MATLAB Central Newsreader - can I avoid a loop here?

can I avoid a loop here?

Ross — Tue, 19 Feb 2008 17:51:41 -0500

Hi,
I face the following problem:

Let j and x be two vectors of same length.
I would like to apply function FUN to the subset of xs, where the
subset is defined by the index j.

--------------
Eg:

j = [1 2 2 2 3 3]'
x = [6 43 3 4 2 5]'

FUN is sum

the result wanted is [6 50 7]'
-----------------

Is there a way to avoid looping?

Thank you
Ross

Re: can I avoid a loop here?

Steven Lord — Tue, 19 Feb 2008 18:10:43 -0500

"Ross" <fed.rossi@gmail.com> wrote in message
news:e01a38fc-431a-415f-9162-8e7ad7a422c0@q33g2000hsh.googlegroups.com...
> Hi,
> I face the following problem:
>
> Let j and x be two vectors of same length.
> I would like to apply function FUN to the subset of xs, where the
> subset is defined by the index j.
>
> --------------
> Eg:
>
> j = [1 2 2 2 3 3]'
> x = [6 43 3 4 2 5]'
>
> FUN is sum
>
> the result wanted is [6 50 7]'
> -----------------
>
> Is there a way to avoid looping?

>> j = [1; 2; 2; 2; 3; 3];
>> x = [6; 43; 3; 4; 2; 5];
>> M = accumarray([j ones(size(j))], x)

M =

     6
    50
     7

--
Steve Lord
slord@mathworks.com

Re: can I avoid a loop here?

Ross — Tue, 19 Feb 2008 18:22:19 -0500

On Feb 19, 12:10 pm, "Steven Lord" <sl...@mathworks.com> wrote:
> "Ross" <fed.ro...@gmail.com> wrote in message
>
> news:e01a38fc-431a-415f-9162-8e7ad7a422c0@q33g2000hsh.googlegroups.com...
>
>
>
> > Hi,
> > I face the following problem:
>
> > Let j and x be two vectors of same length.
> > I would like to apply function FUN to the subset of xs, where the
> > subset is defined by the index j.
>
> > --------------
> > Eg:
>
> > j = [1 2 2 2 3 3]'
> > x = [6 43 3 4 2 5]'
>
> > FUN is sum
>
> > the result wanted is [6 50 7]'
> > -----------------
>
> > Is there a way to avoid looping?
> >> j = [1; 2; 2; 2; 3; 3];
> >> x = [6; 43; 3; 4; 2; 5];
> >> M = accumarray([j ones(size(j))], x)
>
> M =
>
> 6
> 50
> 7
>
> --
> Steve Lord
> sl...@mathworks.com

beautiful.
thanks Steve!