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

[peter dalgaard]

On Sep 10, 2013, at 23:39 , Jim Lemon wrote:

> On 09/11/2013 05:40 AM, Jonathan Greenberg wrote:
>> 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
>> 
> Hi Jonathan,
> While this has already been answered, the question was "why does it work?". As Bill Dunlap pointed out, it is because the "return" does not allow the ifelse to complete. That was not a problem for the student, for it did do what was requested. It is just an unnecessary elaboration of the code, for:
> 
> remainderFunction<-function(x,d) {
> which(x%%d==0)
> }
> 
> works just as well. I think Bill was pointing out the the order of evaluation was important, for:
> 
> remainderFunction<-function(x,d) {
> which(return(x%%d==0))
> }
> 
> doesn't work. The student probably deserves a Rube Goldberg award.


It's slightly trickier: the yes= part of an ifelse is not evaluated unless at least one element of the test is TRUE, so I think the equivalence is closer to

remainderFunction<-function(x,d)
  if (any(x %% d == 0) ) which(x %% d==0) 

except that ifelse() has the curious feature of returning logical(0) if the test vector is logical(0) (evaluating neither of its arguments).

(If you want to be really picky, the above returns NULL _invisibly_ if the any() condition fails. Whether or not the student deserves a little credit for thinking about that case is up in the air, I think. He/she may have failed to grasp the meaning of integer(0).)

> 
> Jim
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com