Keeping track of original indicies when sorting a list of lists by length

Question

You can sort a list of lists by length as follows:

l1 = [1,2,3]
l2 = [1,2,3]
l3 = [1,2]
lists = [l1, l2, l3]
sorted_lists = sorted(lists, key=len)
print sorted_lists  #[[1,2], [1,2,3], [1,2,3]]

I can't figure out how to keep track of the indicies to then match up the contents of sorted_lists with the original list names l1, l2 and l3.

This gets close, but I'm not sure how the solution can be implemented when sorting by length.

score 8 · Accepted Answer · answered Jun 21 '17 at 22:12

8

It is very possible. Just modify the key a bit to specify the right predicate on which len is to be applied.

>>> lists = [l1, l2, l3]
>>> lists = sorted(enumerate(lists), key=lambda x: len(x[1])) # enumerate is a tuple of (index, elem), sort by len(elem)
[(2, [1, 2]), (0, [1, 2, 3]), (1, [1, 2, 3])]

answered Jun 21 '17 at 22:12

cs95

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Can anyone come up with a `lambda`-free solution for this? – cs95 Jun 21 '17 at 22:18
... `def key_func(x): return len(x[1])` :) – juanpa.arrivillaga Jun 21 '17 at 22:31
It's like I learned nothing from my last question. Lol. Thanks. – cs95 Jun 21 '17 at 22:38
@Coldspeed For `lambda` free solution, how about using `arg.sort()` from `numpy`? – niraj Jun 21 '17 at 22:45
@open-source Perfect, thank you. I do believe there is no vanilla one liner without using `lambda`. Numpy is the only other way to go. `>>> np.argsort(lists); array([2, 0, 1])` – cs95 Jun 21 '17 at 22:48
@Coldspeed I just used `list comprehension` for it, just for reference I posted the answer, I don't know how efficient it would be though. – niraj Jun 21 '17 at 22:52

score 1 · Answer 2 · answered Jun 21 '17 at 22:51

1

Using arg.sort() from numpy with list comprehension can be other way:

import numpy

new_list = [(index, lists[index])for index in numpy.argsort(lists)]
print(new_list)

Output:

[(2, [1, 2]), (0, [1, 2, 3]), (1, [1, 2, 3])]

answered Jun 21 '17 at 22:51

niraj

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Seems fine. As for speed, they'd both be the same since they both internally run on C routines. – cs95 Jun 21 '17 at 23:04

Keeping track of original indicies when sorting a list of lists by length

2 Answers2