An ordering question

MRAB google at mrabarnett.plus.com
Fri Mar 13 18:28:54 CET 2009


Kottiyath wrote:
> Hi,
>     I have 2 lists
> a = [(4, 1), (7, 3), (3, 2), (2, 4)]
> b = [2, 4, 1, 3]
> 
>     Now, I want to order _a_ (a[1]) based on _b_.
>     i.e. the second element in tuple should be the same as b.
>     i.e. Output would be [(3, 2), (2, 4), (4, 1), (7, 3)]
> 
>     I did the same as follows:
>>>> l = len(a) * [None]
>>>> for (k, v) in a:
> ...   for i, e in enumerate(b):
> ...     if e == v:
> ...        l[i] = (k, v)
> 
>      This works, but the code -for python- looks very kludgy.
>      I thought for ~2 hours to see whether I can do it in a line or 2,
> but I cannot seem to find a mechanism.
>      Can someone help me out?
> 
How about:

 >>> a = [(4, 1), (7, 3), (3, 2), (2, 4)]
 >>> b = [2, 4, 1, 3]
 >>> d = dict((v, k) for k, v in a)
 >>> c = [(d[s], s) for s in b]
 >>> c
[(3, 2), (2, 4), (4, 1), (7, 3)]

Understanding how it works is left as an exercise for the reader. :-)



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