how to pass keyword arguments to a function via a struct

In recent versions of Matlab, functions can declare optional arguments with default values that get automatically packed into a struct e.g.,

function examplefn(x)

arguments

x.a (1,1) double = 12

x.b (1,1) double = 7

end

With such a declaration, the function is called via examplefn('a', 8, 'b', 9) (or just examplefn('a',8)), and the examplefn function body can now use x.a and x.b as variables. Suppose I have written such a function, and its caller (rather than the function) has the arguments in a struct:

y = struct('a', 8, ,'b', 9);

What is the way to pass y as an input argument to examplefn? I'd like to be able to say something like: examplefn(unpack(y)). Obviously, I could spell it out: examplefn('a', y.a, 'b', y.b) but expanding in this way partly defeats the purpose of having optional and keyword arguments in the first place and makes it fragile to chain a sequence of functions that share keyword/optional arguments. Thanks.

[dpb]

At this time there is no syntax to do anything other than pass the name-value parameter pairs explicitly.

I didn't 'spearmint; you might be able to write an anonymous function that would do the unwrapping to the string to pass, but then you would have to revert to eval to execute the result; MATLAB wouldn't know what to do with the string as an argument list.

[Stephen Vavasis]

Thanks for all the responses! it seems that there are two possible solutions. The first is to use inputParser in examplefn instead of a function argument block. The second is to convert the struct to a cell array, and then use {:} on that array, which converts the cell array into multiple arguments.

Regarding the second approach, I can write an unpack function as follows:

function c1 = unpack(s)

arguments

s (1,1) struct

end

fieldn = fieldnames(s);

fieldv = struct2cell(s);

nfield = length(fieldn);

c1 = cell(2*nfield,1);

c1(1:2:end) = fieldn;

c1(2:2:end) = fieldv;

end

Then I can invoke examplefn via:

unpack_y = unpack(y);

examplefn(unpack_y{:})

Answer 1

[Walter Roberson]

See https://www.mathworks.com/matlabcentral/answers/520619-can-matlab-s-arguments-function-handle-structs#answer_694260

Answer 2

[Matt J]

You'll have to go old school and use inputParser.

y = struct('a',8,'b', 9);
x=examplefn(y)

x = struct with fields:
    a: 8
    b: 9
    c: 100

x=examplefn('a',8,'b',9)

x = struct with fields:
    a: 8
    b: 9
    c: 100

function x=examplefn(varargin)

 p=inputParser();
 addParameter(p,'a',12);
 addParameter(p,'b',7);
 addParameter(p,'c',100);
 parse(p,varargin{:});

 x=p.Results;
end

Answer 3

[Matt J]

@Stephen Vavasis. Your unpack function is already available in Matlab as namedargs2cell.

[Stephen Vavasis]

Great! I will use nameargs2cell, thanks!