MATLAB Central Newsreader - Operate on certain element

Operate on certain element

Diego Lass — Wed, 24 Jun 2009 22:04:01 -0400

Hi,
I want to operate on certain elements in a vector, but leave other elements intact.
Say I know the indexes that I don't want to change, but don't know the indexes I want to change(is the complement of the ones I don't want to change). What is the most efficient way to do this?

In another words, how to single out the indexes, so I can do operation only on these?

Thanks
Diego

Re: Operate on certain element

Alan B — Wed, 24 Jun 2009 22:18:01 -0400

"Diego Lass" <dlISCool@gmail.com> wrote in message <h1u7sh$2e7$1@fred.mathworks.com>...
> Hi,
> I want to operate on certain elements in a vector, but leave other elements intact.
> Say I know the indexes that I don't want to change, but don't know the indexes I want to change(is the complement of the ones I don't want to change). What is the most efficient way to do this?
>
> In another words, how to single out the indexes, so I can do operation only on these?
>
> Thanks
> Diego

How about this?

change = setdiff(1:length(A), dontchange);
B = A;
B(change) = f(A(change));

Re: Operate on certain element

Steven Lord — Thu, 25 Jun 2009 14:06:41 -0400

"Alan B" <monguin61@yahoo.com> wrote in message
news:h1u8mp$nr5$1@fred.mathworks.com...
> "Diego Lass" <dlISCool@gmail.com> wrote in message
> <h1u7sh$2e7$1@fred.mathworks.com>...
>> Hi,
>> I want to operate on certain elements in a vector, but leave other
>> elements intact.
>> Say I know the indexes that I don't want to change, but don't know the
>> indexes I want to change(is the complement of the ones I don't want to
>> change). What is the most efficient way to do this?
>>
>> In another words, how to single out the indexes, so I can do operation
>> only on these?
>>
>> Thanks
>> Diego
>
> How about this?
>
> change = setdiff(1:length(A), dontchange);
> B = A;
> B(change) = f(A(change));

That's one way to do it, although to handle the case where A is a matrix
you'd want to use NUMEL instead of LENGTH in your first line. Alternately,
use logical indexing instead of linear:

elementsToChange = true(size(A));
elementToChange(dontchange) = false;
B = A;
B(elementsToChange) = f(A(elementsToChange));

--
Steve Lord
slord@mathworks.com

Re: Operate on certain element

Matt — Thu, 25 Jun 2009 15:17:01 -0400

"Alan B" <monguin61@yahoo.com> wrote in message <h1u8mp$nr5$1@fred.mathworks.com>...

> change = setdiff(1:length(A), dontchange);

Using setdiff() invokes unnecessary sorting operations.

Steve's method is probably the optimal one , but if you really do need to compute "change" as a set of linear indices, this would probably be better than using setdiff:

change=1:numel(A);
change(dontchange)=[];