Skipping Optional Positional Arguments

I am writing a routine that uses arguments to parse input parameters. After many years of using both inputParser and manual input parsing, I like using arguments. However, I seem to be running into some limitations.

I am trying to get arguments to ignore an optional positional arguments. Take the example function:

function y = foo(a,b,c)
arguments
    a
    b = 2
    c = 3
end
y = a + b + c;
end

Suppose I want to specify arguments for a and c and use the default value of b. With manual parsing, I would just detect an empty value for b by calling foo(1,[],3). When using arguments, I can not achieve the same functionality by calling as foo(1,~,3) or similar syntax.

Is there a way to get this functionality without resorting to manual parsing?

[Stephen23]

"Is there a way to get this functionality without resorting to manual parsing?"

Not as far as I am aware. You should make an enhancement suggestion here:

https://www.mathworks.com/support/contact_us.html

[Stewart]

Just went ahead and created an enhacement suggestion. I wanted to check to see if I was missing a simple workaround, the timeline for getting this into a release definitely won't be quick.

Answer 1

[Star Strider]

I usually use varargin rather than arguments, however your calling syntax may not be appropriate

To illustrate —

out = foo(1,pi,3)

out = 7.1416

out = foo(1,[],3)

out = 6

out = foo(1,~,3)

Using ~ in this context is not supported.

function y = foo(a,varargin)
optargs = cell(1,2);
optargs(1:numel(varargin)) = varargin;
if isempty(optargs{1})
    b = 2;
else
    b = optargs{1};
end
if isempty(optargs{2})
    c = 3;
else
    c = optargs{2};
end
y = a + b + c;
end

Not everything is possible.

.

Answer 2

[Walter Roberson]

Unfortunately you cannot use [] to trigger detection as "missing" for the purpose of default argument processing.

function y = foo(a,b,c)
arguments
    a
    b double {mustBeEmptyOrNumeric(b)} = 2
    c = 3
end
if isempty(b); b = 2; end
y = a + b + c;
end

function mustBeEmptyOrNumeric(b)
      if ~isempty(b) && ~isnumeric(b)
        eidType = 'mustBeEmptyOrNumeric:notEmptyOrNumeric';
        msgType = 'Input must be empty or numeric.';
        error(eidType, msgType);
      end
end

Answer 3

[埃博拉酱]

It is recommended that you use name-value arguments:

function y = foo(a,options)
arguments
    a
    options.b = 2
    options.c = 3
end
y = a + b + c;
end

Then you can specify c without b:

foo(1,c=3);