## Is a Variable Defined?

## Usage

    exists(x, where =NULL, envir=sys.frame(sys.parent()), frame =NULL,
           mode = "any", inherits = TRUE)

## Arguments

`x` | a variable name (given as a quoted string).
`where, envir, frame` | an environent to be searched. By default this is the environment where the call to `envir` takes place.
`mode` | the type of interest for the object.
`inherits` | should the enclosing frames of the environment be inspected.

## Description

This function looks to see if the name `x` has a value bound to it. If `inherits` is `TRUE` and a value is not found for `x`, then the parent frames of `envir` are searched until the name `x` is encountered. If `mode` is specified then only objects of that mode are sought. The function returns `TRUE` if the variable is encountered and `FALSE` if not.

## See Also

`get`.

## Examples

    ##  Define a substitute function if necessary:
    if(!exists("some.fun", mode="function"))
     some.fun <- function(x) { cat("some.fun(x)\n"); x }

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