[R] How to use a variable after $ ($x) for subscripting a list, How to source from a character object (source(x))

drflxms drflxms at googlemail.com
Tue Sep 13 10:11:43 CEST 2011

Dear R colleagues,

as result of a function a huge list is generated. From this result list
I'd like to extract information.

Think of the list i.e. as an object named "listResult" with the
following form:

 [1] [2] [3] [4] [5]

 [1] [2] [3] [4] [5]

 [1] [2] [3] [4] [5]

where levels=c(a,b,c)

I'd like to extract a data.frame like

a [2]
b [2]
c [2]

What I tried, is a function like this:

loopExtract <- function(input) {
foreach(i=1:length(levels)) %do% {listResult[[i]]$input}

where the name of the variable [2] is meant to be the input.

Unfortunately it turned out, that the "input"-variable after the $ is
not substituted as expected. Subscripting the list with a variable after
the $ seems not possible.
The only workaround I found for that is a function like

loopExtract <- function(input) {
codetemplate <- as.character("result <- foreach(i=1:length(levels)) %do%
write(str_replace_all(codetemplate, "input", input), file="tmp.r")

in other words I stored a template of the desired code as characters in
an object, substituted the "input" string in that character object,
wrote the result to a file and sourced the code of that file.

I stored the result in a file cause the expression source(codetemplate)
did not work. From the documentation I learned, that one can only source
"connections". And there seems no chance to establish a connection to an
object. (Probably no one else, except me, has such a strange idea.)

Well, it works that way, but even though I am not a real programmer, I
realize how dirty this solution is. There must be a much simpler and
elegant way.

To sum it all up, my problem can be reduced to the following two questions

(1) How to subscript a list with a variable after $
(2) How to source from an object containing a code template
or (probably the most promising) (3) How to avoid both ;)

Unfortunately I am not experienced enough to find the solution. Please help!

Greetings from sunny Munich, Felix

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