How get the element from the vector which is in even positions (2, 4, 6, 8, etc) IN R

Question

Anyone can help me how to construct the R code. I want to get the element from the vector which is in even position.

The image contains the whole list and i need to pick the elements which is in even position (2,4,6,8,etc).

See also http://stackoverflow.com/questions/5237557/extracting-every-nth-element-of-a-vector — patrickmdnet, Jul 26 '16 at 12:16
Another almost-dup: http://stackoverflow.com/questions/13461829/r-how-to-list-every-other-element — patrickmdnet, Jul 26 '16 at 12:17

score 6 · Answer 1 · answered Jul 26 '16 at 12:11

Two observations can be combined to solve this:

Logical indices allow you to pick an element based on a condition:
```
c(1, 2)[c(TRUE, FALSE)]
```
picks the first element but not the second.
Indices that are shorter than your array are recycled until the end of the array:
```
letters[c(TRUE, FALSE)]
```
is the same as
```
letters[c(TRUE, FALSE, TRUE, FALSE, TRUE, FALSE, …)]
```
and picks, 'a', 'c', 'e', etc.

So you can just use:

winner[c(FALSE, TRUE)]

score 4 · Accepted Answer · edited Jul 26 '16 at 12:21

4

We can use seq

winners[seq(2, length(winners), by = 2)]

Or use %%

winners[seq_along(winners) %%2 == 0]

edited Jul 26 '16 at 12:21

Tensibai

answered Jul 26 '16 at 12:10

akrun

5

Once again I strongly advise against obscure implicit conversions, i.e. instead of `!foo %% 2` I would write `foo %% 2 == 0`. But this approach is in fact the solution I’d use, as I find it the most logical. – Konrad Rudolph Jul 26 '16 at 12:14
2

No codegolf allowed when Konrad's around – David Arenburg Jul 26 '16 at 12:16
3

@DavidArenburg Only on codegolf.stackexchange.com ;-) Maybe I need to change my name to “Konrad Nogolph”. – Konrad Rudolph Jul 26 '16 at 12:17
Now why the heck was this downvoted?! – Konrad Rudolph Jul 26 '16 at 14:50
@KonradRudolph I guess it was for the posting the time earlier. – akrun Jul 26 '16 at 15:11
@akrun Possible, though by the time that was edited out, your post was still at +1. – Konrad Rudolph Jul 26 '16 at 15:15

score 0 · Answer 3 · answered Jul 26 '16 at 12:15

0

You can use the colon operator for this:

winner[2*(1:(length(winner)/2))]

answered Jul 26 '16 at 12:15

patrickmdnet

3 Answers3