Simulation

Wacek Kusnierczyk wrote:

Barry Rowlingson wrote:

On Wed, May 13, 2009 at 5:36 PM, Wacek Kusnierczyk
<Waclaw.Marcin.Kusnierczyk at idi.ntnu.no> wrote:

Barry Rowlingson wrote:

 Soln - "for" loop:

 > z=list()
 > for(i in 1:1000){z[[i]]=rnorm(100,0,1)}

now inspect the individual bits:

 > hist(z[[1]])
 > hist(z[[545]])

If that's the problem, then I suggest she reads an introduction to
R...

i'd suggest reading the r inferno by pat burns [1], where he deals
with
this sort of for-looping lists the way it deserves ;)

 I don't think extending a list this way is too expensive. Not like

indeed, for some, but only for some, values of m and n, it can actually
be half a hair faster than the matrix and the replicate approaches,
proposed earlier by others:

another approach to create a matrix, a bit more efficient than using
matrix() but also clean for beginners IMO, is to directly assign
dimensions to a vector, e.g.,

library(rbenchmark)

n=100; m=100
benchmark(replications=100, columns=c('test', 'elapsed'), order=NULL,
    list={ l=list(); for (i in 1:n) l[[i]] = rnorm(m) },
    liist={ l=vector('list', n); for (i in 1:n) l[[i]] = rnorm(m) },
    matrix=matrix(rnorm(n*m), n, m),
    matrix2 = {mat <- rnorm(n*m); dim(mat) <- c(n, m); mat},
    replicate=replicate(m, rnorm(n))
)