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).
solution and explanation in Mathematica and Scheme lisp, Qi lisp, at http://xahlee.org/UnixResource_dir/writ/notations_mma.html