How to support default parameter in MATLAB FUNCTION ?

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M.K.H. CHANDAN
M.K.H. CHANDAN el 18 de Mayo de 2015
Editada: Nicholas Ayres el 22 de Jun. de 2023
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.

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Stephen23
Stephen23 el 18 de Mayo de 2015
Editada: Stephen23 el 18 de Mayo de 2015
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:
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:
  6 comentarios
Tong Zhao
Tong Zhao el 16 de Jun. de 2022
Thanks from 2022

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Más respuestas (5)

Jan Siegmund
Jan Siegmund el 4 de Jul. de 2020
Editada: Jan Siegmund el 8 de Jul. de 2020
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
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
  2 comentarios
Jan Siegmund
Jan Siegmund el 4 de Jul. de 2020
Ah thank you Stephen, I thought it would have existed for longer.

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Stefan Schuberth
Stefan Schuberth el 23 de En. de 2023
Editada: Stefan Schuberth el 23 de En. de 2023
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

Ingrid
Ingrid el 18 de Mayo de 2015
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
  1 comentario
M.K.H. CHANDAN
M.K.H. CHANDAN el 18 de Mayo de 2015
Editada: M.K.H. CHANDAN el 19 de Nov. de 2015
Yes , It is programmer way , but MATLAB does not support like function myFunction(variable1,variable2,... optionalVarChangeDefaultParam1 =3, ... optionalVarChangeDefaultParam2 =4 , ... optionalVarChangeDefaultParam4 =5)

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shmuel katz
shmuel katz el 8 de Jun. de 2023
Editada: shmuel katz el 8 de Jun. de 2023
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
  2 comentarios
Nicholas Ayres
Nicholas Ayres el 8 de Jun. de 2023
Hi.
I would say that this other comment is strictly better:
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
shmuel katz el 8 de Jun. de 2023
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)

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Captain Karnage
Captain Karnage el 21 de Jun. de 2023
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
  1 comentario
Nicholas Ayres
Nicholas Ayres el 22 de Jun. de 2023
Editada: Nicholas Ayres el 22 de Jun. de 2023
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)

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