sorting "problem"

[Bror Johansson]

Assume having defined two lists. There will be a one-to-one relationship
between elements in the two lists.

Is there a way to - when calling listA.sort() - have elements in listB
shuffled the same as those in listA? When calling listB.sort(), the listA
should be sorted alikewise.

[I know how to circumvent the problem - e.g. by creating a temporary list
with associated elements in tuples, sort this list and eventually extract
the two separate lists.]

Nevertheless, I'm curious whether it is possible - and if it is, how to -
associate two - or more - lists with respect to sorting.

/BJ

 

[Gary Herron]

Assume having defined two lists. There will be a one-to-one relationship
between elements in the two lists.

Is there a way to - when calling listA.sort() - have elements in listB
shuffled the same as those in listA? When calling listB.sort(), the listA
should be sorted alikewise.

[I know how to circumvent the problem - e.g. by creating a temporary list
with associated elements in tuples, sort this list and eventually extract
the two separate lists.]

Nevertheless, I'm curious whether it is possible - and if it is, how to -
associate two - or more - lists with respect to sorting.

/BJ

Here is a way to sort two (or more) associated lists for the price of
one sort. No zipping and unzipping of the associated lists is
necessary. One auxiliary list (of indexes) is used.

# Here are two associated lists
L1 = ('B', 'C', 'A')
L2 = ('b', 'c', 'a')

# A list of indexes (i.e., [0,1,2])
indexes = range(len(L1))

# Sort the indexes according values in L1
indexes.sort(lambda a,b: cmp(L1[a], L1))

# Use the indexes to access either list in sorted order
print [L1 for i in indexes] # prints ['A', 'B', 'C']
print [L2 for i in indexes] # prints ['a', 'b', 'c']




Gary Herron

 

[Mike C. Fletcher]

You can use the Numeric Python extensions for this if you've got a lot
of elements:

>>> from Numeric import *
>>> a array([8, 5, 1, 4, 3])
>>> b array([5, 2, 9, 8, 7])
>>> indices = argsort(a)
>>> indices array([2, 4, 3, 1, 0])
>>> c,d = take(a,indices),take(b,indices)
>>> c array([1, 3, 4, 5, 8])
>>> d

array([9, 7, 8, 2, 5])

Enjoy,
Mike

_______________________________________
Mike C. Fletcher
Designer, VR Plumber, Coder
http://members.rogers.com/mcfletch/