[Bioc-devel] sapply and vapply

[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