CODEWITHSUNDEEP
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Egus
Egus

Reputation: 121

Referencing list elements in R is ambiguous - how to do it right?

Using lists in R seems to be tricky. I was not aware that referencing list elements with identical first part of name is ambiguous: 

opts = list()
opts$value = NULL
opts$valueDefault = c(1,2,3)

print(opts)                # note: displaying the whole list does not reveal 
                           # the element "value"
$valueDefault
[1] 1 2 3 



print(opts$value)          # with this notation I do not get the correct (intended) 
                           # result
[1] 1 2 3

print(opts[["value"]])     # with this notation I do
NULL

Upvotes: 0

Views: 122

Answers (2)

Sven Hohenstein
Sven Hohenstein

Reputation: 81693

If you want a list element to be NULL, you have to create a list with NULL.

opts <- list(value = NULL)

> opts
$value
NULL

Upvotes: 1

Dason
Dason

Reputation: 61933

Note that setting a list element to NULL is the same as removing it from the list. Your list at that point literally only contains a single element called valueDefault. Try setting value to NA instead to see the difference.

Reading over ?"$" will give some more info on this. And to summarize briefly - one difference between referencing via $ and via [[ is that $ will do partial matching. So since your list only contained valueDefault and you said opts$value it realized that you must be referring to valueDefault. opts[["value"]] doesn't try to do partial matching (by default)

Upvotes: 3

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