MATLAB Central Newsreader - Substitute values in data array

Substitute values in data array

Florian — Wed, 04 Nov 2009 11:57:01 -0500

Dear all,
I want to substitute values in data array and there is a matlab function in the toolbox called changem to o so.
However, this function uses a loop:

for k = 1:numel(newval)
B(A == oldval(k)) = newval(k);
end

and my array is very large. I have tried to speed it up several ways, but no success. Does anyone have an idea how this could be done faster?

cheers,
flo

Re: Substitute values in data array

Jan Simon — Wed, 04 Nov 2009 12:37:02 -0500

Dear Florian!

> I want to substitute values in data array and there is a matlab function in the toolbox called changem to o so.
> However, this function uses a loop:
>
> for k = 1:numel(newval)
> B(A == oldval(k)) = newval(k);
> end
>
> and my array is very large.

Please specify the sizes auf B, A, newval.

> I have tried to speed it up several ways, but no success. Does anyone have an idea how this could be done faster?

Please tell us, what you have tried already. Then we do not have to try it again...

Sometimes FINDSTR is faster than the == operator.
Are there repititions in oldval/newval?
Have you tried to use backwar-indexing (e.g. X(oldval)=newval; B = X(A);), as e.g. in IND2RGB?

Good luck, Jan

Re: Substitute values in data array

Bruno Luong — Wed, 04 Nov 2009 12:55:05 -0500

"Florian" <f.pappenberger@NOSPAMlancater.ac.uk> wrote in message <hcrq6d$mn1$1@fred.mathworks.com>...
> Dear all,
> I want to substitute values in data array and there is a matlab function in the toolbox called changem to o so.
> However, this function uses a loop:
>
> for k = 1:numel(newval)
> B(A == oldval(k)) = newval(k);
> end
>

Careful if oldval and newval intersect and arranged in the order that can fail your for-loop.

A=ceil(10*rand(1,12))
oldval=[1 2];
newval=-oldval

B = A;
[tf k]=ismember(A,oldval);
B(tf)=newval(k(tf))

Bruno

Re: Substitute values in data array

Florian — Wed, 04 Nov 2009 13:49:02 -0500

Bruno,
thanks that works - you should submit that also to mathworks as a replacement for changem .m :-)

Thanks a lot!!!!!
flo