Weird behavior of R brackets - r

Question

So I've been trying to get a subset of a character vector for the last hour or so. In my (floundering) attempt to get this working I ran into an interesting characteristic of R. I have data (after JSON parsing) in the form of
[[1]]
[[1]]$business_id
[1] "rncjoVoEFUJGCUoC1JgnUA"
[[1]]$full_address
[1] "8466 W Peoria Ave\nSte 6\nPeoria, AZ 85345"
[[1]]$open
[1] TRUE
[[1]]$categories
[1] "Accountants" "Professional Services" "Tax Services"
[4] "Financial Services"
[[1]]$city
[1] "Peoria"
[[1]]$review_count
[1] 3
[[1]]$name
[1] "Peoria Income Tax Service"
[[1]]$neighborhoods
list()
[[1]]$longitude
[1] -112.2416
[[1]]$state
[1] "AZ"
[[1]]$stars
[1] 5
[[1]]$latitude
[1] 33.58187
[[1]]$type
[1] "business"
Here's the code I'm using
#!/usr/bin/Rscript
require(graphics)
require(RJSONIO)
parsed_data <- lapply(readLines("yelp_phoenix_academic_dataset/yelp_academic_dataset_business.json"), fromJSON)
#parsed_data[,c("categories")]
print(parsed_data[1])
As I was trying to drop everything but the categories column I ran into this interesting behaviour
print(parsed_data[1])
print(parsed_data[1][1])
print(parsed_data[1][1][1][1][1][1])
All produce the same output (the one posted above). Why is that?

Answer 1

This is the difference between [ and [[. It is hard to search for these online, but ?'[' will bring up the help.
When indexing a list with [, a list is returned:
list(a=1:10, b=11:20)[1]
## $a
## [1] 1 2 3 4 5 6 7 8 9 10
This is a list of one element, so repeating the operation again results in the same value:
list(a=1:10, b=11:20)[1][1]
## $a
## [1] 1 2 3 4 5 6 7 8 9 10
[[ returns the element, not a list containing the element. It also only accepts a single index (whereas [ accepts a vector):
list(a=1:10, b=11:20)[[1]]
## [1] 1 2 3 4 5 6 7 8 9 10
And this operation is not idempotent on lists:
list(a=1:10, b=11:20)[[1]][[1]]
## [1] 1

Answer 2

Your JSON data is currently stored in a list, rather than a vector, so the indexing is different.
As Matthew has pointed out, there is a difference between using [] to access an element and using [[]]. For a discussion on this I will refer you to this stack overflow thread:
In R, what is the difference between the [] and [[]] notations for accessing the elements of a list?
Looking at the data print out your data is stored as a nested list:
parsed_data[[1]]
Will give you a list containing each of the columns. To access the categories column you can use any of the following:
parsed_data[[1]][["categories"]]
parsed_data[[1]][[4]]
parsed_data[[1]]$categories
This will give you a vector of names as a you'd expect:
## [1] "Accountants" "Professional Services" "Tax Services"
## [4] "Financial Services"
Note that when accessing by index (either named or numeric) you still have to use the double bracket notation: [[]]. If you use [] instead, it will give you a list instead of a vector:
parsed_data[[1]]["categories"]
## [[1]]
## [1] "Accountants" "Professional Services" "Tax Services"
## [4] "Financial Services"