MATLAB Central Newsreader - Column Index

Column Index

Oleg Komarov — Wed, 01 Jul 2009 20:42:01 -0400

I have a matrix like this:
a = random('bino',1,0.5,[10,5]);

And i want to find the colum index corresponding to the last 1 of each row:
IDX = NaN(size(a,1),1);
for i = 1:size(a,1)
IDX(i) = find(a(i,:),1,'last');
end

Can it be done without the loop?

Thanks a lot

Oleg

Re: Column Index

Jos — Wed, 01 Jul 2009 22:07:01 -0400

"Oleg Komarov" <oleg.komarov@hotmail.it> wrote in message <h2ghmp$t3a$1@fred.mathworks.com>...
> I have a matrix like this:
> a = random('bino',1,0.5,[10,5]);
>
> And i want to find the colum index corresponding to the last 1 of each row:
> IDX = NaN(size(a,1),1);
> for i = 1:size(a,1)
> IDX(i) = find(a(i,:),1,'last');
> end
>
> Can it be done without the loop?
>
> Thanks a lot
>
> Oleg

One of the many approaches:

[i,idx] = max(cumsum(a,2),[],2)

but what if a row has no ones?

Jos

Re: Column Index

Bruno Luong — Wed, 01 Jul 2009 22:24:01 -0400

"Oleg Komarov" <oleg.komarov@hotmail.it> wrote in message <h2ghmp$t3a$1@fred.mathworks.com>...
> I have a matrix like this:
> a = random('bino',1,0.5,[10,5]);
>
> And i want to find the colum index corresponding to the last 1 of each row:
> IDX = NaN(size(a,1),1);
> for i = 1:size(a,1)
> IDX(i) = find(a(i,:),1,'last');
> end
>

Using SplitVec on FEX http://www.mathworks.com/matlabcentral/fileexchange/24255

IDX = NaN(size(a,1),1);

[J I]=find(a.');
last=SplitVec([I J],1,'last'); % On FEX
IDX(I(last)) = J(last)

% Bruno

Re: Column Index

Oleg Komarov — Wed, 01 Jul 2009 22:46:01 -0400

"Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <h2gnm1$o91$1@fred.mathworks.com>...
> "Oleg Komarov" <oleg.komarov@hotmail.it> wrote in message <h2ghmp$t3a$1@fred.mathworks.com>...
> > I have a matrix like this:
> > a = random('bino',1,0.5,[10,5]);
> >
> > And i want to find the colum index corresponding to the last 1 of each row:
> > IDX = NaN(size(a,1),1);
> > for i = 1:size(a,1)
> > IDX(i) = find(a(i,:),1,'last');
> > end
> >
>
> Using SplitVec on FEX http://www.mathworks.com/matlabcentral/fileexchange/24255
>
> IDX = NaN(size(a,1),1);
>
> [J I]=find(a.');
> last=SplitVec([I J],1,'last'); % On FEX
> IDX(I(last)) = J(last)
>
> % Bruno
Thank you very much Jos and Bruno

TO: Jos
I work with matrices which, by construction, in every row have at least one "1".