[R] How to append to a list dynamically?

[David Winsemius]

On Jun 17, 2009, at 7:52 AM, Nick Angelou wrote:

> I have a problem with dynamic appending to a list. Here is the list
> variable:
>
>
> clusters <- vector("list", 0)
>

  library(fortunes)
  fortune("dog")

I would suggest that referring to a character vector as a "list", and  
then also calling an element in that vector "list" is akin to calling  
your cat, "dog", and then calling its tail, "dog", as well.

>
> I extended in the function below:
>
>
> cluster <- function (pair, clusters)
> {
>  found <- FALSE
>  for (i in length(clusters))
>  {
>    if (length(intersect(pair, clusters[i])) > 0)
>    {
>     clusters[i] <- union(clusters[i], pair)
>     found <- TRUE
>    }
>  }
>  if (!found)
>  {
>   clusters <- list(clusters, as.vector(pair))

So the first time cluster() is called, it would need to handle a  
vector correctly, but now you have changed clusters to be a list and  
the second call to clusters will need to handle a list correctly.

>  }
> }
>
>
> The function is executed in a loop:
>
>
> for (i in 1:nrow(adjMatrix))
> {
> for (j in 1:nrow(adjMatrix))
> {
>  if ((i != j) && adjMatrix[i,j] >0) # the matrix element has to be  
> non-zero
> in order to be clustered
>  {
>    cat(rownames(adjMatrix)[i], colnames(adjMatrix)[j], "\n")
>    cluster(as.vector(c(rownames(adjMatrix)[i], colnames(adjMatrix) 
> [j])),
> clusters)

>  }
> }
> }
>
>
> But the list variable remains empty (i.e. length(clusters) = 0) even  
> though
> it should not. Somehow the dynamic extension of the list does not  
> work in
> this case. Any suggestions?

Use appropriate naming and do not conflate "vector" with "list". They  
need different functions to properly access elements or extend their  
lengths.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT