Indexing nested list structure in 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 dat

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.

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Perhaps this post and my answer there is useful for you: Extract nested list elements using bracketed numbers and names?