MATLAB: Nargout of builtin set

nargoutset

The function set() returns either zero output (when 3 arguments are supplied), either 1 output (when 1 argument is supplied). Hence I thought nargout(@set) was negative, since a variable number of outputs is supposed to rely on the varargout mechanism. However, nargout(@set)= 1 !

I don't understand. How can Matlab supply a function with varying number of outputs that does not use varargout ? Is it a bug in nargout ?

Thank you

Best Answer

Varying number of outputs do not need to rely on varargout. It is completely legal to have something like

   function r = set(h, varargin)
     ....
     tr = ....
     if nargout > 0; r = tr; end
  end

varargout is used for arguments that are not named in the output list; ndgrid() is an example where it does not make sense to have the function code name arguments.

Related Solutions

MATLAB: Behaviour of nargout for anonymous functions

You get two different answers because you are testing two different functions:

the function handle to addOne
an anonymous function (which happens to call some other function/s inside it, but these are irrelevant).

Testing the simple function handle returns the number of output arguments supported by that function:

fb = @sort;
nargout(fb)
ans = 2

However nargout only measures the output of the anonymous function itself, not any functions that happen to be called inside it. And anonymous functions attempt to return as many output arguments as are requested when the code is evaluated, i.e. the anonymous function syntax supports any number of output arguments, which the nargout documentation tells us is indicated by a negative output value. For example:

f0 = @(x)disp(x); % DISP has no outputs
f1 = @(x)sqrt(x); % SQRT has one output
f2 = @(x)sort(X); % SORT has two outputs
fn = @(x)ndgrid(x); % NDGRID has any number of outputs
nargout(f0)
ans = -1
nargout(f1)
ans = -1
nargout(f2)
ans = -1
nargout(fn)
ans = -1

These all return -1 because nargout can only check the anonymous function itself, NOT any functions that happen to be called inside it!

MATLAB: Scalar assignment and nargout bump

To me, a better justification (if there is one) for the nargout bump is to make it easier to write wrappers for the many builtin functions tht always return at least 1 argument, even when called with nargout=0. For example, at the command line, SORT always returns at least one output arg, even when one is not explicitly requested,

>> sort([5 1 3]) % call with no arguments
ans =
       1     3     5

Suppose I now want to write a wrapper for sort() such that its 2nd argument returns 0-based indices instead of 1-based indices. Then I can do so, while preserving the above behavior, as easily as

   function varargout=mysort(varargin)
    [varargout{1:nargout}]=sort(varargin{:});
    if nargout>1, varargout{2}=varargout{2}-1;
   end

whereas without the bump, I would have to do something more complicated,

   function varargout=mysort(varargin)
    [varargout{1:max(1,nargout)}]=sort(varargin{:});
    if nargout>1, varargout{2}=varargout{2}-1;
   end

to ensure that at least one output arg was always returned. To my tastes, I would rather endure the second syntax for consistency's sake, but maybe that's just me...

Best Answer

Related Solutions

MATLAB: Behaviour of nargout for anonymous functions

MATLAB: Scalar assignment and nargout bump

Related Question