Thread Subject:

how to get matrix elements...?

Hi

Suppose I have a matrix a = [ 2 3 4; 5 6 7]
and a vector b =[ 2 1 2]

What is the fastest way to obtain elements of the matrix a, where b is the the a's row, i. e.
a(b(1), 1) = 5
a(b(2), 2) = 3
a(b(3), 3) = 7

Thanks!

"stani S." <stanci10@hotmail.com> wrote in message <i9ktr4$dl4$1@fred.mathworks.com>...
> Hi
>
> Suppose I have a matrix a = [ 2 3 4; 5 6 7]
> and a vector b =[ 2 1 2]
>
> What is the fastest way to obtain elements of the matrix a, where b is the the a's row, i. e.
> a(b(1), 1) = 5
> a(b(2), 2) = 3
> a(b(3), 3) = 7
>
> Thanks!

if you want a row vector:
a(sub2ind(size(a),b,1:3))

if you want a column vector, transpose that. (i.e. add .' to the end of the line)

"stani S." <stanci10@hotmail.com> wrote in message <i9ktr4$dl4$1@fred.mathworks.com>...
> Hi
>
> Suppose I have a matrix a = [ 2 3 4; 5 6 7]
> and a vector b =[ 2 1 2]
>
> What is the fastest way to obtain elements of the matrix a, where b is the the a's row, i. e.
> a(b(1), 1) = 5
> a(b(2), 2) = 3
> a(b(3), 3) = 7
>
> Thanks!

if you want a row vector:
a(sub2ind(size(a),b,1:3))

if you want a column vector, transpose that. (i.e. add .' to the end of the line)

stani S. wrote:
> Hi
> Suppose I have a matrix a = [ 2 3 4; 5 6 7]
> and a vector b =[ 2 1 2]
>
> What is the fastest way to obtain elements of the matrix a, where b is
> the the a's row, i. e.
> a(b(1), 1) = 5
> a(b(2), 2) = 3
> a(b(3), 3) = 7

That's about as good as any; otoh, could do sotoo

 >> a(sub2ind(a,b,1:3))
ans =
      5 3 7
 >>

altho not sure it's any "better"...

--

Great! Thanks!
dpb <none@non.net> wrote in message <i9kum4$107$1@news.eternal-september.org>...
> stani S. wrote:
> > Hi
> > Suppose I have a matrix a = [ 2 3 4; 5 6 7]
> > and a vector b =[ 2 1 2]
> >
> > What is the fastest way to obtain elements of the matrix a, where b is
> > the the a's row, i. e.
> > a(b(1), 1) = 5
> > a(b(2), 2) = 3
> > a(b(3), 3) = 7
>
> That's about as good as any; otoh, could do sotoo
>
> >> a(sub2ind(a,b,1:3))
> ans =
> 5 3 7
> >>
>
> altho not sure it's any "better"...
>
> --