R help - Feb 2002 - looping through lists...

Hi, all.  I have a whole group of lists with things like this:

nw.1$cond (a string)
nw.1$RL (a vector of 10 values)
nw.1$IL (a vector of 10 values)

and I have lots of these lists:
nw.201
nw.202
nw.203
...

what I'd like to do is be able to get specific values from all these
lists (like the mean of $RL) for each indivividual list.  I can
certainly do this manually
mean(c(nw.201.1$RL,nw.204.1$RL,nw.206.1$RL)) to get the overall mean
or write some simple code to return a vector of means:
[1] 15.630  2.713 18.811
(the problem with my current function is that I have to hard code the
individual list names which is what I'm trying to avoid).

but what I'd rather do is have a list of lists that I can then cycle
through to get the individual values for.  mostly, I just don't want
to have to type in the (nw.201.1$RL,nw.204.1$RL,nw.206.1$RL) every
time I want the values.  I tried
nw.1 <- c(nw.201.1,nw.204.1,nw.206.1)

but that did not preserve the individual list structure I needed.

and of course I use more than just RL, so I want to be able to
arbitrarily get information from each list.

ideally, I'd like to be able to do something like this:

getmean(nw.1, "RL")
[1] 15.630  2.713 18.811

any suggestions on how to cycle through lists or assign lists to a
variable that keeps their list-structure?

thanks!
greg
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Greg Trafton <trafton at itd.nrl.navy.mil> writes:
> nw.1 <- c(nw.201.1,nw.204.1,nw.206.1)
> 
> but that did not preserve the individual list structure I needed.
> 
> and of course I use more than just RL, so I want to be able to
> arbitrarily get information from each list.
> 
> ideally, I'd like to be able to do something like this:
> 
> getmean(nw.1, "RL")
> [1] 15.630  2.713 18.811
> 
> any suggestions on how to cycle through lists or assign lists to a
> variable that keeps their list-structure?
Something like 

nw.1 <- list(nw.201.1, nw.204.1, nw.206.1)
sapply(nw.1, function(x) mean(x$RL))

should do it, or generally

getmean <- function(l,n) sapply(l, function(x) mean(x[[n]]))

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
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