[Bioc-devel] sapply and vapply

FWIW,
sapply(X, length) - always numeric(1) (integer(1) or double(1) for vectors of more than 2^31 - 1 elements)
Actually, the length of length(x) may not be 1L, e.g.
x <- Formula::Formula(~ x)
length(x)
[1] 0 1
From help("length", package = "base"):
"Warning: Package authors have written methods that return a result of
length other than one (Formula) and that return a vector of type
double (Matrix), even with non-integer values (earlier versions of
sets). Where a single double value is returned that can be represented
as an integer it is returned as a length-one integer vector."

I/we recently learned this the hard way
(https://github.com/HenrikBengtsson/future/issues/395).  It's rather
unfortunate that not even length() is strictly defined here, I'd say.
I think we could move away from this if lengths() would be a generic
so lengths(x) could be used above.  But that's a discussion for
R-devel.

/Henrik

On Wed, Aug 12, 2020 at 9:33 AM Laurent Gatto
Dear all,

I have a quick question regarding the usage of vapply and sapply. The former is recommended to insure that the output is always a vector of a specific type. For example:

df1 <- data.frame(x = 1:3, y = LETTERS[1:3])     ## OK test
df2 <- data.frame(x = 1:3, y = Sys.time() + 1:3) ## Not OK test
sapply(df1, class) ## vector of chars, OK
          x           y
  "integer" "character"
sapply(df2, class) ## ouch, not a vector
$x
[1] "integer"

$y
[1] "POSIXct" "POSIXt"

vapply(df2, class, character(1)) ## prefer an error rather than a list
Error in vapply(df2, class, character(1)) : values must be length 1,
 but FUN(X[[2]]) result is length 2

There are cases, however, were FUN ensures that the output will be of length 1 and of a expected type. For example

- sapply(X, all) - all() always returns logical(1)
- sapply(X, length) - always numeric(1) (integer(1) or double(1) for vectors of more than 2^31 - 1 elements)

or more generally

- sapply(X, slot, "myslot") - slot() will always return a character(1) because @myslot is always character(1) (as defined by the class)

Would you still recommend to use vapply() in such cases?

Thank you in advance.

Laurent

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[Bioc-devel] sapply and vapply

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