
Re: Multiplying each colum of my matrix by a constant
Posted:
Jun 12, 2013 10:22 AM


dpb <none@non.net> wrote in message <kp9vn3$v42$1@speranza.aioe.org>... > On 6/11/2013 6:49 PM, dpb wrote: > > On 6/11/2013 6:00 PM, Wanderson wrote: > >> dpb <none@non.net> wrote in message <kp89gu$ooe$1@speranza.aioe.org>... > > ... > > > >>> If you need speed, the straightahead loop is probably as good as > >>> you're going to do w/o a specialized mex function. (I believe there > >>> may be some submissions on File Exchange that do, in fact, promise > >>> better performance than bsxfun iirc) > >>> > >>>  > >> > >> hehe, my problem is that I am doing a program that evaluates some double > >> integrals, calculate FFT and whanever, but I have a NECESSARY big 'FOR > >> LOOP' inside my program to store each value of these integrals in the > >> right place. And I didn't want to use another loop inside my huge loop > >> rsrsrs. > > > > For 3x3 you can always try the explicit repmat() route, too for > > comparative timings... > > > > m.*repmat(v,size(m,1),1) > > > > And, of course, if you're lucky enough that the v is a constant you > > could obviously save it since memory isn't going to be an issue. > > > > bsxfun() is essentially syntactic candy to save explicitly writing such > > as the above but is basically all it is. Of course, it has the overhead > > of processing inputs and all that. I've not tested but I'd suspect the > > time differential would drop pretty quickly w/ size of the arrays as > > compared to other implementation choices. > > > > BTW... > > If the object is speed in an inner loop, then for the 3x3 case the > original version of writing out the three explicit multiplies may well > be the most efficacious route. For higher dimensions it gets klunky > certainly but for as few as three it's not so bad. > > 
Thanks again dpb, In fact, my matrices dimensions vary inside my program rsrs. I start with an 3x3 matrix and they will growing as (2k+1) x (2k+1) dimension. I'll try the repmat idea, this function seems to be very usefull to many things, I should learn more about it.

