
Ublas : 
Subject: Re: [ublas] Is it possible to construct alternative "views" of vectors and matrices?
From: Jesse Manning (manning.jesse_at_[hidden])
Date: 20090917 14:19:37
the ublas matrix and vector classes both have a constructor that takes the
array_data as the third parameter
// pulled from vector.hpp
vector (size_type size, const array_type &data)
// pulled from matrix.hpp
matrix (size_type size1, size_type size2, const array_type &data)
these take the data by const reference and then make a copy of it to their
internal array_type variable data_ so they don't do exactly what you want
theoretically there could be a different constructor that would allow
pointing to the same array_type data, but this could be risky if you were to
manipulate the sizing of the data by adding or removing elements and this
could be why the view adapters that were mentioned by Nasos allowing read
only access.
On Thu, Sep 17, 2009 at 1:11 PM, Mark Johnson <mj1_at_[hidden]> wrote:
> Thanks Jesse for your answer and the code fragment!
>
> Is there a way of building a matrix or vector out of a preexisting
> array_type? I'd like to be able to view a matrix as a vector, and use
> vector arithmetic on it.
>
> What would be great would be a way of building a matrix or vector out of
> any object v that implements (say) v.size() and v[index].
>
> I also think it would be great to have "computed matrices", i.e., an
> adaptor that constructs a matrix interface for an arbitrary function
> object f that implements (say) f.size1(), f.size2() and f(i,j).
>
> I'd be willing to implement such a thing, but I don't know whether the
> returned object should be a matrix_expression, a matrix_reference or
> whatever, and I don't know what things an object has to define in order
> to be a vector or a matrix. If someone can point me to the appropriate
> documentation that would be great!
>
> Best,
>
> Mark
>
>
> On 01/10/28163 02:59 PM, Jesse Manning wrote:
>
>> For dense matrix types the underlying storage of the matrix is just an
>> array of m*n values. You can access this data directly from the matrix
>> and it will provide the "vector view" so to speak. I am not sure about
>> other types of matrices such as sparse or compressed since I have
>> mostly dealt with dense. Here is some example code:
>>
>> ublas::matrix<int> mat1(3,3);
>>
>> // fill matrix with values
>> mat1(0,0) = 1;
>> mat1(0,1) = 3;
>> mat1(0,2) = 2;
>> mat1(1,0) = 1;
>> mat1(1,1) = 0;
>> mat1(1,2) = 0;
>> mat1(2,0) = 1;
>> mat1(2,1) = 2;
>> mat1(2,2) = 2;
>>
>> // get vector view of matrix elements
>> ublas::matrix<int>::array_type& myarray = mat1.data();
>>
>> // print contents
>> std::cout<< "myarray size: "<< myarray.size()<< std::endl;
>> std::cout<< "[";
>> for (size_t i=0; i< myarray.size(); ++i)
>> {
>> std::cout<< myarray[i];
>>
>> if (i != myarray.size()1)
>> {
>> std::cout<< ",";
>> }
>> }
>> std::cout<< "]"<< std::endl;
>>
>> // modify some contents of the matrix using the vector view
>> myarray[0] = 0;
>> myarray[1] = 0;
>> myarray[2] = 0;
>>
>> // print contents to show changes
>> std::cout<< "myarray size: "<< myarray.size()<< std::endl;
>> std::cout<< "[";
>> for (size_t i=0; i< myarray.size(); ++i)
>> {
>> std::cout<< myarray[i];
>>
>> if (i != myarray.size()1)
>> {
>> std::cout<< ",";
>> }
>> }
>> std::cout<< "]"<< std::endl;
>>
>
>
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