Select every other element from a vector

Question

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.

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, Nov 19 '12 at 20:55

Sven Hohenstein · Accepted Answer · 2012-11-20T08:17:48.213

160

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.

edited Nov 20 '12 at 08:17

answered Nov 19 '12 at 20:59

Sven Hohenstein

80,497
17
145
168

1

Perfectly cromulent R! – DuckPyjamas Oct 29 '22 at 01:25
I guess this works only if the vector has an even number of elements, though. – riccardo-df Feb 11 '23 at 13:12

score 26 · Answer 2 · answered Nov 19 '12 at 20:48

26

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

answered Nov 19 '12 at 20:48

Grega Kešpret

11,827
6
39
44

score 15 · Answer 3 · edited Feb 14 '22 at 09:53

15

Just another alternative:

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

edited Feb 14 '22 at 09:53

andschar

3,504
2
27
35

answered Sep 22 '14 at 21:08

Jilber Urbina

58,147
10
114
138

Select every other element from a vector

3 Answers3

Linked

Related