[R] ifelse question (I'm not sure why this is working)...

[Jonathan Greenberg]

R-helpers:

One of my intrepid students came up with a solution to a problem where
they need to write a function that takes a vector x and a "scalar" d,
and return the indices of the vector x where x %% d is equal to 0 (x
is evenly divisible by d).  I thought I had a good handle on the
potential solutions, but one of my students sent me a function that
WORKS, but for the life of me I can't figure out WHY.  Here is the
solution:

remainderFunction<-function(x,d)
{
   ifelse(x%%d==0,yes=return(which(x%%d==0)),no=return(NULL))
}
remainderFunction(x=c(23:47),d=3)

I've never seen an ifelse statement used that way, and I was fully
expecting that to NOT work, or to place the output of which(x%%d==0)
in each location where the statement x%%d==0 was true.

Any ideas on deconstructing this?

--j

-- 
Jonathan A. Greenberg, PhD
Assistant Professor
Global Environmental Analysis and Remote Sensing (GEARS) Laboratory
Department of Geography and Geographic Information Science
University of Illinois at Urbana-Champaign
607 South Mathews Avenue, MC 150
Urbana, IL 61801
Phone: 217-300-1924
http://www.geog.illinois.edu/~jgrn/
AIM: jgrn307, MSN: jgrn307 at hotmail.com, Gchat: jgrn307, Skype: jgrn3007