CODEWITHSUNDEEP
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Tomas
Tomas

Reputation: 59435

How to check if object (variable) is defined in R?

I'd like to check if some variable is defined in R - without getting an error. How can I do this?

My attempts (not successful):

> is.na(ooxx)
Error: object 'ooxx' not found
> is.finite(ooxx)
Error: object 'ooxx' not found

Thanks!

Upvotes: 378

Views: 247746

Answers (7)

Sebastian Tacora
Sebastian Tacora

Reputation: 71

If you don't mind using quotes, you can use:

exists("x")

If you don't want to use quotes you can use:

exists(deparse(substitute(x)))

Upvotes: 6

Nirmal
Nirmal

Reputation: 687

If you don't want to use quotes, you can use deparse(substitute()) trick which I found in the example section of ?substitute:

is.defined <- function(sym) {
  sym <- deparse(substitute(sym))
  env <- parent.frame()
  exists(sym, env)
}

is.defined(a)
# FALSE
a <- 10
is.defined(a)
# TRUE

Upvotes: 40

smoe
smoe

Reputation: 630

There may be situations in which you do not exactly know the name of the variable you are looking for, like when an array of results have been created by a queuing system. These can possibly be addressed with "ls" and its argument "pattern" that expects a regular expression.

The "exists" function could be reimplemented that way as

exists <-function(variablename) {
   #print(ls(env=globalenv()))
   return(1==length(ls(pattern=paste("^",variablename,"$",sep=""),env=globalenv())))
}

While preparing this answer, I was a bit surprised about the need for the need of the specification of the environment when invoking ls() from within a function. So, thank you for that, stackoverflow! There is also an "all.names" attribute that I should have set to true but have omitted.

Upvotes: 2

sbaldrich
sbaldrich

Reputation: 1142

As others have pointed out, you're looking for exists. Keep in mind that using exists with names used by R's base packages would return true regardless of whether you defined the variable:

> exists("data")
[1] TRUE

To get around this (as pointed out by Bazz; see ?exists), use the inherits argument:

> exists("data", inherits = FALSE)
[1] FALSE

foo <- TRUE
> exists("foo", inherits = FALSE)
[1] TRUE

Of course, if you wanted to search the name spaces of attached packages, this would also fall short:

> exists("data.table")
[1] FALSE
require(data.table)
> exists("data.table", inherits = FALSE)
[1] FALSE
> exists("data.table")
[1] TRUE

The only thing I can think of to get around this -- to search in attached packages but not in base packages -- is the following:

any(sapply(1:(which(search() == "tools:rstudio") - 1L),
           function(pp) exists(_object_name_, where = pp, inherits = FALSE)))

Compare replacing _object_name_ with "data.table" (TRUE) vs. "var" (FALSE)

(of course, if you're not on RStudio, I think the first automatically attached environment is "package:stats")

Upvotes: 68

tim
tim

Reputation: 3608

if you are inside a function, missing() is what you want.

exchequer = function(x) {
    if(missing(x)){
        message("x is missing… :-(")
    }
}

exchequer()
x is missing… :-(

Upvotes: 70

Dirk is no longer here
Dirk is no longer here

Reputation: 368181

You want exists():

R> exists("somethingUnknown")
[1] FALSE
R> somethingUnknown <- 42
R> exists("somethingUnknown")
[1] TRUE
R>

Upvotes: 570

Gavin Simpson
Gavin Simpson

Reputation: 174778

See ?exists, for some definition of "...is defined". E.g.

> exists("foo")
[1] FALSE
> foo <- 1:10
> exists("foo")
[1] TRUE

Upvotes: 135

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