apply for nested lists

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch 
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of 
Alexandre Sanchez Pla
Sent: Wednesday, January 26, 2005 8:50 AM
To: r-help at stat.math.ethz.ch
Subject: [R] apply for nested lists

Hi,

I am working with lists whose terms are lists whose terms are 
lists. Although 
the real ones contain locuslink identifiers and GO 
annotations (I work with the 
Bioconductor GO) package, I have prepared an simplified 
example of what I have 
and what I would like to do with it:

Imagine I have a list such as:

tst.list<-list("1"=list("1A"=list(ID="1A",VAL=172),"1B"=list
(ID="1B",VAL=134),"1C"=list(ID="1C",VAL=0)),"2"=list("2A"=NA),
"3"=list("3A"=list
(ID="3A",VAL=33),"3B"=list(ID="3B",VAL=2)))

I would like, for instance, to be able to extract some values 
such as the 
content of the "VAL" field, which may sometimes not be available.
I may do it using a nested for such as:

x<-character(0)
for (i in 1:length(tst.list)){
    if (!is.na(tst.list[[i]][[1]][[1]])){
        for (j in 1:length(tst.list[[i]]))
            {x<-c(x,tst.list[[i]][[j]]$VAL)}}
    else    
            {x<-c(x, NA)}}

which gives me what I need

x

[1] "172" "134" "0"   NA  "33"  "2"  

According to most R documents this may be done more 
efficiently using apply 
instructions, but I have failed in my temptatives to obtain the same

Thanks for any help.

Alex


-- 
Dr.Alex S?nchez
Departament d'Estad?stica.
Universitat de Barcelona
asanchez at ub.edu