[R] How to assign value to a variable dynamically constructed

[William Dunlap]

You can use an environment instead of a list using the same [[ syntax.  It
is like 'get0(..., inherit=FALSE)' on the left side of the <- and like
'assign(...)' on the right side.   E.g.,
   myData <- new.env()
   varName <- "v1"
   myData[[varName]] <- 1:10
   myData[[varName]][4] <- myData[[varName]][4] * 100
   myData[[varName]]
   #  [1]   1   2   3 400   5   6   7   8   9  10
   names(myData)
   # [1] "v1"
(Before R-3.2.0 or so, you had to use objects(myData,all=TRUE) if
myData was an environment and names(myData) if it was a list.  Now
names() works for environments.)

It is better to use a dedicated environment (or list) for each set of
related
variables so that name collisions do not cause problems.


Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Wed, Jul 8, 2015 at 10:06 AM, Greg Snow <538280 at gmail.com> wrote:

> This is FAQ 7.21.
>
> The most important part of the answer in FAQ 7.21 is the last section
> where it states that it is often easier to use a list rather than
> messing around with trying to dynamically name global variables.
>
> If you tell us what you are trying to accomplish then we may have
> better advice.  The route you are headed down now usually leads to
> inefficient code and hard to find bugs.
>
> On Tue, Jul 7, 2015 at 2:53 PM, Jun Shen <jun.shen.ut at gmail.com> wrote:
> > Dear list,
> >
> > Let's say we have a variable (id), whose name is dynamically constructed.
> > This variable represents a vector or data frame with many elements. Now I
> > want to specifically assign a value to one of the elements. I couldn't
> get
> > it right.
> >
> > test <- 'id' # "id" is dynamically constructed through paste()
> >
> > id <- 1:4
> >
> > # I can get the element by doing
> >
> > get(test)[2]
> >
> > # Now I want to assign a value to the second element of this dynamical
> > variable.
> >
> > get(test)[2] <- 5  # doesn't work.
> >
> > Thanks a lot.
> >
> > Jun Shen
> >
> >         [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > 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
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>

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