Python: Sort a Nested List With Specific Criteria

Question

As a beginner of Python I recently get stuck for a problem of sorting a nested list with specific criteria. I have a nested list like this:

nestedList=[['R2D2','1path1','1path2'], 
            ['R3A1','2path1','2path2'],
            ['R15L2','3path1','3path2']]

I would like this list to be sorted by the first string in each nested list. The result would look like:

nestedList=[['R15L2','3path1','3path2'],
            ['R3A1','2paht1','2path2'],
            ['R2D2','1path1','1path2']]

Currently my solution is only use the sort function with reverse parameter:

nestedList.sort(reverse=True)

I am not sure whether this is safe, because I would like it not sort the list also by the second string.

How could I sort it only by the first string? (e.g. 'R15L2', 'R3A1', etc.)

Thanks a lot for your help!

My method is not really working though... for example, changing '1path1' to '5path1' will not get the expected result... :( — AplusG
– AplusG, Commented Apr 9, 2013 at 9:57
I don't understand the result that you want. 'R3A1' is larger than 'R2D2' but comes before it in your sorted list? — halex
– halex, Commented Apr 9, 2013 at 9:59
hmm... I was considering R15L2>R3A1>R2D2... perhaps this is more complicated than I expected... — AplusG
– AplusG, Commented Apr 9, 2013 at 10:06
If you want to sort by numeric value you have to get numeric values first. Convert "R15L2" to ('R', 15, 'L', 2) and you can sort correctly. — Jochen Ritzel
– Jochen Ritzel, Commented Apr 9, 2013 at 10:17

eumiro · Accepted Answer · 2013-04-09 09:58:47Z

5

You want to sort according to a key (the key is the first element of a list):

nestedList.sort(key=lambda x: x[0])

or

import operator as op

nestedList.sort(key=op.itemgetter(0))

answered Apr 9, 2013 at 9:58

eumiro

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Comments

sourabh21jain · Accepted Answer · 2013-04-09 11:13:11Z

1

 a = [['3f','2f','5a'],['5a','0r','7v'],['4r','58v','5l']]
>>> a
[['3f', '2f', '5a'], ['5a', '0r', '7v'], ['4r', '58v', '5l']]
>>> a.sort()
>>> a
[['3f', '2f', '5a'], ['4r', '58v', '5l'], ['5a', '0r', '7v']]
>>> a.sort(reverse=True)
>>> a
[['5a', '0r', '7v'], ['4r', '58v', '5l'], ['3f', '2f', '5a']]
>>>

answered Apr 9, 2013 at 11:13

sourabh21jain

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1 Comment

AplusG Over a year ago

Sorry.. Not sure what are you trying to say... I have this solution already, but it seems not entirely correct...

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