How to remove elements from a list

Question

I have two lists

first = ['-6.50', '-7.00', '-6.00', '-7.50', '-5.50', '-4.50', '-4.00', '-5.00'] 
second = ['-7.50', '-4.50', '-4.00']

I want to shorten first by every element that occur in second list.

for i in first:
    for j in second:
        if i == j:
            first.remove(i)

Don't know why this did not remove the -4.00

['-6.50', '-7.00', '-6.00', '-5.50', '-4.00', '-5.00']

Any help appreciated :)

A comparison of all the methods suggested in the answers below. http://codebunk.com/bunk#-ItP66mNZpNolwb3ZNX5 — spicavigo, Apr 30 '13 at 09:03
@spicavigo You'll need to run it on larger input to really see the difference. And move the setup lists out of the code too — jamylak, Apr 30 '13 at 09:26

score 3 · Accepted Answer · answered Apr 30 '13 at 08:49

3

>>> first = ['-6.50', '-7.00', '-6.00', '-7.50', '-5.50', '-4.50', '-4.00', '-5.00']
>>> second = ['-7.50', '-4.50', '-4.00']
>>> set_second = set(second) # the set is for fast O(1) amortized lookup
>>> [x for x in first if x not in set_second]
['-6.50', '-7.00', '-6.00', '-5.50', '-5.00']

answered Apr 30 '13 at 08:49

jamylak

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1

Thank you, any idea my solution did not do it? – nutship Apr 30 '13 at 08:50
3

@nutship you shouldn't remove elements from a list while iterating through it :p http://stackoverflow.com/questions/1207406/remove-items-from-a-list-while-iterating-in-python – TerryA Apr 30 '13 at 08:51
1

@nutship what Haidro said ;) – jamylak Apr 30 '13 at 08:53

score 2 · Answer 2 · answered Apr 30 '13 at 08:50

2

Don't modify sequences you are iterating through.

Shortest way to do this:

list(set(first) - set(second))

answered Apr 30 '13 at 08:50

Daniel Roseman

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score 2 · Answer 3 · edited May 23 '17 at 12:12

2

If you don't care about order, use

list(set(temp1) - set(temp2))

References: Get difference between two lists

edited May 23 '17 at 12:12

Community

1
1

answered Apr 30 '13 at 08:55

AutomaticHourglass

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score 1 · Answer 4 · answered Apr 30 '13 at 08:54

Try this code, and you will see that the -4.00s don't come side by side.:

for i in first:
    for j in second:
        print i,j,
        if i == j:
            print 'removed'
            first.remove(i)
        else:
            print

You shouldn't modify sequences you are iterating over.

TO solve your problem, just create a copy of your lists, which can be done by adding [:]:

for i in first[:]:
    for j in second[:]:
        if i == j:
            first.remove(i)

Another way to do this is:

[i for i in first if i not in second]

score 1 · Answer 5 · answered Apr 30 '13 at 08:58

1

l3 = [x for x in first if x not in second]

answered Apr 30 '13 at 08:58

kiriloff

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How to remove elements from a list

5 Answers5