Arguments function: Name-value arguments in default values

I am currently using Matlabs argument validation functionality (arguments) to validation an input struct (via conversion to name-value pair). I was wondering if there is any way to set the default value for name-argument pair as the value from a different name-value pair as below. Ideally this would set the default value of <>.B to be equal to <>.A if the value is not provided.

function my_func(nameValueInputStruct)
    arguments
        nameValueInputStruct.A double = 1
        nameValueInputStruct.B double = nameValueInputStruct.A
    end
...
end

Matlab throws the error when I try to do this, but was hoping that there is an available work-around. It is important that the input is a name-value pair struct. Thank you.

Use of name-value arguments in default values is not supported.

Answer 1

[Shivam Malviya]

Hi Austin,

I see that you are interested in utilizing the "A" name-value pair argument to assign a default value for the "B" name-value pair argument inside the function argument validation block. However, I regret to inform you that this feature is currently unsupported.

To work around this issue, please note that if a value is not provided for a specific name-value pair argument, it will not appear as a field in the name-value pair argument variable. With this in mind, I have created a function which is attached below for your reference and to help you understand this concept better.

function my_func(nameValueInputStruct)
    % Argument validation
    arguments
        nameValueInputStruct.A double = 1
        nameValueInputStruct.B double
    end

    % Call a helper function to parse the input struct
    parseNameValueInputStruct(nameValueInputStruct)
end

function parseNameValueInputStruct(nameValueInputStruct)
%parseNameValueInputStruct Parse the nameValueInputStruct and modify it 
%   accordingly

    % If value for argument "B" is not provided
    if ~isfield(nameValueInputStruct, "B")
        % Set the value of "B" equal to the value of "A"
        nameValueInputStruct.B = nameValueInputStruct.A;
    end
end

Please refer to the following links for a better understanding;

• https://in.mathworks.com/help/matlab/ref/isfield.html
• https://in.mathworks.com/help/matlab/ref/arguments.html

Best regards,

Shivam Malviya

[John]

I know this is an old thread, but I did have a similar question. For me, I have a situation where I want defaults to be contingent on other parameters.

function my_output = my_func(my_data, is_dos, opts)
arguments
    my_data; %I'm going to do something with this
    is_dos (1,1) logical = true;
    opts.directory_delim char = default_delim(is_dos); %Unless explicitly specified, determine this value based on the 'is_dos' argument
end

for which I can write

function delim = default_delim(is_dos)
if is_dos
    delim = '\';
else
    delim = '/';
end

So I'm curious as to whether there's an elegant way to do this. Observations:

• I cannot make default_delim a local function (i.e., inside my_func). Matlab disallows this.
• I can make default_delim its own file, but that just seems a bit clunky.
• This also works, but is a bit of a monstrosity:

[repmat('\',1,is_dos) repmat('/',1,~is_dos)]

• A local helper function (as above) will work, but that breaks the clean simplicity of using arguments for validation/default handling in the first place.

Any other thoughs?

[Walter Roberson]

@John Why not just use filesep ?

[John]

The example is intended as an easily understood illustration of what I want to do. I know there are easier ways to handle this particular circumstance. Essentially I want the default param_B value to be determined by the param_A value.

Answer 2

[Markus Leuthold]

Same with this code, which I expect to work, but it fails with the same error message

arguments
    opts.A = mustBeMember(opts.A, ["x" "y"])
end

[John]

Probably past the time you care about an answer, but the format for lines in an arguments block is:

argName1 (dimensions) class {validators} = defaultValue

So for your example, what you want is something like:

opts.A string {mustBeMember(opts.A, ["x" "y"])} = "x"