Select every other element from a vector

问题:

Let's say I had a vector:

remove <- c(17, 18, 19, 20, 24, 25, 30, 31, 44, 45).

How do I select / extract every second value in the vector? Like so: 17, 19, 24, 30, 44

I'm trying to use the seq function: seq(remove, 2) but it doesn't quite work.

Any help is greatly appreciated.

回答1:

remove[c(TRUE, FALSE)]

will do the trick.

---

How it works?

If logical vectors are used for indexing in R, their values are recycled if the index vector is shorter than the vector containing the values.

Here, the vector remove contains ten values. If the index vector c(TRUE, FALSE) is used, the actual command is: remove[c(TRUE, FALSE, TRUE, FALSE, TRUE, FALSE, TRUE, FALSE, TRUE, FALSE)]

Hence, all values with odd index numbers are selected.

回答2:

remove[seq(1,length(remove),2)]

回答3:

Just another alternative:

> remove[seq_along(remove) %% 2 > 0]
[1] 17 19 24 30 44