10

Say i have a list:

main_list = ['bacon', 'cheese', 'milk', 'cake', 'tomato']

and another list:

second_list = ['cheese', 'tomato']

How can I remove all elements that are found in the second list, from the main list?

Karl Knechtel
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Theadamlt
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4 Answers4

11
new_array = [x for x in main_array if x not in second_array]

However, this is not very performant for large lists. You can optimize by using a set for second_array:

second_array = set(second_array)
new_array = [x for x in main_array if x not in second_array]

If the order of the items does not matter, you can use a set for both arrays:

new_array = list(set(main_array) - set(second_array))
ThiefMaster
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  • I love how expressive python is :). It is like you are speaking english. – Vikas May 12 '12 at 11:30
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    Nice answer, just a minor addition: you could optimize the order preserving variant by changing `second_array` into a set first: `second_set = set(second_array); new_array = [x for x in main_array if x not in second_set]` – roskakori May 12 '12 at 11:33
9

If the order is not important you can use sets:

>>> main_array = ['bacon', 'cheese', 'milk', 'cake', 'tomato']
>>> second_array = ['cheese', 'tomato']
>>> set(main_array) & set(second_array)
set(['tomato', 'cheese'])

Here we use the intersection operator, &. Should you only want items not found in your second list, we can use difference, -:

>>> set(main_array) - set(second_array)
set(['cake', 'bacon', 'milk'])
fraxel
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  • That won't work if the order should be kept. Otherwise this is indeed the best solution. However, I think he wants to *remove* those elements so use `-` instead of `&`. – ThiefMaster May 12 '12 at 11:27
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    @ThiefMaster - true, and didn't interpret `I want to all elements` right! – fraxel May 12 '12 at 11:38
3
main_array = set(['bacon', 'cheese', 'milk', 'cake', 'tomato'])
second_array = (['cheese', 'tomato'])

main_array.difference(second_array)
>>> set(['bacon', 'cake', 'milk'])

main_array.intersection(second_array)
>>> set(['cheese', 'tomato'])
Mp0int
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0
l = [u'SQOOP', u'SOLR', u'SLIDER', u'SFTP', u'PIG', u'NODEMANAGER', u'JSQSH', u'HCAT', u'HBASE_REGIONSERVER', u'GANGLIA_MONITOR', u'FLUME_HANDLER', u'DATANODE', u'BIGSQL_WORKER']

p = [u'SQOOP', u'SOLR', u'SLIDER', u'SFTP']

l = [i for i in l if i not in [j for j in p]]

print l
[u'PIG', u'NODEMANAGER', u'JSQSH', u'HCAT', u'HBASE_REGIONSERVER', u'GANGLIA_MONITOR', u'FLUME_HANDLER', u'DATANODE', u'BIGSQL_WORKER']
Santhosh Balasa
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