Re: Collecting like-labelled sublists of a list

Cortez <relativeflux@xxxxxxxxxxxxx> writes:

I need to traverse a list of lists, where each sublist is labelled by
a number, and 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 -

((a b) (c d i j) (e f k l o p) (g h) (m n q r) (s t))

I do this with the following -

(defun test (list)
(loop for j in list
for index = (first j)
for k = (rest j)
with indices = nil
if (not (member index indices))
do (pushnew index indices)
and collect k into res
else
do (nconc (nth index res) k)
finally (return res)))

I suspect that there is a more efficient and elegant way of doing
this, however. Any suggestions welcome.

Brief background: this is part of a program I've written for reading
data from SDIF files, a binary format which stores sound description
data. The labelled lists represent partials in spectral analysis data
(partial-index, time, frequency).

I'm a junior lisper, so I tried a different approach mostly for my own
education.


(let ((mylist '((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) (5 s t)))
(ar (make-array 10 :adjustable t)))
(mapcar (lambda (l)
(setf (aref ar (first l))
(append (aref ar (first l)) (rest l)))) mylist))


My choice of 10 for the initial size of the array was arbitrary. I
chose the non-destructive append over nconc, but nconc might be ok.


--
-- Steve
.

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