Loren on the Art of MATLAB

May 12th, 2009

type somefun2AltEmptyDefs

function y = somefun2AltEmptyDefs(a,b,varargin)
% Some function that requires 2 inputs and has some optional inputs.

% only want 3 optional inputs at most
numvarargs = length(varargin);
if numvarargs > 3
    error('myfuns:somefun2Alt:TooManyInputs', ...
        'requires at most 3 optional inputs');
end

% set defaults for optional inputs
optargs = {eps 17 @magic};

% skip any new inputs if they are empty
newVals = cellfun(@(x) ~isempty(x), varargin);

% now put these defaults into the valuesToUse cell array, 
% and overwrite the ones specified in varargin.
optargs(newVals) = varargin(newVals);
% or ...
% [optargs{1:numvarargs}] = varargin{:};

% Place optional args in memorable variable names
[tol, mynum, func] = optargs{:};

By Loren Shure

19:57 UTC | Posted in Best Practice, Cell Arrays, Defaults | Permalink | 11 Comments »

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11 Responses to “Optional Arguments Using Empty as Placeholder”

1. Dan replied on May 13th, 2009 at 07:25 UTC :
   

   The past couple of posts have been really helpful to me for dealing with setting input arguments. I am surprised by how the number of optional arguments is being checked. Is there a reason that you did not use the nargchk function?

2. Loren replied on May 13th, 2009 at 11:00 UTC :
   

   Dan-

   I could definitely use nargchk but mentally what I am doing is simple enough that I don’t think to. It doesn’t help with setting the defaults either.

   –Loren

3. Petr replied on May 13th, 2009 at 15:42 UTC :
   

   Hi Loren.

   I have been trying to find the optimal way of dealing with optional arguments and default values for several years, but I find myself of not being able to stick with one best approach. I am trying new ways over and over.

   I must actually say, that I like the Python way of dealing with optional arguments and default values, i.e. specifying them directly in the function header. It would be nice to have this feature in MATLAB as well:

   function foo(reqArg, optArg1=12, optArg2=’whatever’)

   Petr

4. Loren replied on May 13th, 2009 at 15:48 UTC :
   

   Petr-

   I believe R does something similar as well. Can you please put this into our system as an enhancement request here? It would be more meaningful coming directly from a customer. Thanks.

   –Loren

5. Petr replied on May 13th, 2009 at 19:00 UTC :
   

   Loren

   I just posted the feature request as suggested.

   I realized that I did not provide the way I currently consider the most suitable for me. I have converged to an approach very similar to the one in Ben’s comments of the previous article, i.e. using key-value pairs, with defaults defined in a structure in the beginning of a function and then overwriting them with the respective fields of parameters structure.

   The only difference is that I prefer to allow for key-value pairs given in the same way as in many MATLAB graphics commands (i.e. converting the varargin cell array to structure inside the function), rather then directly expecting a structure as an argument.

   Thanks for showing all these possibilities.

6. Jessee replied on May 15th, 2009 at 12:22 UTC :
   

   Hi Loren,
   I’ve been following your blog for several years now and it’s been interesting to see how the format of the website, style-sheets, etc. have changed.
   I’ve noticed lately when you use the “type” command to display the text of a function, it prints it in hard to read italics. I’ve found that when I’m reading code, it’s easier to read and understand if it uses the Matlab Editor’s standard syntax highlighting. Is there a way (maybe using CSS) to have your blog posts format anything between “pre” tags the same way the Editor does it?
   -Jessee

7. Loren replied on May 15th, 2009 at 12:38 UTC :
   

   Hi Jessee-

   Unfortunately I don’t know of a way to do that right now. Having syntax-highlighted code is on our wish list too.

   –Loren

8. João Henriques replied on May 16th, 2009 at 02:48 UTC :
   

   Hi! I just wanted to tell you that I found both the last two posts and their comments very insightful. Excellent food for thought! :)

   The first method is great, however I’m a bit turned off by the fact that the inputs are all lumped into varargin. As I grew accustomed to Matlab, I learned to identify a function’s arguments with a quick glance at the top line.

   I was thinking of accepting the inputs as usual, define a struct with the default values, and pass it to a helper function. The helper function accesses the main function’s variables as defined in the struct, and writes the default values if needed. IMHO it could be an excellent compromise :)

   function testing()
       something('testing ', 'it')
       something()
   end
   
   function z = something(a, b, c)
       defaults.a = 'hey ';
       defaults.b = 'you';
       defaults.c = '!';
       setdefaults(defaults);
   
       z = [a b c];
   end
   
   function setdefaults(defaults)
       var_names = evalin('caller', 'whos');
       default_names = fieldnames(defaults);
       must_replace = ~ismember(default_names, var_names);
       for k = find(must_replace)
           name = default_names(k);
           assignin('caller', name, defaults.(name));
       end
   end
   

9. João Henriques replied on May 16th, 2009 at 23:31 UTC :
   

   Sorry, I was away from a MATLAB console when I wrote that :) Minor corrections to the function:

   function setdefaults(defaults)
       var_names = evalin('caller', 'who');
       default_names = fieldnames(defaults);
       must_replace = ~ismember(default_names, var_names);
       for k = find(must_replace)'
           name = default_names{k};
           assignin('caller', name, defaults.(name));
       end
   end
   

   I’ve been testing it and it’s pretty handy.

10. K_E_M_O replied on November 27th, 2009 at 00:38 UTC :
    

    Thanks a lot.. I am learning how to use matlab and these are very useful for me.

11. zhouyang replied on October 26th, 2010 at 04:52 UTC :
    

    so wonderful!

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