[R] Convert a character string to variable names

Jeff Newmiller
Wed, 09 Feb 2022 03:09:36 -0000

A variable in R can refer to many things, but it cannot be an element of a
vector. It absolutely _can_ refer to a list, a list of lists, a function, an
environment, and any of the various kinds of atomic vectors that you seem to
think of as variables. (R does _not_ name individual elements of vectors, unlike
many other languages.)

The things you can do with the mtcars object may be different than the things
you can do with the object identified by the expression mtcars$disp, but the
former has a variable name in an environment while the latter is embedded within
the former. mtcars$disp is shorthand for the expression mtcars[[
"disp" ]] which searches the names attribute of the mtcars list (a
data frame is a list of columns) to refer to that object.

R allows non-standard evaluation to make elements of lists accessible as though
they were variables in an environment, such as with( mtcars, disp ) or various
tidyverse evaluation conventions. But while the expression mtcars$disp DOES have
a value( it is an atomic vector of 32 integer elements) it is not a variable so
get("mtcars$disp") cannot be expected to work (as it does not). You
may be confusing "variable" with "object" ... lots of
objects have no variable names.

I have done all sorts of complicated data manipulations in R, but I have never
found a situation where a use of get() could not be replaced with a clearer way
to get the job done. Using lists is central to this... avoid making distinct
variables in the first place if you plan to be retrieving them later indirectly
like this.

On February 8, 2022 5:45:39 PM PST, "Ebert,Timothy Aaron" <tebert
at ufl.edu> wrote:
>
>I had thought that mtcars in "mtcars$disp" was the name of a
dataframe and that "disp" was the name of a column in the dataframe.
If I would make a model like horse power = displacement then "disp"
would be a variable in the model and I can find values for this variable in the
"disp" column in the "mtcars" dataframe. I am not sure how I
would use "mtcars" as a variable.
>"mtcars$disp" has no specific value, though it will have a
specific value for any given row of data (assuming rows are observations).
>
>Tim
>
>
>-----Original Message-----
>From: R-help <r-help-bounces at r-project.org> On Behalf Of Richard
O'Keefe
>Sent: Tuesday, February 8, 2022 8:17 PM
>To: Erin Hodgess <erinm.hodgess at gmail.com>
>Cc: r-help at r-project.org
>Subject: Re: [R] Convert a character string to variable names
>
>[External Email]
>
>"mtcars$disp" is not a variable name.
>"mtcars" is a variable name, and
>get("mtcars") will get the value of that variable
assign("mtcars", ~~whatever~~) will set it.
>mtcars$disp is an *expression*,
>where $ is an indexing operator
>https://urldefense.proofpoint.com/v2/url?u=https-3A__cran.r-2Dproject.org_doc_manuals_r-2Drelease_R-2Dlang.html-23Indexing&d=DwICAg&c=sJ6xIWYx-zLMB3EPkvcnVg&r=9PEhQh2kVeAsRzsn7AkP-g&m=CI-7ZdIwlhUvhmOkVD7KJkv3IvSSWy4ix2Iz1netW81V-NUV8aOVVqyn5-fmD6cf&s=RjRC5kve6D8k59qZQYcX-PR-aA4TTu1yfLPBhHxSlWk&e>so
what you want is
>> mtcars <- list(cyl=4, disp=1.8)
>> eval(parse(text="mtcars$disp"))
>[1] 1.8
>
>Though it's easy to do this, it's very seldom a good idea.
>The combination of parse and eval can do ANYTHING, no matter how disastrous.
Less powerful techniques are safer.
>Where do these strings come from in the first place?
>Why isn't it c("disp", "hp", "cyl")?
>
>On Tue, 8 Feb 2022 at 11:56, Erin Hodgess <erinm.hodgess at gmail.com>
wrote:
>
>> Hello!
>>
>> I have a character string that is a vector of variable names.  I would
>> like to use those names to access the variables and create a matrix.
>> I tried the following:
>>
>> > .x
>>
>> [1] "mtcars$disp" "mtcars$hp"
"mtcars$cyl"
>>
>> > .y <- NULL
>>
>> > for(i in 1:3) {
>>
>> + .y[i] <- c(as.name(.x[[i]]))
>>
>> + }
>>
>> > .y
>>
>> [[1]]
>>
>> `mtcars$disp`
>>
>>
>> [[2]]
>>
>> `mtcars$hp`
>>
>>
>> [[3]]
>>
>> `mtcars$cyl`
>>
>>
>> But I am having trouble converting the variables in .y into a matrix.
>>
>>
>> I tried all kinds of stuff with bquote, deparse, do.call, but no good.
>>
>>
>> I have a feeling that it's something simple, and I'm just not
seeing it.
>>
>>
>> Thanks,
>>
>> Erin
>>
>>
>>
>>
>> Erin Hodgess, PhD
>> mailto: erinm.hodgess at gmail.com
>>
>>         [[alternative HTML version deleted]]
>>
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>
>        [[alternative HTML version deleted]]
>
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>
>______________________________________________
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--
Sent from my phone. Please excuse my brevity.