How to avoid error when whether the function has returns is unknown?

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function Output = PairArrayArrayfun(Function,Array1,Array2,ArrayDimension,UniformOutput)
CatDimension=max(ndims(Array1),ndims(Array2))+1;
Output=cellfun(@(Cell)Function(Cell{1},Cell{2}),PackIntoCell(cat(CatDimension,SplitIntoCell(Array1,ArrayDimension),SplitIntoCell(Array2,ArrayDimension)),CatDimension),"UniformOutput",false);
if UniformOutput
Output=cell2mat(Output);
end
end
My function accept a parameter "Function", which is expected to be a function pointer offered by the caller. However, I don't know whether this caller-offered function has any returns. In my current code, if the given function returns nothing, an error will occur.
I want my function to return nothing if the caller's function returns nothing; to return something if the caller's function returns something. How should I code?
Try-catch is a bad idea since the caller's function will be called for more than expected times. This may cause more issues if the given function has side effects.
The given function in the usecase is very likely to be anonymous, so nargout(Function) will always return -1.
The built-in function cellfun should face the same issue, and MATLAB developers successfully solved it. However, its source code is unavailable.

Accepted Answer

埃博拉酱
埃博拉酱 on 30 Oct 2019
This is my workaround:
function Output = PairArrayArrayfun(Function,Array1,Array2,ArrayDimension,UniformOutput)
CatDimension=max(ndims(Array1),ndims(Array2))+1;
if nargout>0
Output=cellfun(@(Cell)Function(Cell{1},Cell{2}),PackIntoCell(cat(CatDimension,SplitIntoCell(Array1,ArrayDimension),SplitIntoCell(Array2,ArrayDimension)),CatDimension),"UniformOutput",false);
if UniformOutput
Output=cell2mat(varargout);
end
else
cellfun(@(Cell)Function(Cell{1},Cell{2}),PackIntoCell(cat(CatDimension,SplitIntoCell(Array1,ArrayDimension),SplitIntoCell(Array2,ArrayDimension)),CatDimension),"UniformOutput",false);
end
end
In this way the responsibility to judge output is kicked back to the caller. It seems clumsy but may be how the built-in cellfun works.
  4 Comments
埃博拉酱
埃博拉酱 on 30 Oct 2019
@Daniel M
nargout(Function) is unreliable. If it returns -1, that means Function returns varargout. A varargout return could be nothing! abs(nargout(Function))>0 doesn't assure the Function of any returns.

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More Answers (2)

Daniel M
Daniel M on 29 Oct 2019
Edited: Daniel M on 29 Oct 2019
You can use nargout(Function) to check the number of output arguments. <https://www.mathworks.com/help/matlab/ref/nargout.html>
  4 Comments
埃博拉酱
埃博拉酱 on 30 Oct 2019
@Walter Roberson Anonymous functions are always supposed to return varargout by nargout, even for those ones that really return nothing. In this usecase the given function is very likely to be anonymous, unfortunately.

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Walter Roberson
Walter Roberson on 30 Oct 2019
try/catch the first execution outside of the cellfun. If the assignment works then cellfun the rest of the iterations. If it fails then you will need to loop instead of cellfun.
cellfun cannot be used with functions that produce no output.
  1 Comment
埃博拉酱
埃博拉酱 on 30 Oct 2019
You're wrong. cellfun really can work with no-return functions, at least for R2019b

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