Sorting list based on values from another list?
Question
I have a list of strings like this:
X = ["a", "b", "c", "d", "e", "f", "g", "h", "i"]
Y = [ 0, 1, 1, 0, 1, 2, 2, 0, 1 ]
What is the shortest way of sorting X using values from Y to get the following output?
["a", "d", "h", "b", "c", "e", "i", "f", "g"]
The order of the elements having the same "key" does not matter. I can resort to the use of for constructs but I am curious if there is a shorter way. Any suggestions?
Accepted Answer
Shortest Code
[x for _,x in sorted(zip(Y,X))]
Example:
X = ["a", "b", "c", "d", "e", "f", "g", "h", "i"]
Y = [ 0, 1, 1, 0, 1, 2, 2, 0, 1]
Z = [x for _,x in sorted(zip(Y,X))]
print(Z) # ["a", "d", "h", "b", "c", "e", "i", "f", "g"]
Generally Speaking
[x for _, x in sorted(zip(Y,X), key=lambda pair: pair[0])]
Explained:
zipthe twolists.- create a new, sorted
listbased on thezipusingsorted(). - using a list comprehension extract the first elements of each pair from the sorted, zipped
list.
For more information on how to set\use the key parameter as well as the sorted function in general, take a look at this.
Read more... Read less...
Zip the two lists together, sort it, then take the parts you want:
>>> yx = zip(Y, X)
>>> yx
[(0, 'a'), (1, 'b'), (1, 'c'), (0, 'd'), (1, 'e'), (2, 'f'), (2, 'g'), (0, 'h'), (1, 'i')]
>>> yx.sort()
>>> yx
[(0, 'a'), (0, 'd'), (0, 'h'), (1, 'b'), (1, 'c'), (1, 'e'), (1, 'i'), (2, 'f'), (2, 'g')]
>>> x_sorted = [x for y, x in yx]
>>> x_sorted
['a', 'd', 'h', 'b', 'c', 'e', 'i', 'f', 'g']
Combine these together to get:
[x for y, x in sorted(zip(Y, X))]
Also, if you don't mind using numpy arrays (or in fact already are dealing with numpy arrays...), here is another nice solution:
people = ['Jim', 'Pam', 'Micheal', 'Dwight']
ages = [27, 25, 4, 9]
import numpy
people = numpy.array(people)
ages = numpy.array(ages)
inds = ages.argsort()
sortedPeople = people[inds]
I found it here: http://scienceoss.com/sort-one-list-by-another-list/
The most obvious solution to me is to use the key keyword arg.
>>> X = ["a", "b", "c", "d", "e", "f", "g", "h", "i"]
>>> Y = [ 0, 1, 1, 0, 1, 2, 2, 0, 1]
>>> keydict = dict(zip(X, Y))
>>> X.sort(key=keydict.get)
>>> X
['a', 'd', 'h', 'b', 'c', 'e', 'i', 'f', 'g']
Note that you can shorten this to a one-liner if you care to:
>>> X.sort(key=dict(zip(X, Y)).get)
more_itertools has a tool for sorting iterables in parallel:
Given
from more_itertools import sort_together
X = ["a", "b", "c", "d", "e", "f", "g", "h", "i"]
Y = [ 0, 1, 1, 0, 1, 2, 2, 0, 1]
Demo
sort_together([Y, X])[1]
# ('a', 'd', 'h', 'b', 'c', 'e', 'i', 'f', 'g')
I actually came here looking to sort a list by a list where the values matched.
list_a = ['foo', 'bar', 'baz']
list_b = ['baz', 'bar', 'foo']
sorted(list_b, key=lambda x: list_a.index(x))
# ['foo', 'bar', 'baz']