plyr (version 1.5.2)

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

Description

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

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.

Value

• list of symbol and language primitives

Details

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).

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)
.(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))