toy list processing problem: collect similar terms

Pascal J. Bourguignon pjb at
Sun Sep 26 16:24:46 CEST 2010

Xah Lee <xahlee at> writes:

> 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:

It's too complex. Just write:

(let ((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))))

  (mapcar (lambda (class) (reduce (function append) class :key (function rest)))
           (com.informatimago.common-lisp.list:equivalence-classes list :key (function first)))


--> ((S T) (Q R M N) (G H) (O P K L E F) (I J C D) (A B))

__Pascal Bourguignon__           

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