[Matlisp-users] Matrix column access

[Jan R. <ja...@ry...>]

Is there a way to efficiently operate on matrix columns and/or rows? I
often need to extract a column (or a row) and do operations on it,
sometimes storing it right back.

I expected to be able to use matrix-ref with a single number for this,
but couldn't find a simple way. Perusing the copious documentation
pointed me to the sequence parameter to matrix ref, so I came up with
this:

(defmacro extract-column (matrix column)
  `(matrix-ref ,matrix (seq 0 1 (1- (first (size ,matrix)))) ,column))

(defmacro extract-row (matrix row)
  `(matrix-ref ,matrix ,row (seq 0 1 (1- (second (size ,matrix))))))

This works (also for (setf (extract-column...))), but is very
inefficient, as it conses a *lot*. This is something I do very, very
often in my code, so the performance impact is huge.

Surely there must be a better way? From what I remember, matlisp stores
matrices in a column-major order, so at least columns could be processed
quickly...

--J.

PS: amusingly enough, it seems that using ATLAS actually slowed down my
program, by about 3%. Rather surprising.