R help - May 2010 - Building a list

Hello,

I need to build a "list of lists"

We have 20 groups we are generating MCMC samples for.  There are 10
coefficients, and 10000 MCMC iterations.

I would like to store each iteration by-group in a list.  My problem is
with the first iteration.

Here is a toy example:

Chain <- list()
for (j in 1:10000){
    coef <- c(1,2,3,4,5,6,7,8,9,10)  #would be actual MCMC samples
    Chain[[j]] <- rbind(Chain[[j]], coef)
}

This returns an error, UNLESS I  initialize the first row of Chain[[j]]
with something.

The idea is that for any group, I can quickly extract, plot, average,
etc the values for each coefficient.

for example:

Chain[[5]][,3] will give me all 10,000 values of coefficient 3 for group
5. 

Again, this seems to work, but I can't initialize the chain with a
random value as it will cause problems with the data summary later.
(Each row in Chain[[j]] will be out of sync by 1, subsequently all
summary and plotting work will have to account for this - it can get
messy in a large program.)

Is there an easier way to do this?  Am I missing something?

Hello Noah,

Does this work for you?

Chain <- vector("list", 10000)
for (j in 1:10000){
   coef <- c(1,2,3,4,5,6,7,8,9,10)  #would be actual MCMC samples
   Chain[[j]] <- rbind(Chain[[j]], coef)

If it does, this has the additional advantage that it tends to be
faster to initialize the list at size rather than expanding it as
needed.

HTH,

Josh

On Sun, May 30, 2010 at 2:52 PM, Noah Silverman <noah at
smartmediacorp.com> wrote:
> Hello,
>
> I need to build a "list of lists"
>
> We have 20 groups we are generating MCMC samples for. ?There are 10
> coefficients, and 10000 MCMC iterations.
>
> I would like to store each iteration by-group in a list. ?My problem is
> with the first iteration.
>
> Here is a toy example:
>
> Chain <- list()
> for (j in 1:10000){
> ? ?coef <- c(1,2,3,4,5,6,7,8,9,10) ?#would be actual MCMC samples
> ? ?Chain[[j]] <- rbind(Chain[[j]], coef)
> }
>
> This returns an error, UNLESS I ?initialize the first row of Chain[[j]]
> with something.
>
> The idea is that for any group, I can quickly extract, plot, average,
> etc the values for each coefficient.
>
> for example:
>
> Chain[[5]][,3] will give me all 10,000 values of coefficient 3 for group
> 5.
>
> Again, this seems to work, but I can't initialize the chain with a
> random value as it will cause problems with the data summary later.
> (Each row in Chain[[j]] will be out of sync by 1, subsequently all
> summary and plotting work will have to account for this - it can get
> messy in a large program.)
>
> Is there an easier way to do this? ?Am I missing something?
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Joshua Wiley
Senior in Psychology
University of California, Riverside
http://www.joshuawiley.com/