CODEWITHSUNDEEP
rfunctionargumentsexists
Rahi
Rahi

Reputation: 175

How to check if any variable is defined in argument (...) in R?

If I have a function with argument (...) and want to check if a variable is defined in the argument. How can I do this? I have already looked at the solution provided at this link: How to check if object (variable) is defined in R?. However, it does not solve my problem.

# Scenario 1
exists("a")
# [1] FALSE

# Scenario 2
a <- 10
exists("a")
# [1] TRUE

# Define a function for remaining scenarios
f = function(...){exists("a", inherits = F)}

# Scenario 3
f()
# [1] FALSE

# Scenario 4
a <- 10
f()
# [1] FALSE

# Scenario 5
a <- 10
f(a = 5)
# [1] FALSE

I want the answer to be TRUE in Scenario 5.

Upvotes: 1

Views: 496

Answers (2)

MrFlick
MrFlick

Reputation: 206167

Generally you use ... when you are passing parameters to other functions, not when you are using them in the function itself. It also makes a difference if you want to evaluate the parameter value or if you want to leave it unevaulated. If you need the latter, then you can do something like

f = function(...) {
  mc <- match.call(expand.dots = TRUE)
  "a" %in% names(mc)
}

This will return true for both

f(a = 4)
f(a = foo)

even when foo doesn't exist.

Upvotes: 3

Limey
Limey

Reputation: 12451

Does this suffice?

# Define a function for remaining scenarios
f = function(...){"a" %in% names(list(...))}

# Scenario 3
f()
# [1] FALSE

# Scenario 4
a <- 10
f()
# [1] FALSE

# Scenario 5
f(a = 5)
# [1] FALSE

f(a = 5)
[1] TRUE

Upvotes: 3

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