R help - Jul 2011 - Looping through data sets to change column from character to numeric

Greetings to all --

I am having a silly problem that I just can't solve.  Someone has given me
an .RData file will hundreds of data frames.  Each data frame has a column
named ResultValue, currently character when it should be numeric.  I want to
loop through all of the data frames to change the variable to numeric, but I
cannot figure out how to do it. My best guess was along the lines of:

frames = ls()
for (frame in frames){
     assign(frame, get(frame), .GlobalEnv)
     frame[,"ResultValue"] =
as.numeric(frame[,"ResultValue"])
     }

It doesn't work.  After the assign() the frame object remains the character
name of the dataframe I am trying to change.  If I do the following, the
TEST object comes out just fine.

frames = ls()
for (frame in frames){
     assign("TEST", get(frame), .GlobalEnv)
     TEST[,"ResultValue"] = as.numeric(TEST[,"ResultValue"])
     }

Seems like it should be simple, but I am misunderstanding something and not
following the logic.  Any insight?

Thanks,

Sarah

	[[alternative HTML version deleted]]

The problem is that you can not assign a variable to itself.

rm(list=ls())
df1 <- data.frame(ResultValue=as.character(1:5))
df2 <- data.frame(ResultValue=as.character(1:10))
frames = ls()
for (frame in frames){
     temp <- get(frame)
     temp[,"ResultValue"] = as.numeric(temp[,"ResultValue"])
     assign(frame,temp)
}

HTH,
  Denes


> Greetings to all --
>
> I am having a silly problem that I just can't solve.  Someone has given
me
> an .RData file will hundreds of data frames.  Each data frame has a column
> named ResultValue, currently character when it should be numeric.  I want
> to
> loop through all of the data frames to change the variable to numeric, but
> I
> cannot figure out how to do it. My best guess was along the lines of:
>
> frames = ls()
> for (frame in frames){
>      assign(frame, get(frame), .GlobalEnv)
>      frame[,"ResultValue"] =
as.numeric(frame[,"ResultValue"])
>      }
>
> It doesn't work.  After the assign() the frame object remains the
> character
> name of the dataframe I am trying to change.  If I do the following, the
> TEST object comes out just fine.
>
> frames = ls()
> for (frame in frames){
>      assign("TEST", get(frame), .GlobalEnv)
>      TEST[,"ResultValue"] =
as.numeric(TEST[,"ResultValue"])
>      }
>
> Seems like it should be simple, but I am misunderstanding something and
> not
> following the logic.  Any insight?
>
> Thanks,
>
> Sarah
>
> 	[[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.
>

On 29/07/11 11:09, Sarah Henderson wrote:
> Greetings to all --
>
> I am having a silly problem that I just can't solve.  Someone has given
me
> an .RData file will hundreds of data frames.  Each data frame has a column
> named ResultValue, currently character when it should be numeric.  I want
to
> loop through all of the data frames to change the variable to numeric, but
I
> cannot figure out how to do it. My best guess was along the lines of:
>
> frames = ls()
> for (frame in frames){
>       assign(frame, get(frame), .GlobalEnv)
>       frame[,"ResultValue"] =
as.numeric(frame[,"ResultValue"])
>       }
>
Note that ``frame'' is an object the value of which is a character
string
naming (successive) data frames. Your call to assign doesn't change the
value of ``frame''.  It *could* (but doesn't actually) change the
value
of the
object *named* by the character string stored in ``frame''.

Suppose that you have data frames clyde, melvin, and irving.  On your 
first pass
through the loop the value of frame is the character string ``clyde''.

Your assign() call in effect does ``clyde <- clyde'' which
doesn't
accomplish
much. :-)

The value of frame after the assign() is still the character string 
``clyde''.
I.e. frame is a character scalar, so frame[,"ResultValue"] doesn't
make any
sense.

What you need to do is something like:

     frames <- ls()
     for(frame in frames) {
         assign("temp",get(frame)) # Don't think you need the 
``.GlobalEnv'', although it doesn't hurt.
         temp[,ResultValue"] <-
as.numeric(temp[,"ResultValue"])
         assign(frame,temp)
     }

This is untested ....

But I hope it helps.

         cheers,

             Rolf Turner

Others have shown how to do this, but a better approach may be to load your
.Rdata file into a new environment rather than the global environment, then you
can work with the environment (and all the objects in it) using lapply, or loops
with [[]] instead of fighting with get and assign.

Something like:
> mydfs <- new.env()
> load( '.Rdata', env=mydfs
> mydfs <- lapply( mydfs, function(x) { x$ResultValue <-
as.numeric(x$ResultValue); x} )
Or
> mydfs <- lapply( mydfs, function(x) within(x, ResultValue <-
as.numeric(ResultValue) ) )
Or
> for( frame in ls(env=mydfs) ) {
+  mydfs[[frame]]$ResultValue <- as.numeric( mydfs[[frame]]$ResultValue )
+ }



-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Sarah Henderson
Sent: Thursday, July 28, 2011 5:09 PM
To: R List
Subject: [R] Looping through data sets to change column from character to
numeric

Greetings to all --

I am having a silly problem that I just can't solve.  Someone has given me
an .RData file will hundreds of data frames.  Each data frame has a column
named ResultValue, currently character when it should be numeric.  I want to
loop through all of the data frames to change the variable to numeric, but I
cannot figure out how to do it. My best guess was along the lines of:

frames = ls()
for (frame in frames){
     assign(frame, get(frame), .GlobalEnv)
     frame[,"ResultValue"] =
as.numeric(frame[,"ResultValue"])
     }

It doesn't work.  After the assign() the frame object remains the character
name of the dataframe I am trying to change.  If I do the following, the
TEST object comes out just fine.

frames = ls()
for (frame in frames){
     assign("TEST", get(frame), .GlobalEnv)
     TEST[,"ResultValue"] = as.numeric(TEST[,"ResultValue"])
     }

Seems like it should be simple, but I am misunderstanding something and not
following the logic.  Any insight?

Thanks,

Sarah

	[[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.