Jonathan Greenberg
2013-Sep-10 19:40 UTC
[R] ifelse question (I'm not sure why this is working)...
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
William Dunlap
2013-Sep-10 19:58 UTC
[R] ifelse question (I'm not sure why this is working)...
> remainderFunction<-function(x,d) > { > ifelse(x%%d==0,yes=return(which(x%%d==0)),no=return(NULL)) > } > remainderFunction(x=c(23:47),d=3)The above call returns c(2, 5, 8, 11, 14, 17, 20, 23), the value of (23:47)%%3. Note that remainderFunction(integer(0), 3) returns logical(0). The return() calls in the call to ifelse cause the problem. The one used for the second argument to ifelse causes remainderFunction to return, abandoning the evaluation of ifself, as soon as ifelse evaluates its second argument. I think that using return this way is bad practice, but have seen it in code like tryCatch(return(something), error=function(e)"Error in something") which is common in support code for RStudio. Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com> -----Original Message----- > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf > Of Jonathan Greenberg > Sent: Tuesday, September 10, 2013 12:40 PM > To: r-help > Subject: [R] ifelse question (I'm not sure why this is working)... > > 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 > > ______________________________________________ > 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.
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. Jim
David Winsemius
2013-Sep-11 00:21 UTC
[R] ifelse question (I'm not sure why this is working)...
On Sep 10, 2013, at 12:40 PM, 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.I think it did what you expected (at east your second expectation). Look at: c(NULL, 1,2,3, NULL, 4,5,6, NULL) # [1] 1 2 3 4 5 6 Obviously teh ifelse is not needed since this is cleaner code: remainderFunction<-function(x,d) { which(x%%d==0) } remainderFunction(x=c(23:47),d=3) # [1] 2 5 8 11 14 17 20 23 -- David.> > 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 > > ______________________________________________ > 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.David Winsemius Alameda, CA, USA