[R] Convert a character string to variable names

Ebert,Timothy Aaron
Sun, 13 Feb 2022 03:24:18 -0000

How does ?a value? differ from ?an object??

From: Richard O'Keefe <raoknz at gmail.com>
Sent: Friday, February 11, 2022 12:25 AM
To: Ebert,Timothy Aaron <tebert at ufl.edu>
Cc: Jeff Newmiller <jdnewmil at dcn.davis.ca.us>; r-help at r-project.org;
Erin Hodgess <erinm.hodgess at gmail.com>
Subject: Re: [R] Convert a character string to variable names

[External Email]
You wrote "32 numbers is not a value".
It is, it really is.  When you have a vector like
 x <- 1:32
you have a simple variable (x) referring to an immutable value
(1, 2, ..., 32).  A vector in R is NOT a collection of mutable
boxes, it is a collection of *numbers* (or strings).  The vector
itself is a good a value as ever twanged.  You cannot change it.
A statement like
 x[i] <- 77
is just shorthand for
 x <- "[<-"(x, i, 77)
which constructs a whole new 32-number value and assigns that to x.
(The actual implementation is cleverer when it can be, but often it
cannot be clever.)
Pure values like vectors can be shared: if x is a vector,
then y <- x is a constant time operation.  If you then change
y, you only change y, not the vector.  x is unchanged.


On Wed, 9 Feb 2022 at 17:06, Ebert,Timothy Aaron <tebert at
ufl.edu<mailto:tebert at ufl.edu>> wrote:
"A variable in R can refer to many things, ..." I agree.
"It absolutely _can_ refer to a list, ..." I partly agree. In R as a
programming language I agree. In R as a statistical analysis tool then only
partly. Typically one would need to limit the list so each variable would be of
the same length and all values within the variable be of the same data type
(integer, real, factor, character). As a programmer yes, as a statistician not
really unless you always qualify the type of list considered and that gets
tiresome.

R does name individual elements using numeric place names: hence df[row,
column]. Each element must have a unique address, and that is true in all
computer languages.

A dataframe is a list of columns of the same length containing the same data
type within a column.

mtcars$disp does not have a value (a value is one number). With 32 elements I
can calculate a mean and the mean is a value. 32 numbers is not a value. I
suppose a single value could be the starting memory address of the name, but I
don't see how that distinction helps unless one is doing Assembly or Machine
language programming.

I have never used get(), so I will keep that in mind. I agree that it makes life
much easier to enter the data in the way it will be analyzed.




-----Original Message-----
From: Jeff Newmiller <jdnewmil at dcn.davis.ca.us<mailto:jdnewmil at
dcn.davis.ca.us>>
Sent: Tuesday, February 8, 2022 10:10 PM
To: r-help at r-project.org<mailto:r-help at r-project.org>; Ebert,Timothy
Aaron <tebert at ufl.edu<mailto:tebert at ufl.edu>>; Richard
O'Keefe <raoknz at gmail.com<mailto:raoknz at gmail.com>>; Erin
Hodgess <erinm.hodgess at gmail.com<mailto:erinm.hodgess at
gmail.com>>
Cc: r-help at r-project.org<mailto:r-help at r-project.org>
Subject: Re: [R] Convert a character string to variable names

[External Email]

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<mailto: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<mailto: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<mailto:erinm.hodgess at
gmail.com>>
>Cc: r-help at r-project.org<mailto: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.o
>rg_doc_manuals_r-2Drelease_R-2Dlang.html-23Indexing&d=DwICAg&c=sJ6xIWYx
>-zLMB3EPkvcnVg&r=9PEhQh2kVeAsRzsn7AkP-g&m=CI-7ZdIwlhUvhmOkVD7KJkv3IvSSW
>y4ix2Iz1netW81V-NUV8aOVVqyn5-fmD6cf&s=RjRC5kve6D8k59qZQYcX-PR-aA4TTu1yf
>LPBhHxSlWk&e>so what you want is
>> mtcars <- list(cyl=4, disp=1.8)
>> eval(parse(text="mtcars$disp"))
>[1] 1.8
>
>Though it's easy to do this, it's very seldom a good idea.
>The combination of parse and eval can do ANYTHING, no matter how disastrous.
Less powerful techniques are safer.
>Where do these strings come from in the first place?
>Why isn't it c("disp", "hp", "cyl")?
>
>On Tue, 8 Feb 2022 at 11:56, Erin Hodgess <erinm.hodgess at
gmail.com<mailto:erinm.hodgess at gmail.com>> wrote:
>
>> Hello!
>>
>> I have a character string that is a vector of variable names.  I
>> would like to use those names to access the variables and create a
matrix.
>> I tried the following:
>>
>> > .x
>>
>> [1] "mtcars$disp" "mtcars$hp"
"mtcars$cyl"
>>
>> > .y <- NULL
>>
>> > for(i in 1:3) {
>>
>> + .y[i] <-
c(as.name<https://urldefense.proofpoint.com/v2/url?u=http-3A__as.name&d=DwMFaQ&c=sJ6xIWYx-zLMB3EPkvcnVg&r=9PEhQh2kVeAsRzsn7AkP-g&m=vzlTpQd9zYJkQ77y8VRROjzMQQJrJce_5rInko9TViGjuIt93PxagLXs9prJsMwy&s=Yrczdj8QHFrWSBSm_k4WyKN7ppY20M360b7tUmMCJaY&e=>(.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<mailto:erinm.hodgess at
gmail.com>
>>
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>>
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-fmD6cf&s=c8oCLZK8TFAAs5d3vhDyB52KR2I9WWSTg6kDjL8orcI&e>> PLEASE
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