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" <teb...@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-boun...@r-project.org> On Behalf Of Richard O'Keefe
>Sent: Tuesday, February 8, 2022 8:17 PM
>To: Erin Hodgess <erinm.hodg...@gmail.com>
>Cc: r-help@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$disp"))
>[1] 1.8
>
>Though it's easy to do this, it's very seldom a good idea.
>The combination of parse and eval can do ANYTHING, no matter how disastrous.
>Less powerful techniques are safer.
>Where do these strings come from in the first place?
>Why isn't it c("disp", "hp", "cyl")?
>
>On Tue, 8 Feb 2022 at 11:56, Erin Hodgess <erinm.hodg...@gmail.com> wrote:
>
>> Hello!
>>
>> I have a character string that is a vector of variable names. I would
>> like to use those names to access the variables and create a matrix.
>> I tried the following:
>>
>> > .x
>>
>> [1] "mtcars$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.hodg...@gmail.com
>>
>> [[alternative HTML version deleted]]
>>
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>
> [[alternative HTML version deleted]]
>
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>PLEASE do read the posting guide
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>
>______________________________________________
>R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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>and provide commented, minimal, self-contained, reproducible code.
--
Sent from my phone. Please excuse my brevity.
______________________________________________
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