[Tutor] Original indices after Sorting

[Alan Gauld]

On 23/07/12 20:55, Ali Torkamani wrote:
> Hi every one,
> How can we get the indices of values in the original list after sorting
> a list?

create a sorted copy

L1 = [...original data...]
L2 = sorted(L1)

Now you can compare indexes except if there are duplicate elements
and you need to know which specific copy is where. In that case
I think you need to use id() but even that fails for small ints and 
other cached values.

If the data is too large to hold two copies you could create a list
of tuples with the original indexes:

L2 = []
n = -1
while L1:
    L2.append( (n,L1.pop()) )
    n -= 1

This creates L2 with a reversed L1 using the negative index of the 
original list. A bit clunky, I'm sure there is a neater way but it seems 
to work. But hopefully memory is not so tight you need to do this!


HTH

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/