[R] Variable definition problem

[Greg Snow]

On Thu, 18 Apr 2002, T. Hotchi wrote:

> Hello, what does this error message indicate and how do I avoid this? 
> (sample code below)
> 
> Thank you.
> 
> -Tosh
> 
> #read in the data table
> co<-read.table("co.txt",header=T,as.is=T)
> 
> for (i in 1:3){
>    paste("logco",i, sep="")<-log(co$co[co$day==i])
> }
> 
> Gives the error:
> Error: Target of assignment expands to non-language object

Another response pointed you to the assign function which is the way to do
what you are trying to do.  However assigning to global variables in a
loop or function is often not a good idea.  If you want to save a bunch of
things and generate their names on the fly then a list is often a better
approach.  Replace your above code with something like:

mylogs <- list() # create an empty list to put data into
for (i in 1:3){
   mylogs[[ paste("logco",i, sep="") ]] <- log(co$co[co$day==i])
}

now you can access the results in several different ways:

mylogs[["logco1"]]
mylogs$logco1

or 

attach(mylogs)
logco1


This approach generaly does what you want and keeps from creating to many
top level global variables.  It is also easier to dump one single list to
a file for backup or tranfer than to do the same with all of your global
variables.

hope this helps,


-- 
Greg Snow, PhD                Office: 223A TMCB
Department of Statistics      Phone:  (801) 378-7049
Brigham Young University      Dept.:  (801) 378-4505
Provo, UT  84602              email:  gls at byu.edu

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