How to test if list element exists?

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This is actually a bit trickier than you’d think. Since a list can actually (with some effort) contain NULL elements, it might not be enough to check is.null(foo$a). A more stringent test might be to check that the name is actually defined in the list:

foo <- list(a=42, b=NULL)
foo

is.null(foo[["a"]]) # FALSE
is.null(foo[["b"]]) # TRUE, but the element "exists"...
is.null(foo[["c"]]) # TRUE

"a" %in% names(foo) # TRUE
"b" %in% names(foo) # TRUE
"c" %in% names(foo) # FALSE

…and foo[["a"]] is safer than foo$a, since the latter uses partial matching and thus might also match a longer name:

x <- list(abc=4)
x$a  # 4, since it partially matches abc
x[["a"]] # NULL, no match

[UPDATE] So, back to the question why exists('foo$a') doesn’t work. The exists function only checks if a variable exists in an environment, not if parts of a object exist. The string "foo$a" is interpreted literary: Is there a variable called “foo$a”? …and the answer is FALSE…

foo <- list(a=42, b=NULL) # variable "foo" with element "a"
"bar$a" <- 42   # A variable actually called "bar$a"...
ls() # will include "foo" and "bar$a" 
exists("foo$a") # FALSE 
exists("bar$a") # TRUE

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