[R] loop question

[Joshua Wiley]

Dear Thomas,

On Tue, Apr 5, 2011 at 8:33 AM, Thomas <thomas.triebs at cantab.net> wrote:
> Dear all,
>
> I am trying to set up a list with 1:c objects each meant to capture the
> coefficients for one coefficient and 100 replications. I receive the
> following error message:
>
> Error in betaboot[[p]] : subscript out of bounds.
>
> My code is below. Where is my mistake?
>
> Many thanks,
>
> Thomas
>
> _________________________________
> betaboot<-list(NULL)

if you know the number of bootstraps (which you seem to later on), a
preferred way to instatiate the list would be:

betaboot <- vector(mode = "list", length = yourlength)

>
> for (i in 1:c) {

because "c()" is such an important function, I would strongly
encourage you not to use it also as a variable.

> betaboot[[i]]<-cbind()

Don't use this to build an empty list.

> }
>
>
> num <- 100 # this is the number of bootstraps
>
> for (i in 1:num) {
>
>    [BOOTSTRAP]
>
>  coef.temp <- coef(model.temp, data=newdata)
>
>  for (p in 1:c){
>  betaboot[[p]] <- cbind(betaboot[[p]], coef.temp[,p])

This should work assuming betaboot is instatiated properly.  That
said, it looks like you have a nested for loop and then just keep
cbind()ing each element of betaboot bigger and bigger.  You may get a
performance increase if you also instantiate each matrix/dataframe
inside betaboot.  Then the call would become something like:

betaboot[[i]][,p] <- coef.temp[,p]

that is, you can use a chained series of extraction operators to get
to the appropriate column in the matrix/dataframe inside the
appropriate list element.  Then rather than constantly using cbind(),
you just place coef.temp[,p] where you want it.  The only requirement
is that you know the sizes of the matrices/dataframes going in so you
can create empty ones from the get go.

Cheers,

Josh

>  }
>
>  }
>
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-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/