Select every other element from a vector

55
votes

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.

3
rvectorseq
remove[seq(1, length(remove), by = 2)] - liuminzhao
The structure for seq is seq(start #, end #, interval). So saying seq(remove,2) is telling R: "start at all the numbers in remove, and count up by the default interval (which is 1) until you get to 2." - Señor O

3 Answers

139
votes
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.

21
votes
remove[seq(1,length(remove),2)]
12
votes

Just another alternative:

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