On 09/05/2011 1:31 PM, David Winsemius wrote:
On May 9, 2011, at 11:57 AM, Ron Michael wrote:> Dear all, I would really appreciate if somebody can help me to > understand what does the phrase "Vectorize your function" mean? And > what is the job of Vectorize() function in doing that? I have read > many threads where experts suggest to Vectorize the function, which > will speed up entire calculation (and more elegant ofcourse.) > > I used to think that vectorizing function means, to create a > function such a way so that if input(s) are vector then this > function will also return a vector output (without having any extra > effort like use of 'for' loop etc.) Is this understanding correct? > > Here I have tried 2 essentially similar functions: > >> fn1<- function(x,y,z) return(x+y+z) >> fn2<- Vectorize(function(x,y,z) return(x+y+z), SIMPLIFY=TRUE) >> fn1(1:3, 4:6,5) > [1] 10 12 14 >> fn2(1:3, 4:6,5) > [1] 10 12 14 > > > You see that, fn1() and fn2() is giving same answer (vectorized?) > despite of the fact that fn1() is not wrapped within Vectorize() > function. Therefore I want to know: what additional thing this > Vectorize() function brings on the table? That is because you used "+" which is "vectorized" to start with. Try is with "sum" and you will not be as "happy" > sum(a=1:3, b=4:6, cc=7:9) [1] 45 It removed all your structure. > Vsum<-function(...) {what<- list(...); lenlist<- length(what); y<-matrix(unlist(what), ncol=lenlist); apply(y,1,sum)} > Vsum(a=1:3, b=4:6, cc=7:9) [1] 12 15 18 # Does not recycle as I might have expected, although it does recycle > (Vsum( a=1:3, b=4:6, cc=7)) [1] 12 8 11 > (mapply(sum, a=1:3, b=4:6, cc=7:9)) # also works and recycles as expected: [1] 12 15 18 > (mapply(sum, a=1:3, b=4:6, cc=7)) [1] 12 14 16 # but _not_ VSum<- Vectorize(sum) for reasons not clear to me behaves the same as sum()
The problem with Vectorize(sum) is that Vectorize doesn`t see any parameters to vectorize in sum(). It needs simple functions with parameters like fn1, and it rewrites them to loop over the values of the parameters.
Duncan Murdoch ______________________________________________ R-help@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.
