toy list processing problem: collect similar terms

Nick Mellor thebalancepro at
Thu Feb 7 00:25:36 CET 2013

Python 3 version:

from collections import defaultdict

data = ((0,'a','b'),(1,'c','d'),(2,'e','f'),(3,'g','h'),(1,'i','j'),(2,'k','l'),(4,'m','n'),(2,'o','p'),(4,'q','r'),(5,'s','t'))

register = defaultdict(list)
for number, *letters in data:
final = []
for i in sorted(register.keys()):
print (final)

NB sorting is "stable" in Python, so sorted() will always keep the 1's, 2's etc in the same order as they were in the unsorted list.


On Sunday, 26 September 2010 14:05:13 UTC+10, Xah Lee  wrote:
> here's a interesting toy list processing problem.
> I have a list of lists, where each sublist is labelled by
> a number. I need to collect together the contents of all sublists
> sharing
> the same label. So if I have the list
> ((0 a b) (1 c d) (2 e f) (3 g h) (1 i j) (2 k l) (4 m n) (2 o p) (4 q
> r) (5 s t))
> where the first element of each sublist is the label, I need to
> produce:
> output:
> ((a b) (c d i j) (e f k l o p) (g h) (m n q r) (s t))
> a Mathematica solution is here:
> R5RS Scheme lisp solution:
> by Sourav Mukherjee
> also, a Common Lisp solution can be found here:
> anyone care to give a solution in Python, Perl, javascript, or other
> lang? am guessing the scheme solution can be much improved... perhaps
> using some lib but that seems to show scheme is pretty weak if the lib
> is non-standard.
>  Xah ∑ ☄

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