Aha! I haven't thought about it. I really like the approach presented
by Bert Gunter in the previous post. It is a good lesson.
I made my previous code a little bit better by building a function that
pulls out only the desired component. At this time, the names of
sublists are changed as below (i.e. the names of 'A', 'B', 'C' in the
example given by Rainer M Krug are changed to 'A', 'Aha', 'alpha' here):
> x1 <- list(A = 11, Aha = 21, alpha = 31)
> x2 <- list(A = 12, Aha = 22, alpha = 32)
> x3 <- list(A = 13, Aha = 23, alpha = 33)
> x4 <- list(A = 14, Aha = 24, alpha = 34)
> y1 <- list(x1 = x1, x2 = x2)
> y2 <- list(x3 = x3, x4 = x4)
> x <- list(f1 = y1, f2 = y2)
>
> extr.1 <- function(x, name){
+ xl <- unlist(x)
+ depth <- sum(unlist(strsplit(names(xl)[1], split="")) == ".") + 1
+ xl[grep(paste("^",name,"$", sep=""), unlist(strsplit(names(xl), ".",
fixed=TRUE)))/depth]
+ }
> extr.1(x=x, name="alpha")
f1.x1.alpha f1.x2.alpha f2.x3.alpha f2.x4.alpha
31 32 33 34
> extr.1(x=x, name="A")
f1.x1.A f1.x2.A f2.x3.A f2.x4.A
11 12 13 14
> extr.1(x=x, name="a")
named numeric(0)
> extr.1(x=x, name="Aha")
f1.x1.Aha f1.x2.Aha f2.x3.Aha f2.x4.Aha
21 22 23 24
>
Hm.... this function 'extr.1()' seems to be (much) slower than the
function 'extr()'.
Chel Hee Lee
On 01/16/2015 11:34 AM, Bert Gunter wrote:
Chee Hee's approach is both simpler and almost surely more efficient,
but I wanted to show another that walks the tree (i.e. the list)
directly using recursion at the R level to pull out the desired
components. This is in keeping with R's "functional" programming
paradigm and avoids the use of regular expressions to extract the
desired components from the unlist() version.
extr <- function(x,nm){
if(is.recursive(x)){
wh <- names(x) %in% nm
c(x[wh],lapply(x[!wh],extr,nm=nm) )
} else NULL
}
## The return value contains a bunch of NULLs; so use unlist() to remove them
unlist(extr(x,"A"))
f1.x1.A f1.x2.A f2.x3.A f2.x4.A
11 12 13 14
I would welcome any possibly "slicker" versions of the above.
Cheers,
Bert
Bert Gunter
Genentech Nonclinical Biostatistics
(650) 467-7374
"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
Clifford Stoll
On Fri, Jan 16, 2015 at 7:23 AM, Chel Hee Lee <chl...@mail.usask.ca> wrote:
This approach may not be fancy as what you are looking for.
xl <- unlist(x)
xl[grep("A", names(xl))]
f1.x1.A f1.x2.A f2.x3.A f2.x4.A
11 12 13 14
I hope this helps.
Chel Hee Lee
On 01/16/2015 04:40 AM, Rainer M Krug wrote:
Hi
Consider the following variable:
--8<---------------cut here---------------start------------->8---
x1 <- list(
A = 11,
B = 21,
C = 31
)
x2 <- list(
A = 12,
B = 22,
C = 32
)
x3 <- list(
A = 13,
B = 23,
C = 33
)
x4 <- list(
A = 14,
B = 24,
C = 34
)
y1 <- list(
x1 = x1,
x2 = x2
)
y2 <- list(
x3 = x3,
x4 = x4
)
x <- list(
f1 = y1,
f2 = y2
)
--8<---------------cut here---------------end--------------->8---
To extract all fields named "A" from y1, I can do
,----
| > sapply(y1, "[[", "A")
| x1 x2
| 11 12
`----
But how can I do the same for x?
I could put an sapply into an sapply, but this would be less then
elegant.
Is there an easier way of doing this?
Thanks,
Rainer
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