```
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 <- parent.frame()
## 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
a <- list(a = 3, b = 4)
with(a, a <- 5) # alters the copy of a from the list, discarded.
##
## Example of evalq()
##
N <- 3
env <- new.env()
assign("N", 27, envir = env)
## this version changes the visible copy of N only, since the argument
## passed to eval is '4'.
eval(N <- 4, env)
N
get("N", envir = env)
## this version does the assignment in env, and changes N only there.
evalq(N <- 5, env)
N
get("N", envir = env)
##
## Uses of local()
##
# Mutually recursive.
# 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))))
```

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