[R] Vectors out of lists?

[Bill.Venables at csiro.au]

That was my thought, and if you are going to be integrating it, you do need to be concerned with efficiency to some extent, I would imagine.

My experience is that Vecotrize() is theoretically interesting and it is great for getting you out of a tight spot like this, but if you can avoid it without too much trouble, so much the better.

Bill V. 

-----Original Message-----
From: Phil Spector [mailto:spector at stat.berkeley.edu] 
Sent: Thursday, 18 November 2010 2:30 AM
To: Venables, Bill (CMIS, Cleveland)
Cc: eduardo.oliveirahorta at gmail.com; r-help at r-project.org
Subject: Re: [R] Vectors out of lists?

Bill -
     An excellent point, especially if you're concerned 
about 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
 					 spector at stat.berkeley.edu


On Wed, 17 Nov 2010, Bill.Venables at 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-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Phil Spector
> Sent: Wednesday, 17 November 2010 10:24 AM
> To: Eduardo de Oliveira Horta
> Cc: r-help at 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
> 					 spector at 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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