Sort()
Bengt Richter
bokr at oz.net
Thu Jun 5 20:26:08 CEST 2003
On Thu, 5 Jun 2003 11:42:07 -0500, sismex01 at hebmex.com wrote:
>> From: Turhan Ozen [mailto:txo at cs.nott.ac.uk]
>> Sent: Thursday, June 05, 2003 11:32 AM
>>
>> I created a class and modified the default _comp_ function.
>> I can sort an array of instances of this class. But I don't
>> want to change the original array. Instead, I want to keep
>> the positions after sorting in an attribute of the instances.
>>
>> Instead of [a1,a2,a3] becoming [a2,a1,a3], I would like the
>> order remain same but
>>
>> a1.rank=2
>> a2.rank=1
>> a3.rank=3
>>
>> How can I do this? Does this functionality already exist? If
>> not could you please give me advice on how to implement it?
>> I have read the HOWTO on sorting, there is an example of sorting
>> classes there. Is there any other place where I can find more
>> examples of using Python functions?
>>
>> Thanks,
>> Turhan.
>>
>
>Hmmm... you could simulate it with:
>
>## First we have...
>L = [ ... original list ... ]
>
>## Now, we create a sorted version of the list:
>SL = L[:]
>SL.sort()
>
>## We now create a dictionary keyed by item, relating to it's index:
>D = dict( zip(SL, range(len(SL))) )
>
>## And now, we transfer the index position as an attribute to
>## the items on the original list:
>for item in L:
> item.rank = D[item]
>
Might be a problem if some objects are not hashable, or if they
hash to identical values (i.e., duplicates in list), so maybe
for i in xrange(len(SL)): setattr(SL[i], 'rank', i)
is safer. E.g.,
>>> class X:
... def __init__(self, v): self.v = v
... def __cmp__(self,other): return cmp(self.v, other.v)
... def __repr__(self): return '<X:%r>'%self.v
...
>>> import random
>>> L = [X(random.choice('abcdefg')) for i in xrange(8)]
>>> L
[<X:'a'>, <X:'a'>, <X:'b'>, <X:'c'>, <X:'b'>, <X:'g'>, <X:'g'>, <X:'d'>]
>>> SL = L[:]
>>> SL
[<X:'a'>, <X:'a'>, <X:'b'>, <X:'c'>, <X:'b'>, <X:'g'>, <X:'g'>, <X:'d'>]
>>> SL.sort()
>>> SL
[<X:'a'>, <X:'a'>, <X:'b'>, <X:'b'>, <X:'c'>, <X:'d'>, <X:'g'>, <X:'g'>]
>>> L
[<X:'a'>, <X:'a'>, <X:'b'>, <X:'c'>, <X:'b'>, <X:'g'>, <X:'g'>, <X:'d'>]
>>> for i in xrange(len(SL)): setattr(SL[i],'rank',i)
...
>>> L
[<X:'a'>, <X:'a'>, <X:'b'>, <X:'c'>, <X:'b'>, <X:'g'>, <X:'g'>, <X:'d'>]
>>> [x.rank for x in L]
[0, 1, 2, 4, 3, 6, 7, 5]
>>> [x.rank for x in SL]
[0, 1, 2, 3, 4, 5, 6, 7]
Regards,
Bengt Richter
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