[Rd] 'vapply' not returning list element names when returned element is a length-1 list

Suharto Anggono Suharto Anggono suharto_anggono at yahoo.com
Thu Aug 6 06:45:19 CEST 2015


> If i have a function that returns a named list with 2 (or more) elements,
> then using 'vapply' retains the names of the elements:
> ....
> But if the function only returns one element, then the name "foo" is lost

vapply _always simplifies_ according to the documentation.

In the first case (function return value contains more than one element, and each ), vapply simplifies to a matrix of two lists (!).  The names "foo" and "hello" have been added to the dimnames so you can tell which is which.

in the second case the function return value is a single list and not a matrix of lists (a simple list is simpler than a matrix of lists). The name of the list ('foo') has nowhere to go; instead, you would be assigning the list to a named variable and you don't need the name 'foo'.

Whether that is inconsistent is something of a matter of perspective. Simplification applied as far as possible will always depend on what simplification is possible for the particular return values, so different return values provide different behaviour.

S Ellison 


In the first case, the result is a matrix of mode list, which is a list with "dim" attribute of length 2.

For comparison, 'sapply' retains the name "foo".

> sapply(1:3, function(x) list("foo" = "bar"))
[1] "bar"

[1] "bar"

[1] "bar"

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