Re: [eigen] Is it really impossible to modify a column of a matrix with in function?

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 30.08.2011 11:58, Helmut Jarausch wrote:
> void set_Col(VectorXi&  V) {
>    V[0]=1;
> }
>
> int main() {
>    MatrixXi A(2,3);
>    A<<  1,2,3,
>         4,5,6;
>    set_Col(A.col(1));
>    std::cout<<  "A=\n"<<  A<<  std::endl;
>
> }
>
>
> It looks as if a column cannot be passed by a non-const reference.
> Why doesn't  the col-method return a non-const reference for a non-
> const object (the col-method could be overloaded with/without the
> const attribute)?

That's because A.col(1) does not return a VectorXi but a view on the 
corresponding column. Unfortunately passing this to another function is 
a bit hackish. To stay generic you need to pass a MatrixBase<Derived>& 
and because A.col() returns a temporary it needs to be a const-ref, 
which you need to const-cast within your method.
Look here for details:
http://eigen.tuxfamily.org/dox/TopicFunctionTakingEigenTypes.html#TopicPlainFunctionsFailing

Christoph


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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
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