MATLAB Answers

Custom Argument Validation Function for NameValueArgs

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David DeVries
David DeVries on 4 Nov 2019
Commented: David DeVries on 30 Jan 2021
While working on a project, I found an interesting error when using custom argument validation functions on name-value arguments. I constructed this contrived example class to demonstrate:
classdef Dog
properties
Name
HouseNumber = []
end
methods
function obj = Dog(Name, NameValueArgs)
arguments
Name (1,1) string
NameValueArgs.HouseNumber double {Dog.mustBeScalarOrEmpty(NameValueArgs.HouseNumber)}
end
obj.Name = Name;
if isfield(NameValueArgs, 'HouseNumber')
obj.HouseNumber = NameValueArgs.HouseNumber;
end
end
end
methods (Static = true)
function mustBeScalarOrEmpty(var)
if ~(isscalar(var) || isempty(var))
error('Not scalar nor empty.');
end
end
end
end
When the following is run:
dog1 = Dog("Test",'HouseNumber',23)
This error is produced:
Error using Dog
Invalid name-value argument 'HouseNumber'. Index exceeds the number of array elements (1).
I'm not understanding why this error is being triggered. Is it not valid to call a static method from the class as a custom argument validation function?

Answers (1)

Roshni Garnayak
Roshni Garnayak on 10 Nov 2019
Try using ‘varargin’ in place of ‘NameValueArgs’. To validate the properties and assign values to them, use the ‘set’ method.
You can refer to the following link to get an idea of how to use Name-value pair arguments in constructor:
For further details on ‘varargin’ and ‘set’ method refer to the following links:
  3 Comments
David DeVries
David DeVries on 30 Jan 2021
@Jan Kappen: Glad to hear that the bug was fixed in MATLAB 2020b.
In the meantime I was using another class and defining "mustBeScalarOrEmpty" in a different class as a public static method, and it seemed to work.
In general, defining functions outside of a classdef breaks many object orientated programming rules, so I avoid that where I can.

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