Re: another quick question

Thanks Rich,

Your code was very helpful, though MSVC6 has some issues with it.

More exactly, it considers
Matrix::OneD A0 = A[i];
To be an assignment between incompatible types.

Using Matrix::OneDRef instead of Matrix::OneD gets rid of the compiler 
error, but simply substituting OneDRef's into your code leads to the same 
old crash.

I've failed to find a simple modification of your swap_row function which 
will work under MSVC6, but I have figured out how to do the swap "by hand"; 
which ought to be good enough given my limited purposes.

-Sven

struct dn {
         double d;
         int n;
         dn(double d, int n) : d(d), n(n) {}
};

//much slower than it needs to be, but still potentially faster than using 
dense matrices...
void swap_row(Matrix& A, int i, int j) {

         std::vector<dn> A0;
         std::vector<dn> A1;

         Matrix::OneDRef::iterator r;
         for(r=A[i].begin();r!=A[i].end();r++)
                 A0.push_back(dn(*r,r.index()));
         for(r=A[j].begin();r!=A[j].end();r++)
                 A1.push_back(dn(*r,r.index()));

         A[i].clear();
         A[j].clear();

         std::vector<dn>::iterator s;
         for(s=A0.begin();s!=A0.end();s++)
                 A[j].push_back(s->n,s->d);
         for(s=A1.begin();s!=A1.end();s++)
                 A[i].push_back(s->n,s->d);
}

At 10:27 PM 12/2/2003 -0800, liequan wrote:
>Hi Sven,
>
>The following code compiles and runs correctly wiht gcc 3.3 for swapping
>two rows in array compressed1D matrix. It is not constant time though.
>
>#include "mtl/matrix.h"
>#include "mtl/mtl.h"
>#include "mtl/utils.h"
>#include <algorithm>
>
>using namespace mtl;
>
>typedef matrix< double, rectangle<>, array< compressed<> >,
>row_major >::type Matrix;
>
>void
>swap_row(Matrix& A, int i, int j)
>{
>   Matrix::OneD A0 = A[i];
>   Matrix::OneD A1 = A[j];
>   mtl::swap(A0, A1);
>   typedef Matrix::OneD::size_type I;
>   std::vector<I>& A0n = const_cast<std::vector<I>&>(A0.nz_struct());
>   std::vector<I>& A1n = const_cast<std::vector<I>&>(A1.nz_struct());
>   std::swap(A0n, A1n);
>}
>
>int main ()
>{
>    //initialize the matrix
>    const int size=4;
>    Matrix A(size,size);
>    A(0,0)=1;
>    A(1,2)=2;
>    A(3,1)=5;
>    A(0,3)=3;
>
>    swap_row(A, 0, 1);
>
>    print_all_matrix(A);
>    return 0;
>}
>
>Andy, I am sorry for misleading information -- I was wrong about
>swapping. In fact, only dense matrices/dense vectors are taken care by
>mtl::swap in mtl2.
>
>--Rich Lee
>
>
>On Tue, 2003-12-02 at 20:38, Sven Olsen wrote:
> > At 07:49 PM 12/2/2003 -0500, you wrote:
> > >I think this example code should swap the two rows in constant time.
> >
> > Hmm.  Well, the example code I gave crashes in terrible ways...  I assumed
> > it was breaking because I wasn't using the library properly, but if you
> > think it should work, then something is going wrong...
> >
> > I've done a little investigation, and it looks like the problems that I've
> > been running into with row swapping are most likely due to the fact that
> > the mtl::swap functions will swap the value vectors of two compressed
> > arrays, but not the index vectors (which leads to all sorts of interesting
> > problems).
> > I have started hacking around with a solution to use in my own code, but I
> > don't know my way around the MTL source, so even if I get something
> > working, it's probably not going to be pretty.
> >
> >  From what I've seen, this is something that anyone who knew the MTL 
> source
> > fairly well would have little trouble fixing.
> >
> > Here's the code that crashes in terrible ways:
> >
> > #include "mtl/matrix.h"
> > #include "mtl/mtl.h"
> > #include "mtl/utils.h"
> >
> > using namespace mtl;
> >
> > typedef matrix< double, rectangle<>, array< compressed<> >,
> > row_major >::type Matrix;
> >
> > int main ()
> > {
> >    using std::cout;
> >    using std::endl;
> >
> >    //initialize the matrix
> >    const int size=4;
> >    Matrix A(size,size);
> >    A(0,0)=1;
> >    A(1,2)=2;
> >    A(3,1)=5;
> >    A(0,3)=3;
> >
> >    //swap the first two rows
> >    mtl::swap(A[0],A[1]);
> >
> >    //output the results
> >    for (int ii=0; ii<A.nrows(); ii++) {
> >      for (int j=0; j<A.ncols(); j++) {
> >        cout.width(6);
> >        cout << A(ii, j) << " ";
> >      }
> >          cout << endl;
> >    }
> >
> >    return 0;
> > }
> >
> > The output that I get is:
> >
> > 2       0       0       2.10535e-314
> > 0       0       1       0
> > 0       0       0       0
> > 0       5       0       0
> >
> > I then crash while trying to execute the cleanup function "_free_dbg".
> >
> > A little more experimenting indicates that while I am swapping the value
> > vectors, I'm not swapping the index vectors-
> > So the matrix defined by
> >    Matrix A(2,2);
> >    A(0,0)=1;
> >    A(1,2)=2;
> > Swaps rows without trouble (which makes sense because the index vectors 
> are
> > the same for each row)
> >
> > While swapping the rows in
> >    Matrix A(2,4);
> >    A(0,0)=1;
> >    A(1,2)=2;
> >    A(1,1)=6;
> >    A(0,3)=3;
> >
> > Yeilds
> > 6       0       0       2
> > 0       1       3       0
> >
> > when it clearly should give
> > 0       6       2       0
> > 1       0       0       3
> >
> > Thanks for your help,
> >          Sven
> >
> >
> > >On Dec 2, 2003, at 5:31 PM, Sven Olsen wrote:
> > >
> > >>If I am working with a matrix of type:
> > >>
> > >>typedef matrix< double, rectangle<>, array< compressed<> >,
> > >>row_major >::type Matrix;
> > >>
> > >>what is the easiest way to swap two of the rows?
> > >>
> > >>I.E., I want to do something like
> > >>Matrix A(4,4);
> > >>/*initialize A*/
> > >>swap(A[0],A[1]);
> > >>
> > >>Thanks,
> > >>         Sven
> > >>
> > >>_______________________________________________
> > >>This list is archived at http://www.osl.iu.edu/MailArchives/mtl-devel/
> > >
> > >_______________________________________________
> > >This list is archived at http://www.osl.iu.edu/MailArchives/mtl-devel/
> >
> > _______________________________________________
> > This list is archived at http://www.osl.iu.edu/MailArchives/mtl-devel/

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