R: Evaluate an (Unevaluated) Expression

eval {base}

R Documentation

Evaluate an (Unevaluated) Expression

Description

Evaluate an R expression in a specified environment.

Usage

eval(expr, envir = sys.frame(sys.parent()), 
     enclos = if(is.list(envir) || is.pairlist(envir)) 
                  sys.frame(sys.parent()))
evalq(expr, envir, enclos)
eval.parent(expr, n = 1)
local(expr, envir = new.env())

Arguments

`expr`	object of mode `expression` or`call` or an ``unevaluated expression''.
`envir`	the `environment` in which `expr` is to be evaluated. May also be a list or an integer as in `sys.call`.
`enclos`	Only relevant if `envir` is a list. Specifies the enclosure, i.e., where R looks for objects not found in `envir`.
`n`	parent generations to go back

Details

eval evaluates the expression expr argument in the environment specified by envir and returns the computed value. If envir is not specified, then sys.frame(sys.parent()), the environment where the call to eval was made is used.

The evalq form is equivalent to eval(quote(expr), ...).

As eval evaluates its first argument before passing it to the evaluator, it allows you to assign complicated expressions to symbols and then evaluate them. evalq avoids this.

eval.parent(expr, n) is a shorthand for eval(expr, parent.frame(n)).

local evaluates an expression in a local environment. It is equivalent to evalq except the its default argument creates a new, empty environment. This is useful to create anonymous recursive functions and as a kind of limited namespace feature since variables defined in the environment are not visible from the outside.

Note

Due to the difference in scoping rules, there are some differences between R and S in this area. In particular, the default enclosure in S is the global environment.

When evaluating expressions in dataframes that has been passed as argument to a function, the relevant enclosure is often the caller's environment, i.e., one needs eval(x, data, sys.frame(sys.parent())).

Examples

eval(2 ^ 2 ^ 3)
mEx <- expression(2^2^3); mEx; 1 + eval(mEx)
eval({ xx <- pi; xx^2}) ; xx

a <- 3 ; aa <- 4 ; evalq(evalq(a+b+aa, list(a=1)), list(b=5)) # == 10
a <- 3 ; aa <- 4 ; evalq(evalq(a+b+aa, 1), list(b=5))         # == 12

ev <- function() {
   e1 <- sys.frame(sys.parent())
   ## Evaluate a in e1
   aa <- eval(expression(a),e1)
   ## evaluate the expression bound to a in e1
   a <- expression(x+y)
   list(aa = aa, eval = eval(a, e1))
}
tst.ev <- function(a = 7) { x <- pi; y <- 1; ev() }
tst.ev()#-> aa : 7,  eval : 4.14

##
## Uses of local()
##

# Mutual recursives. 
# gg gets value of last assignment, an anonymous version of f.

gg <- local({
    k <- function(y)f(y)
    f <- function(x) if(x) x*k(x-1) else 1
})
gg(10)
sapply(1:5, gg)

# Nesting locals. a is private storage accessible to k
gg <- local({
    k <- local({
        a <- 1
        function(y){print(a <<- a+1);f(y)}
    })
    f <- function(x) if(x) x*k(x-1) else 1
})
sapply(1:5, gg)

ls(envir=environment(gg))
ls(envir=environment(get("k", envir=environment(gg))))

[Package Contents]