reduced-tagged (was Re: toy list processing problem: collect similar terms)

Mirko mirko.vukovic at
Mon Sep 27 17:18:22 CEST 2010

On Sep 26, 12:05 am, Xah Lee <xah... at> wrote:

I am hijacking the following post and driving it to Cuba (the Monthy
Python fans will know what I refer to).  I want to create a `reduce'-
like function that can handle similar problems.

Xah said:
> 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))
> stuffed deleted.

Here is my Common Lisp (and I only care about Common Lisp answers)
attempt to create a `reduce'-like function to handle this kind of a
problem (you will see that I am still struggling with the code and the

(defun reduce-tagged (function sequence &key
		 (key-tag #'first)
		 (key-datum #'rest))
  "Use a binary operation, `function' to combine a sequence of tagged
elements.  like-tagged elements are `reduce'd according to `function'
and returned in a list ...

`sequence' is a sequence of tagged elements.  reduce-m will reduce

If `key-tag' is supplied it is used to extract the element tag.  If
`key-tag' is not supplied, the function `first' is used.

If `key-datum' is supplied, it is used to extract the element datum.
If `key-datum' is not supplied, the function `rest' is used.

  (let ((hash (make-hash-table)))
    (dolist (datum sequence)
      (let ((tag (funcall key-tag datum))
	    (values (funcall key-datum datum)))
	(multiple-value-bind (it present)
	    (gethash tag hash)
	  (if present
	      (setf (gethash tag hash)
		    (apply function (gethash tag hash) values))
	      (setf (gethash tag hash) values)))))
    (let (result)
      (dohash (key value hash)
	(push (list key value) result))

Comments, improvements?  I am looking for a general purpose function
like reduce that I
can apply in various situations.



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