在R中使用lapply和str

Question

When I input the following command in R.

lapply(list(1,TRUE),str)

It displays the following results:

num 1

 logi TRUE

[[1]]

NULL


[[2]]

NULL

I know num 1 and logi TRUE are the structure of each element. What is the meaning of the following part?

[[1]]

NULL


[[2]]

NULL

Answer 1

This happens because the return value of str is NULL . Consider:

a <- str(list(1,TRUE))
a # NULL

Now because you use lapply , lapply will return a list with an equal number of elements as the input list. In your case, this is a list of two elements that are both NULL .

Answer 2

The [[N]] output means

1. The first element of this list is a vector with a single element NULL

2. the second element of this list is a vector with a single element NULL

Appending the [[N]] to the variable that holds the list actually returns the element.

Answer 3

In addition to the previous answers, it could be useful to use

invisible(lapply(list(dt1,dt2...), str)) 

so you don't print anything more than the structure of the objects.