R help - Feb 2009 - accessing and preserving list names in lapply

Sometimes I'm iterating over a list where names are keys into another  
data structure, e.g. a related list.  Then I can't use lapply as it  
does [[]] and loses the name.  Then I do something like this:

do.one <- function(ldf) { # list-dataframe item
   key <- names(ldf)
   meat <- ldf[[1]]
   mydf <- some.df[[key]] # related data structure
   r.df <- cbind(meat,new.column=computed)
   r <- list(xxx=r.df)
   names(r) <- key
   r
}

then if I operate on the list L of those ldf's not as lapply(L,...), but

res <- lapply(1:length(L),do.one)

Can this procedure be simplified so that names are preserved?   
Specifically, can the xxx=..., xxx <- key part be eliminated -- how  
can we have a variable on the left-hand side of list(lhs=value)?

Cheers,
Alexy

> res <- lapply(1:length(L),do.one)
Actually, I do

res <- lapply(:length(L),function(x)do.one(L[x]))

-- this is the price of needing the element's name, so I have to both  
make do.one extract the name and the meat separately inside, and  
lapply becomes ugly.  Yet the obvious alternatives -- extracting the  
names separately, attaching them back into list elements, etc., -- are  
even uglier.  Something pretty? :)

Cheers,
Alexy

Hi,

Perhaps Hadley's plyr package can help,
> library(plyr)
> temp <- list(x=2,y=3,x=4)
> llply(temp, function(x) x^2 )
>
> $x
> [1] 4
>
> $y
> [1] 9
>
> $x
> [1] 16

baptiste

On 27 Feb 2009, at 03:07, Alexy Khrabrov wrote:
> Sometimes I'm iterating over a list where names are keys into another
> data structure, e.g. a related list.  Then I can't use lapply as it
> does [[]] and loses the name.  Then I do something like this:
>
> do.one <- function(ldf) { # list-dataframe item
>   key <- names(ldf)
>   meat <- ldf[[1]]
>   mydf <- some.df[[key]] # related data structure
>   r.df <- cbind(meat,new.column=computed)
>   r <- list(xxx=r.df)
>   names(r) <- key
>   r
> }
>
> then if I operate on the list L of those ldf's not as lapply(L,...),  
> but
>
> res <- lapply(1:length(L),do.one)
>
> Can this procedure be simplified so that names are preserved?
> Specifically, can the xxx=..., xxx <- key part be eliminated -- how
> can we have a variable on the left-hand side of list(lhs=value)?
>
> Cheers,
> Alexy
>
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_____________________________

Baptiste Augui?

School of Physics
University of Exeter
Stocker Road,
Exeter, Devon,
EX4 4QL, UK

Phone: +44 1392 264187

http://newton.ex.ac.uk/research/emag