[R] do.call vs. lapply for lists

[Muenchen, Robert A (Bob)]

Hi All,

I'm trying to understand the difference between do.call and lapply for
applying a function to a list. Below is one of the variations of
programs (by Marc Schwartz) discussed here recently to select the first
and last n observations per group.

I've looked in several books, the R FAQ and searched the archives, but I
can't find enough to figure out why lapply doesn't do what do.call does
in this case. The help files & newsletter descriptions of do.call sound
like it would do the same thing, but I'm sure that's due to my lack of
understanding about their specific terminology. I would appreciate it if
you could take a moment to enlighten me. 

Thanks,
Bob

mydata <- data.frame(
  id      = c('001','001','001','002','003','003'),
  math    = c(80,75,70,65,65,70),
  reading = c(65,70,88,NA,90,NA)
)
mydata

mylast <- lapply( split(mydata,mydata$id), tail, n=1)
mylast
class(mylast) #It's a list, so lapply will so *something* with it.

#This gets the desired result:
do.call("rbind", mylast)

#This doesn't do the same thing, which confuses me:
lapply(mylast,rbind)

#...and data.frame won't fix it as I've seen it do in other
circumstances:
data.frame( lapply(mylast,rbind) )

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