Re: [R] Vectors out of lists?

Phil Spector Wed, 17 Nov 2010 08:31:37 -0800

Bill -

An excellent point, especially if you're concernedabout efficiency:

Y = list(sqrt,sin,function(u)u/2)
Ybar0 = function(u)mean(sapply(Y,function(fun)fun(u)))
Ybar1 = function(u) rowMeans(sapply(Y, function(fun) fun(u)))
system.time(one <- Vectorize(Ybar0)(seq(0,1,length=10000)))

   user  system elapsed

1.324 0.000 1.323

system.time(two <- Ybar1(seq(0,1,length=10000)))

   user  system elapsed
  0.004   0.000   0.002

                                        - Phil Spector
                                         Statistical Computing Facility
                                         Department of Statistics
                                         UC Berkeley
                                         spec...@stat.berkeley.edu


On Wed, 17 Nov 2010, bill.venab...@csiro.au wrote:

Another approach would be

Y <- list(sqrt, sin, function(u) u/2)
Ybar <- function(u) rowMeans(sapply(Y, function(fun) fun(u)))

integrate(Ybar, 0, 1)

0.4587882 with absolute error < 5.6e-05


i.e. make the function vectorized directly.

Note, however, that if you had

Y[[4]] <- function(u) 1

you would need to be careful and use something like

Y[[4]] <- function(u) rep(1, length(u))

or indeed

Y[[4]] <- Vectorize(function(u) 1)

for the process to work.

Bill Venables.

-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On 
Behalf Of Phil Spector
Sent: Wednesday, 17 November 2010 10:24 AM
To: Eduardo de Oliveira Horta
Cc: r-help@r-project.org
Subject: Re: [R] Vectors out of lists?

Eduardo -
   Thanks for the reproducible example!

Y<-list()
Y[[1]]<-function(u) sqrt(u)
Y[[2]]<-function(u) sin(u)
Y[[3]]<-function(u) 1/2*u
Ybar = function(u)mean(sapply(Y,function(fun)fun(u)))


Since integrate requires a function which accepts a vector
and returns a vector, we'd need to use Vectorize() before
trying to integrate:

integrate(Vectorize(Ybar),0,1)

0.4587882 with absolute error < 5.6e-05

                                        - Phil Spector
                                         Statistical Computing Facility
                                         Department of Statistics
                                         UC Berkeley
                                         spec...@stat.berkeley.edu




On Tue, 16 Nov 2010, Eduardo de Oliveira Horta wrote:

Thanks, guys... but it seems these suggestions won't work.

Let me try to be more specific with a simple example:

Y<-list()
Y[[1]]<-function(u) sqrt(u)
Y[[2]]<-function(u) sin(u)
Y[[3]]<-function(u) 1/2*u

I wanted something equivalent to

Ybar<-function(u){
   1/3*(Y[[1]](u) + Y[[2]](u) + Y[[3]](u))
}

but with arbitrary length(Y) and without using any loops. Also, I can't allow
for discretization, since I must be able to evaluate Ybar at any u, as I'm
going to integrate it with the function "integrate".

Thanks again,

Eduardo Horta

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