如何每3个索引对R中的列表进行子集化？

Question

I have a list which has 500 elements, but I want to get every 3rd element and save it to a variable. So I'd want list[1] , list[4] , list[7] , list[10] and etc saved to one variable.

I tried sub.list <- list[1:500, by = 3] but this doesn't work.

Answer 1

L <- as.list(1:500) # create a list

L[seq(1, length(L), 3)]
# or, use recycling
L[c(TRUE, FALSE, FALSE)]  

Answer 2

尝试这个：

   sub.list<-myList[seq_along(myList)%%3==1]

Answer 3

You can also use Filter:

L <- as.list(1:500) # create a list
Filter(function(i) {i %% 3 == 1},seq_along(L))

But won't work if you have NA's.