matrix transpose (was Re: mini / maxi functions)

Barton Willis escribi?:
> For a n x n matrix, the running time for transpose is O(n^3). Here is a proposed O(n^2) replacement:
>
> ;; Transpose a list of lists ll and return a list of lists. Example:
> ;; ((1 2) (3 4)) --> ((1 3) (2 4)).
>
> (defun transpose (ll)
>   (let ((acc nil))
>     (while (car ll)
>       (push (mapcar #'car ll) acc)
>       (setq ll (mapcar #'cdr ll)))
>     (nreverse acc)))
>
> To transpose the 800 x 800 matrix (i,j) --> i - j:
>
>   current transpose:  26.21 seconds
>   proposed transpose:  0.80 seconds.
>
> Matrix multiplication also uses transpose.
>
> Comments?
>   

I am in favor of this improvement. Big matrices are common in data 
analysis and hard to handle with the descriptive package. Function 
transpose is called frequently inside package descriptive.

--
Mario