| force {base} | R Documentation |
Forces the evaluation of a function argument.
force(x)
x |
a formal argument of the enclosing function. |
force forces the evaluation of a formal argument. This can
be useful if the argument will be captured in a closure by the lexical
scoping rules and will later be altered by an explicit assignment or
an implicit assignment in a loop or an apply function.
This is semantic sugar: just evaluating the symbol will do the same thing (see the examples).
force does not force the evaluation of other
promises. (It works by forcing the promise that
is created when the actual arguments of a call are matched to the
formal arguments of a closure, the mechanism which implememnts
lazy evaluation.)
f <- function(y) function() y
lf <- vector("list", 5)
for (i in seq(along = lf)) lf[[i]] <- f(i)
lf[[1]]() # returns 5
g <- function(y) { force(y); function() y }
lg <- vector("list", 5)
for (i in seq(along = lg)) lg[[i]] <- g(i)
lg[[1]]() # returns 1
## This is identical to
g <- function(y) { y; function() y }