Re: [ublas] inner_prod

From: Gunter Winkler (guwi17_at_[hidden])
Date: 2005-08-20 05:49:30

Am Freitag, 19. August 2005 22:00 schrieb Shawn D. Pautz:
> Is there a way to use inner_prod (or something similar) for vectors
> of more complicated types?  I would like to form the inner product of
> a vector<double> and a vector<matrix<double> >, with the following
> semantics:
>
> vector<double> vec;
> vector<matrix<double> > vecOfMatrices;
> matrix<double> result;  // = 0
> // result = inner_product(vec, vecOfMatrices);   // won't compile
> for (int i = 0; i < vec.size(); ++i)
>   result += vec[i]*vecOfMatrices[i];
>
> What's the best way to perform the above operation without resorting
> to writing loops?

Currently this is not directly supported. The inner_prod uses the 
operator * (x, y) function to compute the products which should work 
for scalar times Matrix. However, the return type of the expression can 
not be calculated automatically. You have to create a specialization of 
promote_traits.

  typedef double          SCALAR;
  typedef matrix<SCALAR>  MATRIX;

  template<>
  struct promote_traits< SCALAR, MATRIX > {
    typedef MATRIX base_type;
    typedef MATRIX promote_type;
  };

Unfortunatly this only fixes the return type deduction. The second 
problem is that the return type has to be constructible from (int) 0. 
This can be achieved by using a type derived from MATRIX with an 
appropriate constructor.

class my_matrix : public MATRIX {
  public: my_matrix(const size_t zero) : 
MATRIX( zero_matrix<double>(5,5) ) { }
}

I hope you got the idea.

mfg
Gunter

PS: How far is the work for type_traits<MATRIX>::zero ?

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