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Elegantly refer to the second output from a function

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Xin
Xin 9 Feb 2018
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

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답변(3개)

Hank
Hank 6 Feb 2020
편집: Hank 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

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Stephen Cobeldick
Stephen Cobeldick 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 9 Feb 2018
편집: James Tursa 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.

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Walter Roberson
Walter Roberson 9 Feb 2018
Right. Unfortunately there is no way in MATLAB to specify that you want to extract a particular output number of an expression and use that: you have to write the outputs to variables.
James Tursa
James Tursa 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 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.

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