I thought I had tried something similar but must have made a typo somewhere. Thank you for the extremely fast response.

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