Elegantly refer to the second output from a function

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Hello everyone. I have a function (matlab function, say "ismember") and I want to use the second output from this function as index in a vector. For example:
data = rand(1,10);
[A,B] = ismember(1,[0,1,2,3,4,5]);
result = data(B);
My question is how do I skip the second line and directly use B in line 3.
Many thanks

Answers (5)

Hank
Hank on 6 Feb 2020
Edited: Hank on 6 Feb 2020
I wrote two or three functions that allow me to do this. I agree, its much nicer to be able to one line certain values from builtin functions. I hate writing
[~,~,extension] = fileparts( file );
[~,imax] = max(x);
When i want to use extension or imax directly in a statement. So put these functions in your path:
function out = out2(fun) % gets the second output argument of anonymous function fun
[~,out] = fun(); % fun must have no input arguments
end
function out = out3(fun) % gets third
[~,~,out] = fun();
end
% make a function for 4 or 5 or however many output arguments you have!
Then call these with
ext = out3(@() fileparts(file));
imax = out2(@() max(x));
Note whats happening here is that you're wrapping up your original function call in an anonymous function with no arguments. Then this gets called inside the outN function and the correct output argument is returned.
The syntax is a little more clunky, but hey, its one line, it allows something like:
switch out3(@() fileparts(file))
case '.csv'; % csv stuff
case '.txt'; % txt stuff
end
  1 Comment
Stephen23
Stephen23 on 6 Feb 2020
Edited: Stephen23 on 6 Feb 2020
See also this FEX submission, which is one function that selects any output argument/s and does not require an anonymous function (just a simple function handle):

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James Tursa
James Tursa on 9 Feb 2018
Edited: James Tursa on 9 Feb 2018
Unless the function has an input argument syntax that specifies only returning B, you can't do this. You can throw away that A immediately with this syntax however:
[~,B] = ismember(1,[0,1,2,3,4,5]);
That won't prevent the function from calculating that 1st result, it is just thrown away immediately upon return so that it doesn't clutter your workspace.
  2 Comments
James Tursa
James Tursa on 9 Feb 2018
It would be nice to have an "argout_wanted" that can be queried from within the function to find out which outputs the caller actually wants returned.

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Grigory Tagiltsev
Grigory Tagiltsev on 9 Feb 2018
You have to call the function anyway. You can do [~,B] = ismember(1,[0,1,2,3,4,5]); and avoid A. It doesn't decrease a number of lines though. You can also cheat a bit and do [B,B] = ismember(1,[0,1,2,3,4,5]); in this case only the second output will be written to B.

Han Zerui
Han Zerui on 24 Feb 2021
Edited: Han Zerui on 24 Feb 2021
Define your own ismember function:
data = rand(1,10);
result = data(myismember(1,[0,1,2,3,4,5]));
function Locb = myismember(varargin)
[~, Locb] = ismember(varargin);
end

Jeremy
Jeremy on 22 Aug 2022
none of these proved the requested solution. MATLAB is a command line environement. Not being able to call a specific argument from a function makes the programming more expanded. This really should be a option for all function calls.
Why not just use the solutions mentioned?
  1. As stated, it requires multple command line executions or a wrapper function that would be unnecssary if this was available by default
  2. If the comman [~,X,~,~]=func(A,B,C) has to be called in an m file just to be able to use X, X is now stored in the workspace
  3. If a funciton has to be writted to wrap "func", X has to be computed in func, returned by func then returned by the wrapper function, so inherently less effecient.
  2 Comments
Walter Roberson
Walter Roberson on 22 Aug 2022
None of them provide the requested solution because MATLAB does not support the requested functionality.

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