[R] [FORGED] Having trouble understanding the sapply/vapply documentation and behaviour of USE.NAMES

[Rolf Turner]

On 10/04/20 10:59 am, petr smirnov wrote:

> I am having trouble parsing the documentation for sapply and vapply,
> and I cannot understand if it explains the different behaviour of
> USE.NAMES between the two.
> 
> I noticed the following different behaviour between the two functions:
> 
>> sapply(c("1"=1, "2"=2, "3"=3), function(x) {r <- list(x); r}, USE.NAMES=FALSE)
> $`1`
> [1] 1
> 
> $`2`
> [1] 2
> 
> $`3`
> [1] 3
> 
>> vapply(c("1"=1, "2"=2, "3"=3), function(x) { r <- list(x); r}, FUN.VALUE=list(1), USE.NAMES=FALSE)
> [[1]]
> [1] 1
> 
> [[2]]
> [1] 2
> 
> [[3]]
> [1] 3
> 
> In the sapply case, the names of the input vector are retained. In the
> vapply case, they are dropped. Note that this is not true when
> USE.NAMES=TRUE:
> 
>> vapply(c("1"=1, "2"=2, "3"=3), function(x) { r <- list(x); r}, FUN.VALUE=list(1), USE.NAMES=TRUE)
> $`1`
> [1] 1
> 
> $`2`
> [1] 2
> 
> $`3`
> [1] 3
> 
> The manual page explains this for the names of the result of vapply:
> 
> The (Dim)names of the array value are taken from the FUN.VALUE if it
> is named, otherwise from the result of the first function call. Column
> names of the matrix or more generally the names of the last dimension
> of the array value or names of the vector value are set from X as in
> sapply.
> 
> 
> If this explains the behaviour, could someone break it down for me and
> help me understand the reasoning?
> Otherwise, is this different behaviour intentional? Should it be
> documented more clearly?

IMHO there is an error in the documentation here.  Clearly USE.NAMES has 
a different impact on vapply() than it has on sapply() and the 
documentation does not indicate this, in fact quite the opposite.

It might be appropriate to submit a bug report (see
https://www.r-project.org/bugs.html) but it's probably not worth the 
hassle.  It's a pretty minor error.

cheers,

Rolf Turner

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276