Force the number of outputs from function call without setting output variables

Question

For any function, I am able to dictate the variables to be returned by explicitly defining how many outputs I am expecting [out1,out2,...,outn] = ...

Edit: Being able to maximize potential # of outputs would also be useful

Example problem

The following code does exactly what is expected (yes, it is redundant with myArray(IND) = 1;)

[I,J] = ind2sub(size(myArray),IND)
myArray(I,J) = 1;

When I try to pass the function arguments directly, I don't get the results I wannt

myArray(ind2sub(size(myArray),IND)) = 1;

I effectively get myArray(I) = 1; when I wanted myArray(I,J) = 1;

Question

How can I dictate how many output variables are returned without explicitly defining my output arguments?

I expect some function in the eval() family or some typecasting [],{},(:), etc. will do the trick but I have not seen any documentation or gotten any of them to work.

Community · Accepted Answer · 2017-05-23 10:30:55Z

5

The immediate issue with using the output of ind2sub directly as an index into myArray without an intermediate variable is that only the first output of ind2sub is requested by subsindex when indexing into myArray. The first output is then used as a linear index to index into myArray and assign values which resulting in your unexpected behavior.

Instead, you can use a cell array to capture as many outputs as you want and then rely on {} indexing to yield a comma-separated list.

[outputs{1:ndims(myArray)}] = ind2sub(size(myArray), IND);

You can then use this cell array containing all outputs to then forward all values as inputs or subscripts elsewhere:

myArray(outputs{:}) = 1;

That being said, in the example that you've shown, you really don't need to do any of this as you can use the linear indices IND to index into myArray directly. In fact, using the output of ind2sub will likely give you the incorrect assignment as every permutation of the subscripts will be used when assigning values rather than the element-wise pairing of subscripts.

myArray(IND) = 1;

In general, you can use this technique along with nargout to request all output arguments if there is a non-variable number of outputs.

[outputs{1:nargout('func')}] = func(inputs);

edited May 23, 2017 at 10:30

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answered Mar 31, 2017 at 15:59

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9 Comments

Brendan Frick Over a year ago

Ah thank you, you've highlighted why there is a need for explicit output definitions for builtin functions with variable outputs. I was hoping for an in-line solution that allows explicit output definitions without variable assignment (i.e. eval('foo(...),"nargout", 2') ) but I have not seen any evidence that one exists

Suever Over a year ago

@BrendanFrick I'm confused. There is. it's [outputs{1:2}] = thing

Brendan Frick Over a year ago

Can you call that inline? foo1([outputs{1:2}] = foo2(...))

Suever Over a year ago

@BrendanFrick No you need the temporary variable. I see what you are referring to now

Suever Over a year ago

@BrendanFrick Did you read my entire answer? The outputs that you're forwarding the way that you are aren't going to yield the correct result. See the linked post

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