force {base} | R Documentation

## Force Evaluation of an Argument

### Description

Forces the evaluation of a function argument.

### Usage

    force(x)

### Arguments

`x` | a formal argument of the enclosing function.

### Details

`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.

### Note

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 implements lazy evaluation.)

### Examples

    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 }

[Package base version 2.15.1 Index]