$value in unidimensional integrals in R [duplicate] - r

Question

I have transitioned from STATA to R, and I was experimenting with different data types so that R's data structures are clear in my mind.
Here's how I set up my data structure:
b<-list(u=5,v=12)
c<-list(u=7)
j<-list(name="Joe",salary=55000,union=T)
bcj<-list(b,c,j)
Now, I was trying to figure out different ways to access u=5. I believe there are three ways:
Try1:
bcj[[1]][[1]]
I got 5. Correct!
Try2:
bcj[[1]][["u"]]
I got 5. Correct!
Try3:
bcj[[1]]$u
I got 5. Correct!
Try4
bcj[[1]][1][1]
Here's what I got:
bcj[[1]][1][1]
$u
[1] 5
class(bcj[[1]][1][1])
[1] "list"
Question 1: Why did this happen?
Also, I experimented with the following:
bcj[[1]][1][1][1][1][1]
$u
[1] 5
class(bcj[[1]][1][1][1][1][1])
[1] "list"
Question 2: I would have expected an error because I don't think so many lists exist in bcj, but R gave me a list. Why did this happen?
PS: I did look at this thread on SO, but it's talking about a different issue.

Answer 1

I think this is sufficient to answer your question. Consider a length-1 list:
x <- list(u = 5)
#$u
#[1] 5
length(x)
#[1] 1
x[1]
x[1][1]
x[1][1][1]
...
always gives you the same:
#$u
#[1] 5
In other words, x[1] will be identical to x, and you fall into infinite recursion. No matter how many [1] you write, you just get x itself.
If I create t1<-list(u=5,v=7), and then do t1[2][1][1][1]...this works as well. However, t1[[2]][2] gives NA
That is the difference between [[ and [ when indexing a list. Using [ will always end up with a list, while [[ will take out the content. Compare:
z1 <- t1[2]
## this is a length-1 list
#$v
#[1] 7
class(z1)
# "list"
z2 <- t1[[2]]
## this takes out the content; in this case, a vector
#[1] 7
class(z2)
#[1] "numeric"
When you do z1[1][1]..., as discussed above, you always end up with z1 itself. While if you do z2[2], you surely get an NA, because z2 has only one element, and you are asking for the 2nd element.
Perhaps this post and my answer there is useful for you: Extract nested list elements using bracketed numbers and names?