How to elegantly ignore some return values of a MATLAB function

Jordi_48648 0
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Is it possible to get the 'nth' return value from a function without having to create dummy variables for all n-1 return values before it?

Let's say, I have the following function in MATLAB:

function [a,b,c,d] = func()
a = 1;
b = 2;
c = 3;
d = 4;

Now suppose, I'm only interested in the third return value. This can be accomplished by creating one dummy variable:

[dummy, dummy, variableThatIWillUse, dummy] = func;
clear dummy;

But I think this is kind of ugly. I would think that you might be able to do something like one of the following things, but you can't:

[_, _, variableThatIWillUse, _] = func;

[, , variableThatIWillUse, ] = func;

variableThatIWillUse = func(3);

variableThatIWillUse = func()(3);

Are there any elegant ways to do this that do work?

So far, the best solution is to simply use the variableThatIWillUse as a dummy variable. This saves me from having to create a real dummy variable that pollutes the work-space (or that I would need to clear). In short: the solution is to use the variableThatIWillUse for every return value up until the interesting one. Return values after can simply be ignored:

[variableThatIWillUse, variableThatIWillUse, variableThatIWillUse] = func;

I still think this is very ugly code.

return-value matlab function
8 Answers

This is somewhat of a hack, but it works:

First a quick example function:

Func3 = @() deal(1,2,3);
[a,b,c]=Func3();
% yields a=1, b=2, c=3

Now the key here is that if you use a variable twice on the left-hand side of a multiple-expression assignment, an earlier assignment is clobbered by the later assignment:

[b,b,c]=Func3();
% yields b=2, c=3

[c,c,c]=Func3();
% yields c=3

(Just to check, I also verified that this technique works with [mu,mu,mu]=polyfit(x,y,n) if all you care about from polyfit is the third argument.)

There's a better approach; see ManWithSleeve's answer instead.

user_443300

If you wish to use a style where a variable will be left to fall into the bit bucket, then a reasonable alternative is

[ans, ans, variableThatIWillUse] = myfun(inputs);

ans is of course the default junk variable for MATLAB, getting overwritten often in the course of a session.

While I do like the new trick that MATLAB now allows, using a ~ to designate an ignored return variable, this is a problem for backwards compatibility, in that users of older releases will be unable to use your code.

I generally avoid using new things like that until at least a few MATLAB releases have been issued to ensure there will be very few users left in the lurch. For example, even now I find people are still using an old enough MATLAB release that they cannot use anonymous functions.

Admin1

Here's another option you can use. First make a cell array to capture all the outputs (you can use the NARGOUT function to determine how many outputs a given function returns):

a = cell(1,3);  % For capturing 3 outputs
% OR...
a = cell(1,nargout(@func));  % For capturing all outputs from "func"

Then call the function as follows:

[a{:}] = func();

Then simply remove the element from a that you want, and overwrite a:

a = a{3};  % Get the third output
gnovice_527380

You can make a function (or anonymous function) that only returns selected outputs, e.g.

select = @(a,b) a(b);

Then you can call your function like this:

select(func,2);
select(func,1:3);

Or you can assign the output to a variable:

output(1,2:4) = select(func,1:3);
user_12861090
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