Re: a\b operation slow

[David Bateman]

Hartmut Behrens a écrit :

> hello all,
>  
> I have two matrices, a = (20x1) and b = (1x20) for which i would like 
> to calculate M = a\b. The problem is that this operation takes an 
> extremely long time (greater than an 1hour) on a Pentium4 (Celeron 
> 2.6GHz) PC. Do you have any suggestions on how to speed up this 
> calculation in octave?
>  
> Kind Regards,
> hartmut

I think there is a problem here. For "a\b" the number of rows in "a" 
must equal the number of rows in "b". So I suspect that either "a" or 
"b" must be transposed. In both cases I get reasonable solve times. Consider

octave:1> a = randn(20,1); b = randn(1,20);
octave:2> tic; a\b; toc
error: operator \: nonconformant arguments (op1 is 20x1, op2 is 1x20)
error: evaluating binary operator `\' near line 2, column 6
octave:3> tic; a'\b; toc
ans = 0.031426
octave:4> tic; a\b'; toc
ans = 0.0033250
octave:5> OCTAVE_VERSION
CTAVE_VERSION = 2.1.64

What version of octave are you using? If you really can do a\b with the 
version of octave you are using, its a certain segmentation violation 
and then all bets are off as to how lapack/atlas will react to that, and 
one hour of computation might be possible...

D.



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