Thread Subject: Elementary Row Operations !

Hi -

If I wanted to loop through column k in matrix A, and swap
rows with zeros with rows without zeros, is there a faster
way to achieve this than using multiple loops?

Example: If A=[ 20 23 5 8
                92 0 16 14
                60 0 35 88
                44 25 83 19
                44 76 74 20]

Then if I want to operate on column 2 (i.e k=2),the new A
will should look like:
             A=[ 20 23 5 8
                 44 25 83 19
                 44 76 74 20
                 92 0 16 14
                 60 0 35 88]

That is, the new A is the same except rows 2 and 3 of
column 2 were interchanged with rows 4 and 5 of column 2
because the former contained zeros.

Thanks
Alan

"alan dinno" <alan_dinno@hotmail.com> wrote in message <fl3ttt$nce
$1@fred.mathworks.com>...
> Hi -
>
> If I wanted to loop through column k in matrix A, and swap
> rows with zeros with rows without zeros, is there a faster
> way to achieve this than using multiple loops?
>
> Example: If A=[ 20 23 5 8
> 92 0 16 14
> 60 0 35 88
> 44 25 83 19
> 44 76 74 20]
>
> Then if I want to operate on column 2 (i.e k=2),the new A
> will should look like:
> A=[ 20 23 5 8
> 44 25 83 19
> 44 76 74 20
> 92 0 16 14
> 60 0 35 88]
>
> That is, the new A is the same except rows 2 and 3 of
> column 2 were interchanged with rows 4 and 5 of column 2
> because the former contained zeros.
>
> Thanks
> Alan
------
Try this:

  [ignore,p] = sort(A(:,k)==0);
 A = A(p,:);

Roger Stafford

"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in
message <fl3us9$ant$1@fred.mathworks.com>...
> "alan dinno" <alan_dinno@hotmail.com> wrote in message <fl3ttt$nce
> $1@fred.mathworks.com>...
> > Hi -
> >
> > If I wanted to loop through column k in matrix A, and swap
> > rows with zeros with rows without zeros, is there a faster
> > way to achieve this than using multiple loops?
> >
> > Example: If A=[ 20 23 5 8
> > 92 0 16 14
> > 60 0 35 88
> > 44 25 83 19
> > 44 76 74 20]
> >
> > Then if I want to operate on column 2 (i.e k=2),the new A
> > will should look like:
> > A=[ 20 23 5 8
> > 44 25 83 19
> > 44 76 74 20
> > 92 0 16 14
> > 60 0 35 88]
> >
> > That is, the new A is the same except rows 2 and 3 of
> > column 2 were interchanged with rows 4 and 5 of column 2
> > because the former contained zeros.
> >
> > Thanks
> > Alan
> ------
> Try this:
>
> [ignore,p] = sort(A(:,k)==0);
> A = A(p,:);
>
> Roger Stafford
-------
Or you could avoid the sort with:

 A = A([find(A(:,k)~=0);find(A(:,k)==0)],:);

Roger Stafford

"Roger Stafford"
<ellieandrogerxyzzy@mindspring.com.invalid> wrote in
message <fl4039$de8$1@fred.mathworks.com>...
> "Roger Stafford"
<ellieandrogerxyzzy@mindspring.com.invalid> wrote in
> message <fl3us9$ant$1@fred.mathworks.com>...
> > "alan dinno" <alan_dinno@hotmail.com> wrote in message
<fl3ttt$nce
> > $1@fred.mathworks.com>...
> > > Hi -
> > >
> > > If I wanted to loop through column k in matrix A,
and swap
> > > rows with zeros with rows without zeros, is there a
faster
> > > way to achieve this than using multiple loops?
> > >
> > > Example: If A=[ 20 23 5 8
> > > 92 0 16 14
> > > 60 0 35 88
> > > 44 25 83 19
> > > 44 76 74 20]
> > >
> > > Then if I want to operate on column 2 (i.e k=2),the
new A
> > > will should look like:
> > > A=[ 20 23 5 8
> > > 44 25 83 19
> > > 44 76 74 20
> > > 92 0 16 14
> > > 60 0 35 88]
> > >
> > > That is, the new A is the same except rows 2 and 3
of
> > > column 2 were interchanged with rows 4 and 5 of
column 2
> > > because the former contained zeros.
> > >
> > > Thanks
> > > Alan
> > ------
> > Try this:
> >
> > [ignore,p] = sort(A(:,k)==0);
> > A = A(p,:);
> >
> > Roger Stafford
> -------
> Or you could avoid the sort with:
>
> A = A([find(A(:,k)~=0);find(A(:,k)==0)],:);
>
> Roger Stafford
>

This is excellent, Roger!
I like the one without the sort() function...it is more
efficient for me.

Thanks.
Alan