Fwd: problem with solve-sv

Yann:
This looks fishy...
- L.

----------  Forwarded Message  ----------

Subject: problem with solve-sv
Date: Thursday 25 August 2005 07:58 am
From: "Thomas Knudsen" <[email protected]>
To: "Lush Programming" <[email protected]>


I have some problems getting solve-sv in lush version 1.1
to run properly. I am, however, a Lisp/lush newbie
(at time of writing having written a total of approximately
30 lines of lush code), so due to cluelessness, I may do
something obvious wrong. This is my code for testing the
least squares solver:

.......................................................................
svdetc.lsh
.......................................................................

(libload "libnum/linalgebra")

; generate toy problem
(setq A [[1 3 2][4 5 6][7 8 9][10 12 11]])
(setq x [1 2 3])
(setq b (idx-m2dotm1 A x ))

; check dimensions of A and b
(printf "A: %d %d (%d)\n" (idx-dim A 0) (idx-dim A 1) (idx-ndim A))
(printf "b:") (idx-bloop ((ii b)) (printf "%f " (ii))) (printf "\n")


(solve-sv A b)
(printf "b:") (idx-bloop ((ii b)) (printf "%f " (ii))) (printf "\n")

.......................................................................
This is the response from lush:
.......................................................................

? (load "svdetc.lsh")
A: 4 3 (2)
b:13 32 50 67


*** lisp_c runtime error: incompatible vector/matrix sizes for SVD
solver
** in: C_solve_sv

*** solve-sv : Run-time error in compiled code
Debug toplevel [y/N] ?
.......................................................................

(apropos "solve-sv") replies:
------------------------------------------------------------------------
(solve-sv <a> <b>)
[DH](packages/libnum/linalgebra.lsh)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
[adapted from the GSL manual]: This function solves the linear system
A*x=b using singular value decomposition. the result is put in b on
output. Matrix a is also destroyed in the process. Only non-zero
singular values are used in computing the solution. The parts of the
solution corresponding to singular values of zero are ignored. In the
over-determined case where A has more rows than columns the system is
solved in the least squares sense , returning the solution x which
minimizes the square norm of (A*x-b).
.......................................................................


Since A has 4 rows, b has 4 elements, this seems to be OK

Doing (solve-sv A (transpose b)) instead, results in:
*** solve-sv : (lisp_c) INDEX has wrong number of dimensions :
::INDEX2:<1x4>

Doing a rather pointless thing:  (solve-sv A x)  actually satisfies
the lush level bounds checking, but results in gsl aborting the
process, i.e. bombing lush, returning me to the shell prompt:

.......................................................................
gsl: svd.c:371: ERROR: first dimension of matrix U must size of vector
b
Default GSL error handler invoked.
Aborted
.......................................................................

To me, this indicates

(1) a bug in the lush dimension check and
(2) that lush should register its own error handler
    with the gsl error handling code

But (as already mentioned) I am a lisp/lush newbie, I may be very
wrong, so ANY comments/suggestions are much appreciated!

Thomas


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