How to support default parameter in MATLAB FUNCTION ?

1 . As I know , there is no default parameter support on MATLAB FUNCTION like other high level programming language c,c++, Is there any other methodology through which it can be done like other compiler C,C++? or programmer has to do using his own logic through varargin.

Answer 1

[Stephen23]

MATLAB is an interpreted language, not a compiled language, so there is no compiler as such.

In any case there is no inbuilt syntax to allow such things as:

function out = fun(val_1=def_val, val_2=def_val2, ...)

But you could use the input parser class, or use one of the methods listed here:

http://blogs.mathworks.com/loren/2009/05/05/nice-way-to-set-function-defaults/

The most common ways of dealing with this are:

• Use nargin and some switch or if statements to detemrine the default parameters.
• Use varargin and logical indexing to detect non-specified arguments (e.g. empty).
• Use key-value pairs (usually with varargin) and strcmpi to detect the keys.
• Use a scalar structure to hold all options.

Using varargin with nargin or logical indexing is very easy:

function foobar(varargin)
Defaults = {A,B,C...};
Defaults(1:nargin) = varargin;

OR

function foobar(varargin)
Defaults = {A,B,C...};
idx = ~cellfun('isempty',varargin);
Defaults(idx) = varargin(idx);

MATLAB themselves use different methods in different functions. Have a look at the ODE functions to see the structure syntax, many plotting functions use the key-value syntax.

You can easily combine key-value and scalar-structure in one function, see my FEX submission here to see how:

http://www.mathworks.com/matlabcentral/fileexchange/47221-numeric-to-english-words

[M.K.H. CHANDAN]

Yes, It is a nice idea, on programer way Defaults(1:nargin) = varargin; we can solve it , also we can store it on an cell array and can be used for further use.

[Jan Siegmund]

This answer is not up to date in my opinion. The arguments Block can deal with default arguments in modern MATLAB.

[Nicholas Ayres]

Jan Siegmund.

Please explain?

[Steven Lord]

New functionality introduced in release R2019b.

[Tong Zhao]

Thanks from 2022

[Ludwig Behringer]

nice

Answer 2

[Jan Siegmund]

The Answers are not up to date. Modern MATLAB supports the arguments Block:

function out = foo(in)
    arguments
        in(1,1) double {mustBeFinite} = 0;
    end
    out = in + 1;
end

For more Info check this Page: https://de.mathworks.com/help/matlab/matlab_prog/function-argument-validation-1.html

Be careful with the validation functions though. They are not "is"-functions returning logical, rather they throw an error. Here is a list of predefined ones: https://de.mathworks.com/help/matlab/matlab_prog/argument-validation-functions.html

If the default argument is a class you may want to use <Class>.empty:

arguments
    nameValueArgs.BaseFigure matlab.ui.Figure = matlab.ui.Figure.empty;
end

[Stephen23]

https://blogs.mathworks.com/pick/2019/09/27/new-with-r2019b-function-argument-validation

Note that the arguments syntax was first introduced in R2019b.

[Jan Siegmund]

Ah thank you Stephen, I thought it would have existed for longer.

Answer 3

[Stefan Schuberth]

https://de.mathworks.com/help/matlab/ref/arguments.html?searchHighlight=arguments&s_tid=srchtitle_arguments_1

function testFun(a,b)

arguments a=10; end % default value

arguments b(3,3) double {mustBePositive} = 10; end

% arguments a(size1dim,size2dim) varType {validationFunction} = defaultValue; end

disp(a);

end

Answer 4

[Ingrid]

in your function you need to check how many parameters have been passed

function myFunction(variable1,variable2, optionalVarChangeDefaultParam)
if nargin > 2
  defaultParam = optionalVarChangeDefaultParam;
else
  defaultParam = 2;
end

[M.K.H. CHANDAN]

Yes , It is programmer way , but MATLAB does not support like function myFunction(variable1,variable2,... optionalVarChangeDefaultParam1 =3, ... optionalVarChangeDefaultParam2 =4 , ... optionalVarChangeDefaultParam4 =5)

Answer 5

[shmuel katz]

I don't know if it helps, but if you know the name of the variables you enter, you can try the following:

function myFunction(var1,var2,var3,var4,...)
    defaults.var1 = 1;
    defaults.var2 = 2;
    defaults.var3 = 3;
    defaults.var4 = 4;
    
    defaultNames = fieldnames(defaults);
    
    for nInputName = 1:numel(defaultNames)
        variableName = defaultNames{nInputName};
        if ~exist(variableName,'var')
            eval([variableName,'=','defaults.(variableName);']);
        end  
    end   
    ...
    ...
    ...
end

[Nicholas Ayres]

Hi.

I would say that this other comment is strictly better:

https://uk.mathworks.com/matlabcentral/answers/217363-how-to-support-default-parameter-in-matlab-function#answer_460970

Argument blocks allow you even to set default values for name-value pairs (do not need to be positional at all).

Although if you WERE to take a non-argumentbloack approach (argument blocks are only a relatively recent addition), personally, I would use nargin, rather than checking if things exist.

If nargin == 2, you know vars 3+ do not exist.

Something like this may suffice.

function myFunction(var1, var2, var3, var4, ...)
    if nargin < 1
        var1 = 1;
    end
    if nargin < 2
        var2 = 2;
    end
    if nargin < 3
        var3 = 3;
    end
    ...
    ...
end

Also, there is no reason to use eval here. Eval is slow and unsafe. There is no good reason here to dynamically create variables.

This is your function. You have set defaults. This means that these variables are always needed and you have not the need to automate this code section. It just hurts everybody.

Varargin is also useful, but make sure you know why you're using it.

[shmuel katz]

You are right, but sometimes you need a more readable and concise code. If you have 10 variables for one function it is a very cumbersome solution. This said for the Old MATLAB, the new ones have the argument choice (but you need to keep the order)

Answer 6

[Captain Karnage]

Yet another update. As of R2021a, MATLAB supports name/value arguments using either the old, quoted name, value list syntax or name = value syntax. https://www.mathworks.com/help/matlab/matlab_prog/validate-name-value-arguments.html

[Nicholas Ayres]

While this is fun information, it is just syntactic sugar. I don't believe it is relevant to the actual question here, which is about default parameters. Would probably do better as a comment on someone else's answer, rather than its own.

(although the link you have provided and not actually discussed is very on-topic)