Re: [boost] Re: slicing and ranges of matrices

From: Kresimir Fresl (fresl_at_[hidden])
Date: 2002-01-20 04:57:06

jhrwalter wrote:

> Toon Knapen <toon.knapen_at_s...> wrote:

>> But another concept I'm missing, and which is in neither of the 
>> three libraries, is creating a sub-array (of the same order as the 

>> array) that contains a specific selection of indices for every 

>> dimension of the array. e.g. in the 2D case I would like to do

>>   matrix< double > A(8,8);
>>   boost::array<3> i1 = { 0,1,6 };
>>   boost::array<5> i2 = { 5,6,7 };
>>   sub_matrix sub_A = submatrix(A, i1,i2 );

>> where sub_A contains all element that are in columns 5,6 and 7 and 
>> in rows 0,1 and 6. The difference with a range is thus that this 
>> subarray is not a continuous block in the original array. The diff with the 
>> current definition of slicing is that the selection does not need to be 
>> strided.

In 1D case this is very similar to `indirect_array' view of `std::valarray'.  

>> Is there a reason this kind of subarray does not exist ? 

> Interesting question. Should this sub matrix support dense matrices 
> only or sparse matrices also (the old question of genericity ;-)?

Both. FEM matrices, for example, are usually sparse. 

>> Currently, my only loop is for assembling my FEM matrix from all 
>> local element matrices which comes down basically to assigning 
>> a subarray with non-strided indices.

fres