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()
Thanks for your time.
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David Winsemius, MD
West Hartford, CT
______________________________________________
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