|
|
Blitz Support : |
From: Othmar Frey (ofrey_at_[hidden])
Date: 2004-08-03 09:18:58
Hi Julian,
many thanks for your detailed answer. I still have a couple of
questions, though.
(I have been away from mail for 2 weeks,
that's why I bring up these question but now...)
But first of all:
>Let me experiment with this a bit and see how much work would be needed to
>make such extensions.
I am very glad to see that you are interested in extending the array
indirection
functionality.
I would greatly appreciate such an extension and hope other users would,
too.
Questions:
1.
>In the meantime, I'm afraid you will need to assign
>the selected elements from your 1d dataVector object into a temporary 2d
>dataMatrix object first, and then use the temporary dataMatrix object in
>your array expression.
Is there another, more efficient way of assigning the selected elements
than assigning them elementwise in for-loops?
2.
If assigning by for-loops is the only way, is it still worth,
with respect to performance, to 1. assign all the elements and 2. calculate
the array expression instead of replacing the whole thing by
an elementwise approach like the following?
for (i=0; i<sizeX; i++)
{
for (j=0; j<sizeY; j++)
{
// calculate index for element i,j
index = ...;
dataMatrix2(i,j) = dataMatrix1(i,j) + dataVector(index);
}
}
3.
When I use VectorPick, which options do I have to assign the data to a
Vector or an Array object to
ease further calculations?
Thank you.
Regards,
Othmar
-- ----------------------------------------------------------------------- Othmar Frey Remote Sensing Laboratories RSL ofrey_at_[hidden] University of Zurich http://www.geo.unizh.ch/~ofrey Winterthurerstrasse 190 phone +41 44 635 51 64 CH-8057 Zurich fax +41 44 635 68 46 Switzerland ----------------------------------------------------------------------- Julian C. Cummings wrote: >Hello Othmar, > >I think what you are looking for is support for array indirection, where you >reference elements of an Array using indices stored in another Array. There >is some support for this built into blitz already, but it is currently >limited to using the indirect array view as an lvalue (on the left-hand side >of an assignment). This would need to be extended so that the IndirectArray >class (see <blitz/array/indirect.h>) was made to derive from ArrayExpr and >could be used as part of an array expression. There are also some code >comments in the <blitz/array-impl.h> header about possibly extending the >operator() methods used for array slicing to support the use of Vector<int> >indexing objects to produce an ArrayPick type analogous to VectorPick. >Presumably there would be one >Vector<int> argument for each Array dimension and each Vector would have the >same length. >Instead of using a set of Vector<int> arguments, one ought to be able to use >an Array of ints as well. > >Let me experiment with this a bit and see how much work would be needed to >make such extensions. In the meantime, I'm afraid you will need to assign >the selected elements from your 1d dataVector object into a temporary 2d >dataMatrix object first, and then use the temporary dataMatrix object in >your array expression. Or if you wish to get your hands a little dirty, you >could write a loop using iterators over the dataMatrix objects and the >arrayOfIndices object and handle the indirection yourself: > >Array<float,2>::iterator pdm1 = dataMatrix1.begin(), pdm2 = >dataMatrix2.begin(), > pend = dataMatrix2.end(); >Array<int,2>::iterator pind = arrayOfIndices.begin(); >for (; pdm2 != pend; ++pdm1, ++pdm2, ++pind) > *pdm2 = *pdm1 + dataVector(*pind); > > >Regards, Julian C. > >Dr. Julian C. Cummings >Staff Scientist, CACR/Caltech >(626) 395-2543 >cummings_at_[hidden] > > > > >>-----Original Message----- >>From: blitz-support-bounces_at_[hidden] >>[mailto:blitz-support-bounces_at_[hidden]] On Behalf Of Othmar Frey >>Sent: Friday, July 16, 2004 8:06 AM >>To: blitz-support_at_[hidden] >>Subject: [Blitz-support] array of indices applied to a vector >>(or 1-dimarray) >> >> >>Hi. >> >>I am rather new to blitz++. >> >>I would like to evaluate a one dimensional array >>at indices that are given in a - let's say - 2-dimensional >>array of indices. >> >>What I would love to have working is something like: >>(something I am really fond of that it works in Matlab) >>------------------------------------------------------- >> ... >> >> Array<float,1> dataVector(100); >> Array<float,2> dataMatrix1(20,20); >> Array<float,2> dataMatrix2(20,20); >> Array<int,2> arrayOfIndices(20,20); >> >> ... >> // fill dataVector and dataMatrix1 with some data >> // and set the indices in arrayOfIndices according to my needs >> ... >> >> dataMatrix2 = dataMatrix1 + dataVector(arrayOfIndices); >> >> ... >>--------------------------------------------------------- >> >> I've learned from the example pick.cpp at >> >> http://www.oonumerics.org/blitz/examples/blitz/pick.cpp >> >> that there's a possibility to pick >> values from a vector according to a given index vector by using >> VectorPick. >> But then, all the nice elementwise operators don't work >>as the do for >> arrays. >> >> Is there straightforward solution to this problem, which I missed? >> (Maybe one that is even mentioned in the documentation?) >> >> Many thanks for your help. >> >> Cheers >> >> Othmar >> >>-- >>-------------------------------------------------------------- >>--------- >>Othmar Frey Remote Sensing >>Laboratories RSL >>ofrey_at_[hidden] University of Zurich >>http://www.geo.unizh.ch/~ofrey Winterthurerstrasse 190 >>phone +41 44 635 51 64 CH-8057 Zurich >>fax +41 44 635 68 46 Switzerland >>-------------------------------------------------------------- >>--------- >> >> >>_______________________________________________ >>Blitz-support mailing list >>Blitz-support_at_[hidden] >>http://www.oonumerics.org/mailman/listinfo.cgi/blitz-support >> >> >> > > >_______________________________________________ >Blitz-support mailing list >Blitz-support_at_[hidden] >http://www.oonumerics.org/mailman/listinfo.cgi/blitz-support > >