Write function with multiple output, like sort()

B = sort(A);
[B, I] = sort(A);

How can I write such a function? Is it as simple as return both B and I, but the first line just uses B and ignore I, or there's some polymorphism trick under the hood?

Answer 1

[Tommy]

function [out1, out2, out3] = myfunc
    ...
end

As an example, this function has 3 outputs. You can call it with

[a, b, c] = myfunc

if you want, or you can use

a = myfunc

to just get out1, or perhaps

[~, b, c] = myfunc

to just get out2 and out3. Inside myfunc, each output simply needs to be assigned a value before the end of the function. Is this what you mean?

[nvmnghia]

Okay, so is this the only way to do it?

I have some software engineering background, and this is an acceptable way to do it. However, I would prefer something like this:

function a = sort(A)
    % bla bla
end

function [a, I] = sort(A)
    % bla bla, but also compute I
end

It seems that Matlab can't do this.

[Rik]

It can:

function [a,I] = sort(A)
% bla bla    
if nargout>1
    %also compute I
end
end

[Tommy]

Ah I see. If you tried to have both within the same file, you would get an error

'Function with duplicate name "sort" cannot be defined.'

and if they were in separate files, then MATLAB would just find the closest one and ignore the other one.

In MATLAB, it is up to whoever invokes a function to control which output variables are assigned, and that is where the flexibility lies. If you have some function

function [a, I] = sort(A)
    % bla bla, but also compute I
end

you can't force whoever invokes it to request both outputs. It is valid for someone to call this function with just one output argument (which is wonderful). Therefore, if you also have the function

function a = sort(A)
    % bla bla
end

and you call

b = sort(A)

how should MATLAB know which sort you are trying to call? MATLAB instead decides on its own, based solely on where the functions are located, and you'd better use syntax which is compatible with the one that MATLAB decides to go with.

[Steven Lord]

You can't force someone to call your function with the desired number of outputs, but you could encourage then to do so or refuse to cooperate if they don't.

function [B, I] = mysort(A)
if nargout < 2
    warning('You really should call mysort with two output arguments')
    % or
    error('You must call mysort with two output arguments')
end
% Do stuff to compute B and I
end

[Tommy]

That's a good point!