Re: [Matlisp-users] Matrix column access

[Nicolas N. <Nic...@iw...>]

Jan Rychter <ja...@ry...> writes:

> 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.

Probably, the easiest way for you to go is to call the BLAS routine DCOPY
(or similar) with suitable parameters:

MATLISP(9): (apropos "dcopy")
BLAS::FORTRAN-DCOPY [function] (N DX INCX DY ...)
DCOPY               [function] (N DX INCX DY ...)


If you want generic functions working on every matlisp matrix, you will
probably have to write a suitable set of functions yourself.  In my app
Femlisp (which contains a part written in CL giving basic matlisp
functionality) I have written (generic) functions

  minject ((x standard-matrix) (y standard-matrix) row-off col-off)

injecting x into y, and

  mextract ((x standard-matrix) (y standard-matrix) row-off col-off)

extracting y from x.

Yours, Nicolas.

P.S: Timings:

APPLICATION> (time (let ((A (eye 1000)) (x (zeros 1000 1)))
	       (loop repeat 1000 do (mextract A x 0 5))))
; cpu time (non-gc) 50 msec user, 0 msec system
; cpu time (gc)     10 msec user, 0 msec system
; cpu time (total)  60 msec user, 0 msec system
; real time  54 msec
; space allocation:
;  18,218 cons cells, 8,026,592 other bytes, 0 static bytes
NIL

compared with:

MATLISP(11): (time (let ((a (eye 1000)))
(loop repeat 1000 do (extract-column a 5))))
; cpu time (non-gc) 1,580 msec user, 0 msec system
; cpu time (gc)     90 msec user, 0 msec system
; cpu time (total)  1,670 msec user, 0 msec system
; real time  1,675 msec
; space allocation:
;  1,292,128 cons cells, 41,409,488 other bytes, 0 static bytes
NIL