156

I would like to create a vector in which each element is the i+6th element of another vector.

For example, in a vector of length 120 I want to create another vector of length 20 in which each element is value i, i+6, i+12, i+18... of the initial vector, i.e. I want to extract every 6th element of the original.

zx8754
  • 52,746
  • 12
  • 114
  • 209
ThallyHo
  • 2,667
  • 5
  • 22
  • 19

5 Answers5

194
a <- 1:120
b <- a[seq(1, length(a), 6)]
nico
  • 50,859
  • 17
  • 87
  • 112
  • 11
    It is better to use seq.int(1L, length(a), 6L), at least for long vectors – Wojciech Sobala Mar 08 '11 at 20:45
  • 1
    @WojciechSobala Could you comment why it is better? – dpel May 09 '16 at 10:07
  • 1
    @DavidPell `seq.int` is faster in microbenchmarks, but I suspect that any performance increases in an actual program would be dwarfed by the running time of other parts. – Sean1708 Jul 22 '16 at 14:37
  • I hate to compare Python with R, but how great could PyRon be? `a = 1:120; b = [::6]`. Python can't do the former, R not the latter. – bers Jul 29 '20 at 10:56
52

Another trick for getting sequential pieces (beyond the seq solution already mentioned) is to use a short logical vector and use vector recycling:

foo[ c( rep(FALSE, 5), TRUE ) ]
zx8754
  • 52,746
  • 12
  • 114
  • 209
Greg Snow
  • 48,497
  • 6
  • 83
  • 110
  • 4
    An advantage of this approach is it can be used on a temporary; in order to use `seq` you have to be able to call `length` on the vector. `letters[letters < 'm'][c(TRUE, FALSE, FALSE)]` – Matt Chambers Sep 14 '17 at 21:07
29

I think you are asking two things which are not necessarily the same

I want to extract every 6th element of the original

You can do this by indexing a sequence:

foo <- 1:120
foo[1:20*6]

I would like to create a vector in which each element is the i+6th element of another vector.

An easy way to do this is to supplement a logical factor with FALSEs until i+6:

foo <- 1:120
i <- 1
foo[1:(i+6)==(i+6)]
[1]   7  14  21  28  35  42  49  56  63  70  77  84  91  98 105 112 119

i <- 10
foo[1:(i+6)==(i+6)]
[1]  16  32  48  64  80  96 112
Sacha Epskamp
  • 46,463
  • 20
  • 113
  • 131
2

To select every nth element from any starting position in the vector

nth_element <- function(vector, starting_position, n) { 
  vector[seq(starting_position, length(vector), n)] 
  }

# E.g.
vec <- 1:12

nth_element(vec, 1, 3)
# [1]  1  4  7 10

nth_element(vec, 2, 3)
# [1]  2  5  8 11
stevec
  • 41,291
  • 27
  • 223
  • 311
2

To select every n-th element with an offset/shift of f=0,...,n-1, use

vec[mod(1:length(vec), n)==f]

Of course, you can wrap this in a nice function:

nth_element <- function(vec, interval, offset=0){
    vec[mod(1:length(vec), interval)==mod(offset, interval)]
}
user11130854
  • 333
  • 2
  • 9