MATLAB Central Newsreader - How to do vectorize this simple code?

How to do vectorize this simple code?

Henry — Fri, 14 Nov 2008 22:09:03 -0500

I want to vectorize this simple snippet:

for i=1:num_elements
y(i)=my_matrix(row(i),col(i));
end

I tried y=my_matrix(row,col)
and y=my_matrix([row col])

but nothing seems to work. Any pointers? Thanks.

Re: How to do vectorize this simple code?

Kenneth Eaton — Fri, 14 Nov 2008 22:15:07 -0500

"Henry " <henry@fullspectrumengineering.com> wrote in message <gfkstu$1id$1@fred.mathworks.com>...
> I want to vectorize this simple snippet:
>
> for i=1:num_elements
> y(i)=my_matrix(row(i),col(i));
> end
>
> I tried y=my_matrix(row,col)
> and y=my_matrix([row col])
>
> but nothing seems to work. Any pointers? Thanks.

Check out the SUB2IND function.

hth,
Ken

Re: How to do vectorize this simple code?

Scott — Fri, 14 Nov 2008 22:18:01 -0500

If I understand you correctly, all you need is the diagonal. I believe this is the simplest way (at least that I'm aware of).

y(i)=diag(my_matrix(row(i),col(i)));

Re: How to do vectorize this simple code?

John D'Errico — Fri, 14 Nov 2008 22:35:03 -0500

"Scott " <crazyivan84__remove__@hotmail.com> wrote in message <gfktep$9al$1@fred.mathworks.com>...
> If I understand you correctly, all you need is the diagonal. I believe this is the simplest way (at least that I'm aware of).
>
> y(i)=diag(my_matrix(row(i),col(i)));

This is an extremely inefficient way to do it of course.

John

Re: How to do vectorize this simple code?

Mahdieh — Fri, 14 Nov 2008 23:17:03 -0500

Re: How to do vectorize this simple code?

Henry — Fri, 14 Nov 2008 23:36:02 -0500

Hmm all those suggestions don't seem to be very efficient compared to my original code of:
for i=1:num_elements
y(i)=my_matrix(row(i),col(i));
end

I just thought I could write it as a single line like y=my_matrix(row,col) where row and col are vectors and it would match them up as pairs and return a vector.

Seems like a common procedure and I'm surprised matlab doesn't have it built in...

"Mahdieh" <mahdieh.emrani@capitalhealth.ca> wrote in message <gfl0tf$srs$1@fred.mathworks.com>...
> "Henry " <henry@fullspectrumengineering.com> wrote in message <gfkstu$1id$1@fred.mathworks.com>...
> > I want to vectorize this simple snippet:
> >
> > for i=1:num_elements
> > y(i)=my_matrix(row(i),col(i));
> > end
> >
> > I tried y=my_matrix(row,col)
> > and y=my_matrix([row col])
> >
> > but nothing seems to work. Any pointers? Thanks.
>
> Hi Henry
>
> I guess this works:
>
> U = mymatrix(row,:);
> V = U(:,col);
> y = diag(V);
>
> -Mah

Re: How to do vectorize this simple code?

Scott — Fri, 14 Nov 2008 23:43:01 -0500

"John D'Errico" <woodchips@rochester.rr.com> wrote in message <gfkuen$m7v$1@fred.mathworks.com>...
> "Scott " <crazyivan84__remove__@hotmail.com> wrote in message <gfktep$9al$1@fred.mathworks.com>...
> > If I understand you correctly, all you need is the diagonal. I believe this is the simplest way (at least that I'm aware of).
> >
> > y(i)=diag(my_matrix(row(i),col(i)));
>
> This is an extremely inefficient way to do it of course.
>
> John

Actually, I've always found sub2ind to be quite slow.
Consider:

rows=randint(5,100000,[1,10]);
cols=randint(5,100000,[1,10]);
testMat=magic(10);

%Sub2ind
tic
resultsInd=zeros(100000,5);
for i=1:100000
    index=sub2ind(size(testMat),rows(:,i),cols(:,i));
    resultsInd(i,:)=testMat(index);
end
toc

%Diag
tic
resultsDiag=zeros(100000,5);
for i=1:100000
    resultsDiag(i,:)=diag(testMat(rows(:,i),cols(:,i)));
end
toc

isequal(resultsInd,resultsDiag)

With the result:
Elapsed time is 6.405682 seconds. (sub2ind)
Elapsed time is 0.293780 seconds. (diag)

ans =

     1

Re: How to do vectorize this simple code?

Husam Aldahiyat — Sat, 15 Nov 2008 03:24:02 -0500

Your comparison is invalid.

Re: How to do vectorize this simple code?

Kenneth Eaton — Sat, 15 Nov 2008 03:58:01 -0500

"Henry " <henry@fullspectrumengineering.com> wrote in message <gfl212$m0t$1@fred.mathworks.com>...
> Hmm all those suggestions don't seem to be very efficient compared to my original code of:
> for i=1:num_elements
> y(i)=my_matrix(row(i),col(i));
> end
>
> I just thought I could write it as a single line like y=my_matrix(row,col) where row and col are vectors and it would match them up as pairs and return a vector.
>
> Seems like a common procedure and I'm surprised matlab doesn't have it built in...

I don't know if SUB2IND is vectorized. If not, try this:

N = size(my_matrix,1);
ind = N*(col-1)+row;
y = my_matrix(size(my_matrix,1)*(col-1)+row);

hth,
Ken

Re: How to do vectorize this simple code?

Kenneth Eaton — Sat, 15 Nov 2008 04:20:19 -0500

"Kenneth Eaton" <Kenneth.dot.Eaton@cchmc.dot.org> wrote in message <gflhc9$h1d$1@fred.mathworks.com>...
>
> I don't know if SUB2IND is vectorized. If not, try this:
>
> N = size(my_matrix,1);
> ind = N*(col-1)+row;
> y = my_matrix(size(my_matrix,1)*(col-1)+row);

Oops, mistyped that a little; you can remove the first 2 lines, using the third as a one-liner, or make the third line: y = my_matrix(ind);

Ken

Re: How to do vectorize this simple code?

Per Sundqvist — Sat, 15 Nov 2008 10:39:02 -0500