R help - Dec 2004 - using get() in assign()

I'm trying to rename the columns in a list of data.frames using the 
following...

for(i in 1:length(filenames)) {
assign(names(get(filenames[i])), c("name", "infood",
"time") ) }

R returns no errors, but the names are unchanged in the data.frames.

The original names were things like

 > names(get(filenames[2]))
[1] "Tc45w4.V1" "Tc45w4.V2" "Tc45w4.V3"

after the above procedure they are still those names.

Ideas appreciated.  Thanks.

Thomas



	[[alternative text/enriched version deleted]]

Consider the following: 

 > changeNames <- function(dfName,
+          newNames=c("A1", "B1"), pos.=1){
+   DF. <- get(dfName)
+   names(DF.) <- newNames
+   assign(dfName, DF., pos=pos.)
+   "done"
+ }
 >
 > DF <- data.frame(a=1, b=2)
 > changeNames("DF")
[1] "done"
 > DF
  A1 B1
1  1  2
 >
      There's probably a way to do this with "do.call", but the
above
seems to work. 

      hope this helps.  spencer graves

thomas hills wrote:
>I'm trying to rename the columns in a list of data.frames using the 
>following...
>
>for(i in 1:length(filenames)) {
>assign(names(get(filenames[i])), c("name", "infood",
"time") ) }
>
>R returns no errors, but the names are unchanged in the data.frames.
>
>The original names were things like
>
> > names(get(filenames[2]))
>[1] "Tc45w4.V1" "Tc45w4.V2" "Tc45w4.V3"
>
>after the above procedure they are still those names.
>
>Ideas appreciated.  Thanks.
>
>Thomas
>
>
>
>	[[alternative text/enriched version deleted]]
>
>______________________________________________
>R-help at stat.math.ethz.ch mailing list
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>PLEASE do read the posting guide!
http://www.R-project.org/posting-guide.html
>  
>
-- 
Spencer Graves, PhD, Senior Development Engineer
O:  (408)938-4420;  mobile:  (408)655-4567

thomas hills <thills at mail.utexas.edu> writes:
> I'm trying to rename the columns in a list of data.frames using the 
> following...
> 
> for(i in 1:length(filenames)) {
> assign(names(get(filenames[i])), c("name", "infood",
"time") ) }
> 
> R returns no errors, but the names are unchanged in the data.frames.
> 
> The original names were things like
> 
>  > names(get(filenames[2]))
> [1] "Tc45w4.V1" "Tc45w4.V2" "Tc45w4.V3"
> 
> after the above procedure they are still those names.
> 
> Ideas appreciated.  Thanks.
assign() takes a character string as its first argument, so you
probably now have a variable called "Tc45w4.V1" (it's a semi-bug
that
it doesn't barf when passed a character vector of length > 1). What
you seem to want is

for(i in 1:length(filenames)) {
  t <- get(filenames[i])
  names(t) <-  c("name", "infood", "time")
  assign(filenames[i], t)
}

It is easier with a list

for (i in seq(along=listoftables)) 
   names(listoftables[[i]]) <- c("name", "infood",
"time")

but *not* "for (i in listoftables) names(i) <- ..." because
you'd be
changing the names of a copy of the actual elements.

I suspect the jury is still out on whether it is good programming
practice to do 

listoftables <- lapply(listoftables, "names<-",
c("name", "infood", "time"))

but it is tempting in cases like this. (The awkward bit is that for
performance reasons, assignment functions -- names<-, [<-, etc. --
might not leave their argument unchanged.)
 
-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907

Its probably easier to keep your data frames in a list 
and to just specify the col.names= argument on 
read.table when you read them in:

cn <- c("name", "infood", "time")
L <- sapply(filenames, read.table, col.names = cn, simplify = FALSE)

You can add other read.table arguments, if you need them, 
in the same way that col.names= was added above.

Note that we used sapply(...whatever..., simplify = FALSE)
instead of lapply since that sapply will label the components
of list L with the filenames whereas a lapply will not.  


Date:   Tue, 28 Dec 2004 21:27:21 -0600 
From:   thomas hills <thills at mail.utexas.edu>
To:   <r-help at stat.math.ethz.ch> 
Subject:   [R] using get() in assign() 

 
I'm trying to rename the columns in a list of data.frames using the 
following...

for(i in 1:length(filenames)) {
assign(names(get(filenames[i])), c("name", "infood",
"time") ) }

R returns no errors, but the names are unchanged in the data.frames.

The original names were things like
> names(get(filenames[2]))
[1] "Tc45w4.V1" "Tc45w4.V2" "Tc45w4.V3"

after the above procedure they are still those names.

Ideas appreciated. Thanks.

Thomas