[R] Odp: Book: The New S Language

[Petr PIKAL]

Hi

r-help-bounces at r-project.org napsal dne 12.12.2007 07:45:54:

> I'm struggling a bit with R with understanding functions and what's 
going
> on under the hood.
> 
> For example, I was given this snippet of code via this mailing list:
> 
> DF <- data.frame(A =c("A", "A", "A", "B", "C"), B=c(1,1,2,2,0))
> g <- paste(DF$A, DF$B)
> s <- split(DF, g)
> 
> split appears to be taken the string provided by paste, e.g. "A 1", and
> then deciding that the first character "A" in this string relates to 
DF$A,
> and the second character "1" to DF$B.
> 
> The split help pages don't really tell me why split is working like this 
-
> maybe I'm just looking in the wrong place in the help page.

Maybe you misunderstand what split does:

> DF <- data.frame(A =c("A", "A", "A", "B", "C"), B=c(1,1,2,2,0))
a data frame with 2 columns

> g <- paste(DF$A, DF$B)

a character vector (which is internally turned into factor). And split 
splits your data frame DF into groups according to levels of "g". It does 
not at all lookup into columns of your data frame (and the help page does 
not even give you a suspicion that it shall behave like that).

first sentence in help page tells you
split divides the data in the vector x into the groups defined by f.

compare your result with
> g.f<-factor(g)
> str(g.f)
 Factor w/ 4 levels "A 1","A 2","B 2",..: 1 1 2 3 4
> g.n<-as.numeric(g.f)
> g.n
[1] 1 1 2 3 4

split(DF, g.f)
split(DF, g.n)

And try to explain with your idea how split works this
DF$A<-rnorm(5)
s <- split(DF, g)

Regards
Petr

> 
> I've got plenty of books on R, but only one of them gives a bit of info 
on
> the use of split - not in enough detail though.  Does the book 
referenced
> in the help pages ("The New S Language") give more insight into
> this and other functions?
> 
> Will I just get an Aha! moment with R after using it for a while?
> 
> Thanks in advance...
> 
> Chris
> 
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