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## Re: Dot product (nx1).*(nxm)

 From: Joao Cardoso Subject: Re: Dot product (nx1).*(nxm) Date: Mon, 06 Dec 1999 14:50:25 +0000

```Mike Miller wrote:
...

> octave:4> t0=cputime; for i=1:1000, p2=p*q; end , cputime-t0

> octave:5> t0=cputime; for i=1:1000, p2=p(:,q); end , cputime-t0

...

> > As I never really understood the line 5 syntax, I always use the line
> > 4 method -- I understand it.
>
> Joao--
>
> I think it's pretty simple to understand.

...

> You can repeat rows or columns if you like:
>
> A(:,[2 2 2 4])
>
> That will return the all rows of A (the colon ':' means 'all'), but it
> will repeat the second column three times followed by the fourth column.

> There will be no other columns.  So, if we have a column vector 'p' and we

> write this:
>
> p(:,[1 1 1 1])
>
> That yields a matrix containing only four copies of p in the four columns
> of the matrix.  So, of course,

ah, yes, now I understand it. But your explanation skips an important step:
you was speaking of matrix A, and suddenly starts speaking of column vector p
:-)
My problem was (is!) that the notation p(:,[1 1 1]) is applied to matrices,
and if p is a column vector, how can one specify its columns?
...

> Does that clarify the meaning of line 5 above?

yes, thanks. But the notation is not self-evident, as I said.
...

> So far so good, but in MATLAB you can get those elements out of the matrix
> and into a column vector using this command:
>
> A(find(A<.5))
>
> That doesn't work in Octave.  Is there some other way that Octave can
> extract those numbers from the matrix?  Is it as efficient?

As John Eaton said, use do_fortran_indexing=1 for this case.

Thanks,
Joao

> Regards,
>
> Mike

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