I do not know how to formulate it, but I can say what I expected:
Using std::vector I can write the vector-extraction in the following form:
//Using std::vector
std::vector<double> std_vec_1(9);
std::vector<double> std_vec_2(std_vec_1.size()/3);
for(size_t i = 0; i < std_vec_1.size(); ++i)
std_vec_1[i] = i+200;
for(size_t i = 0; i < std_vec_2.size(); ++i)
std_vec_2[i] = 0;
std::cout << "Before: \n";
for(size_t i = 0; i < std_vec_1.size(); ++i)
std::cout << std_vec_1[i] << ' ';
std::cout << '\n';
for(size_t i = 0; i < std_vec_2.size(); ++i)
std::cout << std_vec_2[i] << ' ';
std::cout << '\n';
std::cout << "After: \n";
std_vec_2 = std::vector<double>(std_vec_1.begin()+std_vec_2.size(),
std_vec_1.begin()+std_vec_2.size()*2);
for(size_t i = 0; i < std_vec_1.size(); ++i)
std::cout << std_vec_1[i] << ' ';
std::cout << '\n';
for(size_t i = 0; i < std_vec_2.size(); ++i)
std::cout << std_vec_2[i] << ' ';
std::cout << '\n';
i.e. begin() always points at the beginning of the vector, regardless of
content. But with dealii::Vector I have to write it like
//Using dealII
Vector<double> vec_1 = Vector<double>(9);
Vector<double> vec_2 = Vector<double>(vec_1.size()/3);
for(size_t i = 0; i < vec_1.size(); ++i)
vec_1(i) = i+200;
vec_2 = 0.;
std::vector< int > indices(vec_2.size());
for(size_t i = 0; i < vec_2.size(); ++i)
indices[i] = i+vec_2.size();
const std::vector<int> const_indices(indices);
std::cout << "Before: \n";
std::cout << vec_1 << '\n';
std::cout << vec_2 << '\n';
std::cout << "After: \n";
vec_1.extract_subvector_to(const_indices.begin(), const_indices.end(),
vec_2.begin());
std::cout << vec_1 << '\n';
std::cout << vec_2 << '\n';
i.e. the begin()-iterator of the dealii::Vector does not always point at
the beginning of the Vector. That's what I found confusing.
I hope that helps already!
Am Donnerstag, 20. Juli 2017 16:25:37 UTC+2 schrieb Wolfgang Bangerth:
>
> On 07/20/2017 05:42 AM, Jean-Paul Pelteret wrote:
> > (I would have expected something else)
> >
> >
> > We're always happy to receive contributions
> > <https://github.com/dealii/dealii/compare> that improve the
> documentation! :-)
>
> Indeed, seriously! The documentation for that function is in the file
> include/deal.II/lac/vector.h. It would be great if you could use how you
> misunderstood what it says to improve the documentation!
>
> Best
> Wolfgang
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth email: [email protected]
> <javascript:>
> www: http://www.math.colostate.edu/~bangerth/
>
>
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