I want to sort a list of two lists, where the elements in the two lists are pairs.
I want to sort the lists by the second element in these pairs.
For example if I have
a_list = [[51132, 55274, 58132], [190, 140, 180]]
and want
sorted_list = [[55274, 58132, 51132], [140, 180, 190]]
Is there a simpler way than the following in Python2.7?
from operator import itemgetter
sorted_list = map(list, zip(*sorted(map(list,zip(*a_list)), key=itemgetter(1))))
Best regards, Øystein
I am a bit reluctant to post this as an answer, but why not, actually?
No, there is no simpler way to achieve your sort in Python -- except that you can drop that inner map:
>>> map(list, zip(*sorted(zip(*a_list), key=itemgetter(1))))
[[55274, 58132, 51132], [140, 180, 190]]
It may seem a bit convoluted at first (though not as much as with that additional map), but actually it's totally clear: You zip the list, sort by the second element, and zip it back. Everyone who knows Python should understand what the code does.
If you want to make it even clearer, either add a line comment describing what the sort does, or wrap it inside a function with a descriptive name and the same or a more extensive comment.
What you want according to my understanding is to sort each pair depending on the second element of the pair; that is to say, the ordering of the second element (of the pair) is to determine the ordering of the first element. The following code uses the corresponding position of the second element as the sorting key:
EDIT: I have made a revision that should solve the problem with duplicate elements, thanks for pointing that out:
class _Value:
def __init__(self, pos, val):
self.pos, self.val = pos, val
def __lt__(self, other):
return self.val < other.val
a_list = [[51132, 55274, 58132], [190, 140, 180]]
a_list = [[_Value(i, x) for i, x in enumerate(l)] for l in a_list]
sorted_list = [sorted(l, key=lambda x: a_list[1][x.pos]) for l in a_list]
sorted_list = [[x.val for x in l] for l in sorted_list]
assert sorted_list == [[55274, 58132, 51132], [140, 180, 190]]
index causes quadratic performance degradation on large inputs and incorrect sort results when the input contains duplicates._Value class.
map, apart from that it looks okay to me.