Re: questions on ublas ?

From: jhrwalter (walter_at_[hidden])
Date: 2001-12-12 17:07:55

--- In boost_at_y..., Peter Schmitteckert (boost) <boost_at_s...> wrote:
> Salut,
> 
> On Wednesday 12 December 2001 11:20, you wrote:
> 
> > >
> > > Then I don't understand, why capacity1()/capacity2() is non-
generic,
> > > since I'm not allowed to make use of capacity() in a sense, 
that I
> > > reshape the matrix, so capacity() is useless and we have
> > > capacity1()/capacity2() only.
> >
> > Let's take examples:
> >
> > Case 1: the storage for the first matrix type is an 1-D array, 
i.e. m
> > (i, j) = a [i * size + j]. Then client code could perform 
something
> > like the following operation with capacity():
> >
> > if (new_size1 * new_size2 <= m.capacity ())
> >     m.resize (new_size1, new_size2);
> > else
> >     std::cout << "oops: reallocation necessary" << std::endl;
> >
> > Case 2: the storage for the next matrix type is a 2-D array, i.e. 
m
> > (i, j) = a [i] [j]. Then client code could perform the following
> > operation with capacity1()/capacity2():
> >
> > if (new_size1 <=  m.capacity1 () && new_size2 <= m.capacity2 ()) {
> >     m.resize (new_size1, new_size2);
> > else
> >     std::cout << "oops: reallocation necessary" << std::endl;
> >
> 
> 
> But, since I'm not allowed to reshape the Matrix, case 1 ist an
> insufficient test. Sure, it assures that one doesn't get a core 
dump,
> but what happens if new_size1() is larger than capacity1(), still
> preserving that
> new_size1 * new_size2 < capacity() = capacity1() * capacity2()  ?
> The code is still correct in the sense that it won't access 
> not allocated regions, but the matrix is reshaped, i.e. it's not
> a sub matrix anymore. 
> 
> So, case 1 should look like
> 
> 
> if (new_size1 <=  m.capacity1 () && new_size2 <= m.capacity2 ())
>     m.resize (new_size1, new_size2);
> else if (new_size1 * new_size2 <= m.capacity ()) 
> {
>     std::cout << "Warning: Matrix is reshaped" << std::endl;
>     m.resize (new_size1, new_size2);
 
Isn't it necessary to reallocate here (internally), if the matrix m 
is (internally) organized as a 2-D container like 
vector<vector<double> >  and new_size1 > m.size1(), for example?

> } else
>     std::cout << "oops: reallocation necessary" << std::endl;
> 
> Best wishes,
> Peter
 
Regards
 
Joerg