Create function with given number of input arguments

Question

Here's the situation: I need to create a function that takes a function handle fun which is of CONSTANT input-length (that is nargin(fun)>=0), does some transformation on the inputs and then calls fun. Pseudo-Code:

function g = transformFun(fun)
n = nargin(fun);
g = @(v_1, ..., v_n) ...
%           ^ NOT REAL MATLAB - THE MAIN PROBLEM
    fun(someCalculationsWithSameSizeOfOutput(v_1,...v_n){:});
% CAN BE ACHIEVED WITH TEMPORARY CELL IN HELPER FUNCTION ^
end

Now the problem: the output function's handle (g = transformFun(concreteFun)) is then passed to other code that relies on the fact that the function is of constant length (assumes nargin(g)>=0), thus a variable-input-length function is unacceptable (the "easy" solution).

This transformation is called with many functions with every possible number of arguments (n is unbounded), so covering a finite number of possibilities is also not possible.

Is there a (simple?) way to achieve that?

[I've searched the internet for a few hours and could only come up with a nasty hack involving the deprecated inline function, which I couldn't make work; maybe I have the wrong terminology].

I think your wording is making the problem way more complicated. You want fun to be able to handle variable amount of inputs, not constant, right? — Ander Biguri
– Ander Biguri, Commented Dec 2, 2016 at 14:17
@AnderBiguri Sorry about that; fun is the input and can handle only finite amount, the problem is as fun varies so does this finite amount — Omer Rosler
– Omer Rosler, Commented Dec 2, 2016 at 14:23

Suever · Accepted Answer · 2016-12-02 14:47:31Z

4

So typically you could use varargin to handle this sort of thing, but since you need nargin(g) to return the actual number of inputs it's a little trickier.

You could use str2func to create the anonymous function as a string and then convert it to a function handle.

% Create a list or arguments: x1, x2, x3, x4, ...
args = sprintf('x%d,', 1:nargin(func));
args(end) = '';

% Place these arguments into a string which indicates the function call
str = sprintf('@(%s)fun(someCalculationsWithSameSizeOfOutput(%s))', args, args);

% Now create an anonymous function from this string
g = str2func(str);

Based on the attrocity above, it may be worth considering an alternative way of dealing with your function handles.

edited Dec 2, 2016 at 14:47

answered Dec 2, 2016 at 14:28

Suever

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2 Comments

Omer Rosler Over a year ago

Brilliant! Thank You!

Omer Rosler Over a year ago

It's communicating with other people's code so it's a little tricky, but I couldn't agree more

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