On Fri, 18 Dec 2009, Gabriel Becker wrote:
My understanding is that all the really fast vectorized operations are
implemented down in C code, not in R. Thus if you wanted to write a
vectorized switch, which I agree would be rather nice to have, you'd need to
do it down there and then write a .Call wrapper for it in R.
The ifelse (C) code would probably be a good place to start looking in terms
of how to write it.
Gabe: ifelse() is not in C, nor is it really fast (though it is better than it
used to be).
Stavros: I don't think there is a standard idiom. I tend to use match() to
work out which choice applies for each element, or nested ifelse if there are
only three choices.
-thomas
Gabe
On Fri, Dec 18, 2009 at 11:07 AM, Stavros Macrakis <macra...@alum.mit.edu>wrote:
What is the 'idiomatic' way of writing a vectorized switch statement?
That is, I would like to write, e.g.,
vswitch( c('a','x','b','a'),
a= 1:4,
b=11:14,
100 )
=> c(1, 100, 13, 4 )
equivalent to
ifelse( c('a','x','b','a') == 'a', 1:4,
ifelse( c('a','x','b','a') == 'b', 11:14,
100 ) )
A simple way of doing this is (leaving aside the default case):
colchoose <- function(frame,selector)
mapply(function(a,b)frame[a,b],seq_along(frame[1]),selector))
colchoose( data.frame(a=1:4,b=11:14), c('a','b','b','a'))
=> c(1,11,11,1)
But of course this is not very efficient compared to the way ifelse works.
Is there a standard function or idiom for this (am I missing something
obvious?), or should I write my own?
-s
[[alternative HTML version deleted]]
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
[[alternative HTML version deleted]]
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
