Comments on: What’s the Big deal?

By: Oliver Woodford

Oliver Woodford — Fri, 03 Apr 2009 14:38:50 +0000

Hoi Wong: Note that the A and B that you generate are different. The deal method initializes every cell in A with zeros(140). To do this using the num2cell method you should do:
>> tic, B=squeeze(num2cell(zeros(140,140,140,140),1:2)); toc
??? Error using ==> zeros
Out of memory. Type HELP MEMORY for your options.

However, to replicate what you did using num2cell (initializing every cell to 0) using deal, I would do:
>> tic, A=cell(140); [A{:}]=deal(0); toc
Elapsed time is 1.740839 seconds.
>> tic, B=num2cell(zeros(140)); toc
Elapsed time is 0.012501 seconds.

So your query about turning cells into comma separated lists is still valid.

Oliver

By: Hoi Wong

Hoi Wong — Thu, 02 Apr 2009 09:27:15 +0000

Hi Oliver,

Very slick idea, and I was tempted to use it myself for large data set, but I have to do a performance test first:

>> tic, A=cell(140); [A{:}]=deal(zeros(140)); toc
Elapsed time is 0.936412 seconds.
>> tic, B=cell(140); B=num2cell(zeros(140)); toc
Elapsed time is 0.008790 seconds.

Loren: can you comment on the performance of unpacking cells into comma separated lists? It’s a handy tool, but I’m a little concerned that overusing it can lead to slow performance.

By: Oliver Woodford

Oliver Woodford — Wed, 01 Apr 2009 11:12:25 +0000

Sometimes I use deal to initialize the cells of a cell array. E.g.
A = cell(2,3);
[A{:}] = deal(zeros(3));

By: Hoi Wong

Hoi Wong — Mon, 30 Mar 2009 19:14:34 +0000

I haven’t thought I’d end up using struct2vars() myself, but it just happened today when I’m trying to cache all the needed variables in a struct inside appdata(0) and release them back into workspace.

Because I want variable transparency in my code, I’m reluctant to use assigin, and here’s a complete usage of struct2vars:

variableNames = fieldnames(S);
[variableNames{:}]=struct2vars(S);

With that, I don’t even have to keep track of the order of the variable names.

By: Loren

Loren — Mon, 16 Mar 2009 15:47:10 +0000

Hoi Wong-

At the moment, MATLAB doesn’t allow indexing into temporary variables. They must be assigned first. We have the enhancement suggestion on the books, but I have no timeline for introducing the feature.

–Loren

By: Hoi Wong

Hoi Wong — Sun, 15 Mar 2009 23:50:08 +0000

Hi Loren, after another user suggested using

C = struct2cell(S) ; [a,b,c] = deal(C{:})

to replace struct2vars(), I have another question for you: is there any reason why MATLAB doesn’t allow something this?

[a, b, c]=(struct2cell(S)){:}

In other words, if it makes sense to say C=magic(2)+rand(2) [plus() operator], why can’t I say (magic(2))(:,2) or (magic(2)){:,2} [subsref() operator]?

By: Loren

Loren — Wed, 11 Mar 2009 11:43:43 +0000

Hoi-

Thanks for code for Kled. We have the enhancement requested logged to create a way to dump unwanted outputs. Nothing like this is yet in a shipping MATLAB, including R2009a which is now out.

–Loren

By: Hoi Wong

Hoi Wong — Wed, 11 Mar 2009 08:22:07 +0000

Regarding Kled’s comment, here’s the function to achieve this:

function varargout=struct2vars(S)
C = struct2cell(S);
varargout = {C{:}};
end

However, the output follows the ordering of your struct.

By: Hoi Wong

Hoi Wong — Wed, 11 Mar 2009 08:16:44 +0000

Hi Loren, regarding Oktay’s proble, would you consider in future versions of MATLAB, create a ‘dummy’ or ‘unwanted’ class or keyword for people to dump unwanted output arguments into a black hole, and have MATLAB warn you when you are trying to reuse it? This will avoid a lot of sloppy code changes when a program is maintained by multiple people.

By: Loren

Loren — Thu, 03 Jul 2008 19:43:38 +0000

Oktay-

Depending what version of MATLAB you use, you shouldn’t see the mlint warning (the later the version, the better for that).

There is no way to ask for just the 2nd output, so you can’t do what you want in a single statement unless you make your own function to index into outputs and grab the one you want with an extra function wrapped around the function f you are calling.

We have it as an enhancement in our feature database to allow skipping assignment to earlier output variables.

Today some people suggest doing something like this.

[B,B] = f(...)

so B gets overwritten. I don’t care for that as it assumes behavior of MATLAB (left-to-right) that we never wrote down and documented so could conceivably change it should that allow something new to be possible where it wouldn’t otherwise.

–Loren