[R] another do.call() problem.

[Peter Dalgaard]

Robin Hankin <rksh at soc.soton.ac.uk> writes:

> Hi everyone
> 
> suppose I have
> 
> a <- array(1:256,rep(4,4))
> 
> and want to access a[1,2,3,1] by the vector c(1,2,3,1).  As per
> yesterday, I can use do.call():
> 
> a[1,2,3,1] == do.call("[",c(list(a),c(1,2,3,1)))
> 
> Now how do I apply the above technique (or indeed any other technique!)  to get
> 
> a[1,2,3,]
> 
> [1]  37 101 165 229
> 
> from a vector like c(1,2,3,0) or c(1,2,3,NULL) or c(1,2,3,NA)?

The middle one is impossible. The other two can be done like this

  v <- c(1,2,3,NA)
  al <- lapply(v,function(x) if(is.na(x)) alist(a=)$a else x )
  do.call("[",c(list(a),al))

This is in treacherous territory: The missing argument indicator has
some very strange semantics, for instance if you save it in a
variable, then any subsequent use of that variable will trigger the
same error as when you access a missing argument in a function:

  mis <- alist(a=)$a
  lapply(v,function(x)if(is.na(x))mis else x )
Error in FUN(X[[4]], ...) : Argument "mis" is missing, with no default

Technically, the missing argument is implemented as a zero-length
variable name 

> as.character(alist(a=)$a)
[1] ""
> mode(alist(a=)$a)
[1] "name"

However, I don't think it is a good idea to rely on that, and anyway,
as.name("") is forbidden:

> as.name("")
Error in as.name("") : attempt to use zero-length variable name

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907