use name (not values!) of a dataframe inside a funktion

An embedded and charset-unspecified text was scrubbed...
Name: not available
URL: <https://stat.ethz.ch/pipermail/r-help/attachments/20130201/e2fc49ae/attachment.pl>
Another R-ish way of modifying an object with a function is to use
'replacement functions' (there must be other names, I'm not sure
what the standard is) that let you use syntax like
     someProperty(myData, ...) <- newProperty
To do this define a function called `someProperty<-` whose last argument
is named 'value' and which returns a modified version of its first argument.
When R sees the above syntax it does the equivalent of
   myData <- `someProperty<-`(myData, ..., value=newProperty)

E.g.,
`useLog2Scale<-` <- function(dataframe, value) {
if (value) dataframe[] <- lapply(dataframe, log2)
       else dataframe[] <- lapply(dataframe, function(x)2^x)
       dataframe
    }
d <- data.frame(x=1:5, y=1/(1:5))
useLog2Scale(d) <- TRUE
d
x         y
1 0.000000  0.000000
2 1.000000 -1.000000
3 1.584963 -1.584963
4 2.000000 -2.000000
5 2.321928 -2.321928
useLog2Scale(d) <- FALSE
d
x         y
1 1 1.0000000
2 2 0.5000000
3 3 0.3333333
4 4 0.2500000
5 5 0.2000000

(Usually such a function is used to set a property in the data, such as a logScale
flag, that is used or queried later.)

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com
-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
Of Greg Snow
Sent: Friday, February 01, 2013 8:54 AM
To: Winfried Moser
Cc: r-help
Subject: Re: [R] use name (not values!) of a dataframe inside a funktion

It is strongly discouraged in R to have functions that change data values
in the global workspace (or any location other than their local
environment).

The usual procedure in R is to have your function return a modified version
of the object and the user then decides what to do with it.  They can
assign it back to the same original object so that there is still only one
copy and it has changed (but the user made that decision, not the
programmer), or they can save it to a different name and not lose the
original.

If you really want to change the original copy (and there are sometimes
when the exception to the rule makes sense) then you can either use
environments (which don't copy on modify) or use macros instead of
functions.  Given your examples I would look at the macro approach first.
 There is a 'defmacro' function in the 'gtools' package and the reference
on the help page for 'defmacro' leads to the original R news (now R
Journal) article describing the use of macros in R (definitely read this if
you are considering this approach).

On Thu, Jan 31, 2013 at 7:34 AM, Winfried Moser <winfried.moser at gmail.com>wrote:

Dear Listers,

can anyone help me, please.

Since several days i try to figure out, how to assign values, vectors,
functions etc to variables with dynamically generated names inside of
functions.
Sometimes I succeed, but the success is rather arbitrary, it seems. up to
now i don't fully understand, why things like get, assign, <<- etc do
sometimes work, and sometimes not.

here's one of my daily examples, i am stuck with: Example 1 does work, but
example 2 doesn't?
How kann i tell R, that i want it to expand the string "dfb" to "dfb[,2]"
inside the function.
In the end i want the function to change the second variable of the
dataframe dfb permanently to factor (not just inside the function).

Thanks in advance!

Winfried

Example 1:
dfa <- data.frame(a=c(1:4),b=c(1:4))
dfa[,2] <- factor(dfa[,2])
is.factor(dfa[,2])
TRUE
Example 2:
dfb <- data.frame(a=c(1:4),b=c(1:4))
f.fact <- function(x) {x[,2] <<- factor(x[,2])}
f.fact(dfb)
is.factor(dfb[,2])
FALSE

PS: I tried a whole lot of other things like, ...
I really don't know where to keep on searching.

dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {get(x)[,2] <<- factor(get(x)[,2])}
f.fact("dfb")
is.factor(dfb[,2])
"Object 'x' nicht gefunden
dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {get(x[,2]) <<- factor(x[,2])}
f.fact(dfb)
is.factor(dfb[,2])
"Object 'x' nicht gefunden
dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {get(x)[,2] <<- factor(x[,2])}
f.fact(dfb)
is.factor(dfb[,2])
"Object 'x' nicht gefunden
dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {assign(x[,2], factor(x[,2]))}
f.fact(dfb)
is.factor(dfb[,2])
Ung?ltiges erstes Argument
dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {quote(x)[,2], factor(x[,2])}
f.fact(dfb)
is.factor(dfb[,2])
Unerwartetes ','
dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {
name <- paste0(quote(x),"[,2]")
 assign(name, factor(x[,2]))}
f.fact(dfb)
is.factor(dfb[,2])
FALSE
dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {
name <- paste0(get(x),"[,2]")
assign(name, factor(x[,2]))}
f.fact("dfb")
is.factor(dfb[,2])
Falsche Anzahl von Dimensionen
dfb <- data.frame(a=c(1,2,3,4),b=c(1,2,3,4))
f.fact <- function(x) {
 name <- paste0(x,"[,2]")
assign(name, factor(x[,2]))}
f.fact("dfb")
is.factor(dfb[,2])
Falsche Anzahl von Dimensionen
?chz ...

        [[alternative HTML version deleted]]

______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

--
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com

	[[alternative HTML version deleted]]