String to Variable Name in R

Convert string to a variable name

assign is what you are looking for.

assign("x", 5)

x
[1] 5

but buyer beware.

See R FAQ 7.21
http://cran.r-project.org/doc/FAQ/R-FAQ.html#How-can-I-turn-a-string-into-a-variable_003f

Pass a string as variable name in dplyr::filter

!! or UQ evaluates the variable, so mtcars %>% filter(!!var == 4) is the same as mtcars %>% filter('cyl' == 4) where the condition always evaluates to false; You can prove this by printing !!var in the filter function:

mtcars %>% filter({ print(!!var); (!!var) == 4 })
# [1] "cyl"
#  [1] mpg  cyl  disp hp   drat wt   qsec vs   am   gear carb
# <0 rows> (or 0-length row.names)

To evaluate var to the cyl column, you need to convert var to a symbol of cyl first, then evaluate the symbol cyl to a column:

Using rlang:

library(rlang)
var <- 'cyl'
mtcars %>% filter((!!sym(var)) == 4)

#    mpg cyl  disp  hp drat    wt  qsec vs am gear carb
#1  22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
#2  24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
#3  22.8   4 140.8  95 3.92 3.150 22.90  1  0    4    2
# ...

Or use as.symbol/as.name from baseR:

mtcars %>% filter((!!as.symbol(var)) == 4)

mtcars %>% filter((!!as.name(var)) == 4)

Convert string to variable name in R

Firstly, it's a backtick (`), not an apostrophe ('). In R, backticks occasionally denote variable names; apostrophes work as single quotes for denoting strings.

The issue you're having is that your variables start with a number, which is not allowed in R. Since you somehow made it happen anyway, you need to use backticks to tell R not to interpret 2011_Q4 as a number, but as a variable.

From ?Quotes:

Names and Identifiers

Identifiers consist of a sequence of letters, digits, the period (.)
and the underscore. They must not start with a digit nor underscore,
nor with a period followed by a digit. Reserved words are not valid
identifiers.

The definition of a letter depends on the current locale, but only
ASCII digits are considered to be digits.

Such identifiers are also known as syntactic names and may be used
directly in R code. Almost always, other names can be used provided
they are quoted. The preferred quote is the backtick (`), and deparse
will normally use it, but under many circumstances single or double
quotes can be used (as a character constant will often be converted to
a name). One place where backticks may be essential is to delimit
variable names in formulae: see formula.

The best solution to your issue is simply to change your variable names to something that starts with a character, e.g. Y2011_Q4.

How to use character as variable name in arguments? [R]

We could use setNames

out <- do.call(dict, setNames(rep(list(1), length(x)), x))
out$keys
[1] "1" "2"

Or we may use invoke or exec

library(purrr)
out <- invoke(dict, setNames(rep(1, length(x)), x))
out <- exec(dict, !!!setNames(rep(1, length(x)), x))

For the second case, also setNames works

setNames(list(1), y)
$happy
[1] 1

or we can use dplyr::lst

dplyr::lst(!! y := 1)
$happy
[1] 1

R - use function argument as variable name and string value

Here, it may be better to use .data with [[ to subset instead of {{}} as the input passed is character string

testfun <- function(modelnum){
  covars <- modelinfo %>% 
    filter(.data[[modelnum]]==1)
  
  data <- modeldata %>% 
    filter(model==modelnum)
  
  output=c(covars,data)
  return(output)
  
}

-testing

testfun(modelnum="model 1")
$covars
[1] "x1"

$`model 1`
[1] 1

$`model 2`
[1] 0

$x1
[1] 3 1

$x2
[1] 3 3

Pass a string as variable name in dplyr::mutate

This operation can be carried out with := while evaluating (!!) and using the conversion to symbol and evaluating on the rhs of assignment

library(dplyr)
my_mtcars <- mtcars %>% 
       mutate(!! var  := factor(!! rlang::sym(var)))
class(my_mtcars$vs)
#[1] "factor"

Or without thinking too much, use mutate_at, which can take strings in vars and apply the function of interest

my_mtcars2 <- mtcars %>% 
                    mutate_at(vars(var), factor)