R: Quote variables to create a list of unevaluated expressions...
.
R Documentation
Quote variables to create a list of unevaluated expressions for later
evaluation.
Description
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).
Usage
.(..., .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.
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.
Value
list of symbol and language primitives
Examples
.(a, b, c)
.(first = a, second = b, third = c)
.(a ^ 2, b - d, log(c))
as.quoted(~ a + b + c)
as.quoted(a ~ b + c)
as.quoted(c("a", "b", "c"))
# Some examples using ddply - look at the column names
ddply(mtcars, "cyl", each(nrow, ncol))
ddply(mtcars, ~ cyl, each(nrow, ncol))
ddply(mtcars, .(cyl), each(nrow, ncol))
ddply(mtcars, .(log(cyl)), each(nrow, ncol))
ddply(mtcars, .(logcyl = log(cyl)), each(nrow, ncol))
ddply(mtcars, .(vs + am), each(nrow, ncol))
ddply(mtcars, .(vsam = vs + am), each(nrow, ncol))