Quote variables to create a list of unevaluated expressions for later evaluation.

Source: R/quote.r

This function is similar to ~ in that it is used to capture the name of variables, not their current value. This is used throughout plyr to specify the names of variables (or more complicated expressions).

.(..., .env = parent.frame())

Arguments

...

unevaluated expressions to be recorded. Specify names if you want the set the names of the resultant variables
.env

environment in which unbound symbols in ... should be evaluated. Defaults to the environment in which . was executed.

Value

list of symbol and language primitives

Details

Similar tricks can be performed with substitute, but when functions can be called in multiple ways it becomes increasingly tricky to ensure that the values are extracted from the correct frame. Substitute tricks also make it difficult to program against the functions that use them, while the quoted class provides as.quoted.character to convert strings to the appropriate data structure.

Examples

.(a, b, c)
#> List of 3
#>  $ a: symbol a
#>  $ b: symbol b
#>  $ c: symbol c
#>  - attr(*, "env")=<environment: 0x79d15a0> 
#>  - attr(*, "class")= chr "quoted"
.(first = a, second = b, third = c)
#> List of 3
#>  $ first : symbol a
#>  $ second: symbol b
#>  $ third : symbol c
#>  - attr(*, "env")=<environment: 0x79d15a0> 
#>  - attr(*, "class")= chr "quoted"
.(a ^ 2, b - d, log(c))
#> List of 3
#>  $ a^2   : language a^2
#>  $ b - d : language b - d
#>  $ log(c): language log(c)
#>  - attr(*, "env")=<environment: 0x79d15a0> 
#>  - attr(*, "class")= chr "quoted"
as.quoted(~ a + b + c)
#> List of 3
#>  $ a: symbol a
#>  $ b: symbol b
#>  $ c: symbol c
#>  - attr(*, "env")=<environment: 0x79d15a0> 
#>  - attr(*, "class")= chr "quoted"
as.quoted(a ~ b + c)
#> List of 3
#>  $ a: symbol a
#>  $ b: symbol b
#>  $ c: symbol c
#>  - attr(*, "env")=<environment: 0x79d15a0> 
#>  - attr(*, "class")= chr "quoted"
as.quoted(c("a", "b", "c"))
#> List of 3
#>  $ a: symbol a
#>  $ b: symbol b
#>  $ c: symbol c
#>  - attr(*, "env")=<environment: 0x79d15a0> 
#>  - attr(*, "class")= chr "quoted"

# Some examples using ddply - look at the column names
ddply(mtcars, "cyl", each(nrow, ncol))
#>   cyl nrow ncol
#> 1   4   11   11
#> 2   6    7   11
#> 3   8   14   11
ddply(mtcars, ~ cyl, each(nrow, ncol))
#>   cyl nrow ncol
#> 1   4   11   11
#> 2   6    7   11
#> 3   8   14   11
ddply(mtcars, .(cyl), each(nrow, ncol))
#>   cyl nrow ncol
#> 1   4   11   11
#> 2   6    7   11
#> 3   8   14   11
ddply(mtcars, .(log(cyl)), each(nrow, ncol))
#>   log(cyl) nrow ncol
#> 1 1.386294   11   11
#> 2 1.791759    7   11
#> 3 2.079442   14   11
ddply(mtcars, .(logcyl = log(cyl)), each(nrow, ncol))
#>     logcyl nrow ncol
#> 1 1.386294   11   11
#> 2 1.791759    7   11
#> 3 2.079442   14   11
ddply(mtcars, .(vs + am), each(nrow, ncol))
#>   vs + am nrow ncol
#> 1       0   12   11
#> 2       1   13   11
#> 3       2    7   11
ddply(mtcars, .(vsam = vs + am), each(nrow, ncol))
#>   vsam nrow ncol
#> 1    0   12   11
#> 2    1   13   11
#> 3    2    7   11