Thread Subject: accessing the same elements repeatedly in a vectorized fashion?

Hello,

I would like to vectorize the following loop.

for i = 1:length(c)
  a(b(i)) = a(b(i)) + c(i)
end

However, a simple

 a(b) = a(b) + c

will not work as 'b' is not one-to-one. Matlab will only add one (the last, I think) value of 'c' to each corresponding 'a' element. I would like to add all the corresponding values in 'c', just like in the loop.

Any ideas?

Please note that in my particular case, the multiplicity of 'b' is not fixed or constant. That is, each element of 'a' is accessed a different number of times (between 3 to, say, 8 times) by 'b'.

Thanks.

The function accumarray is what you need.


"Ori " <john.doe.nospam@mathworks.com> wrote in message <f8r2uh$oib$1@fred.mathworks.com>...
> Hello,
>
> I would like to vectorize the following loop.
>
> for i = 1:length(c)
> a(b(i)) = a(b(i)) + c(i)
> end
>
> However, a simple
>
> a(b) = a(b) + c
>
> will not work as 'b' is not one-to-one. Matlab will only add one (the last, I think) value of 'c' to each corresponding 'a' element. I would like to add all the corresponding values in 'c', just like in the loop.
>
> Any ideas?
>
> Please note that in my particular case, the multiplicity of 'b' is not fixed or constant. That is, each element of 'a' is accessed a different number of times (between 3 to, say, 8 times) by 'b'.
>
> Thanks.

Thanks!

"Naor Movshovitz" <lazy_n@yahoo.com> wrote in message <f8r3o3$7qj$1@fred.mathworks.com>...
> The function accumarray is what you need.
>
>
> "Ori " <john.doe.nospam@mathworks.com> wrote in message <f8r2uh$oib$1@fred.mathworks.com>...
> > Hello,
> >
> > I would like to vectorize the following loop.
> >
> > for i = 1:length(c)
> > a(b(i)) = a(b(i)) + c(i)
> > end
> >
> > However, a simple
> >
> > a(b) = a(b) + c
> >
> > will not work as 'b' is not one-to-one. Matlab will only add one (the last, I think) value of 'c' to each corresponding 'a' element. I would like to add all the corresponding values in 'c', just like in the loop.
> >
> > Any ideas?
> >
> > Please note that in my particular case, the multiplicity of 'b' is not fixed or constant. That is, each element of 'a' is accessed a different number of times (between 3 to, say, 8 times) by 'b'.
> >
> > Thanks.
>

Ori:
<SNIP wants to accumulate his/her values...

as others said

     help accumarray;

-and- look at <loren shure>'s contribution here

http://blogs.mathworks.com/loren/2007/07/26/assignment-with-repeated-indices/

us