MATLAB: Best practice for saving only one output from a vector-outputting function

best practicesvector output

What's the proper way to request only one variable from a vector outputting function, i.e.

[WantThis DontWantThis] = myfun(x, y, ... )

In the past I've just saved the DontWantThis output as a variable called trash and not made reference to it again. This seems rather unprofessional. Is there a cleaner way?

Best Answer

Simply

 WantThis = myfun(x, y, ... )

if you just need/want the first output. More generally

 [a, b, c, d] = myfun(x, y, ... )

even if there could be more output args ( e, f, g, ..).

If don't want to name args that you don't need, which are preceding args that you do need, you can use ~ as a placeholder:

 [~, ~, c, ~, e] = myfun(x, y, ... )

Note that ~ is supported since 2009 if I remember well (edit: ref).

Related Solutions

MATLAB: Can a function known whether/which specific output is ignored (tilde operator)

"Can a function known whether/which specific output is ignored ?"

No. nargout tells you the number of output arguments specified in the caller but it counts outputs suppressed with a tilde (~). nargout(fun) tells you the number of ouputs a specific function has to offer but that tells you nothing about the caller.

What are the alternatives?

1. Pass an input to the function that specifies which outputs you're requesting. The tricky part is to remember to change that input value if a different set of outputs is requested.

2. Prioritize your outputs and use nargout to determine which outputs to produce.

You can also try the new and improved (vs.2.0.0) detectOutputSuppression() from the file exchange. It reads the caller line and parses the outputs to determine if any are suppressed.

Example

The main() function calls myFunc() and suppresses the 2nd and 3rd outputs.

function main()
[mst(1), ~, ~, data] = myFunc();
function [a, b, c, d] = myFunc()
a = 1;  b = 2;  c = 3;  d = 4;
isTilde = detectOutputSuppression(nargout);

Result: isTilde = [0 1 1 0].

Supported syntaxes

[a,~,~,d] = myFunc();
a = myFunc();
[a(1),a(2),~] = myFunc();
[~,~,c(1:20),~] = myFunc();
[~,b{3}] = myFunc();
[~,b{3,1,5}] = myFunc();
[~,~,~,g{1}{2}{4}(1)] = myFunc();
q=cell(1); [~,q{:}] = myFunc(); % because q is 1x1.
T=table(); [T.a,~] = myFunc();
[j,u,n,k]; [~,~,~,~] = myFunc(); [j,u,n,k]
[j,u,n,k], [~,~,~,~] = myFunc(), [j,u,n,k]
S=struct; [S.x,~,S.t(4)] = myFunc();
[v.a, v.b{1}, v.c(2,2), ~] = myFunc();
myFunc + 1;
y = [1, myFunc(), 1];
assert(~isempty(myFunc()))
[a,b,~,~] = myFunc(...
inputs); %split in multiple lines after function name
and more....

Syntaxes not supported

An error message is thrown when these syntaxes are detected.

Outputs not separated by commas: [a b c] = myFunc()
When myFunc() is not called from within an m-file or wasn't the most recent command called from the command window.
When myFunc() is called in debug mode but isn't the line currently paused or executed by MATLAB.
Output assignment to comma separated lists: x=cell(1,5); [x{:}] = myFunc();
Caller line is split into multiple lines before the inputs (a split after the function name is OK).
When myFunc() is wrapped in an anonymous function: f = @myFunc; f()
When the parser matches more than one [...]=myFunc per line of code.

MATLAB: Additional output with fsolve

Is the G you're really calculating costly to evaluate? If so, I'd suggest a different approach. Define your function as

function [F G] = myfunc(x)
F = x^2;
if nargout > 1
  G = 2;
end

so it doesn't evaluate G during the solving (which would involve, generally, several calls to myfunc ). Note that you can use a more compact notation:

xsol = fsolve(@myfunc,guess);

Then go ahead and calculate G once:

[F G] = myfunc(x)

You'll probably save a lot more computation with this approach.

Best Answer

Related Solutions

MATLAB: Can a function known whether/which specific output is ignored (tilde operator)

MATLAB: Additional output with fsolve

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