R difference between [[]] and []

Question

I want to clearly understand difference between [] and [[]] and I ran below snippet of code. I know that [[]] returns a individual member of list while [] returns list of elements. But then why do i get an error when I run "all_data[1]=list(5,6)" but no error when I run "all_data[[1]]=list(5,6)" or when I run "all_data[2]=5"

all_data <- list()
all_data[2]=5
all_data[1]=list(5,6)



all_data[[1]]=list(5,6)
all_data

as per the fist comment of the first answer, adding a line of a code which helps to understand further

all_data[1:2] <- list(5,6)

This blog post explains subsetting operations (including subsetting and assignment) very clearly http://adv-r.had.co.nz/Subsetting.html. @Senor O I just wanted to copy the link in a comment rather than copy paste from the blog and answer the question. There is a section there "subsetting and assignment" which addresses this specific question. — konvas, Jun 02 '14 at 16:25

score 5 · Accepted Answer · answered Jun 02 '14 at 16:34

5

all_data[1]=list(5,6) gives you a Warning (not an error) that the lengths aren't the same. You can't set a one-element list to a two-element list. It's like trying x <- 1; x[1] <- 1:2.

But you can set one element of a list to contain another list, which is why all_data[[1]]=list(5,6) works.

answered Jun 02 '14 at 16:34

Joshua Ulrich

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I think adding the example `all_data[1:2] – Dason Jun 02 '14 at 16:36
I understand how this works, but it doesn't seem intuitive. Why must `a` in `List[1] – Señor O Jun 02 '14 at 16:43
@SeñorO And what should something like `x – Dason Jun 02 '14 at 18:17
@Dason nothing, because `x[1]` would never return a length 2 vector, whereas `List[1]` can return a length 2 list. – Señor O Jun 02 '14 at 18:26
@SeñorO - No. List[1] returns a 1 element list. That element itself could be a list that has multiple elements but List[1] will always give a sublist of length 1. – Dason Jun 02 '14 at 18:48
@Dason I see it now - that makes sense – Señor O Jun 02 '14 at 19:06

R difference between [[]] and []

1 Answers1