CODEWITHSUNDEEP
r
MOHAMMED
MOHAMMED

Reputation: 400

R select specific length from a sequence

Say I have a sequence from 1 to (a+b)*q, I would like to select the 1st "a" elements and skip the following "b" elements and repeat. For example

a = 5; b = 4; q = 3
seq(1, (a+b)*q)
1:27
a <- c(1:5, 10:14, 19:23)

For sequences > 1000, it is hard to do it manually, so is there a way to with a code?

Upvotes: 2

Views: 179

Answers (1)

r2evans
r2evans

Reputation: 160417

We can take advantage of "recycling" of arguments/indices:

seq(1, (a+b)*q)[c(rep(TRUE,5),rep(FALSE,4))]
#  [1]  1  2  3  4  5 10 11 12 13 14 19 20 21 22 23

Recycling here means that the indices are of length 9, but the vector itself is length 27, a multiple of 9. This means that the 9 indices are repeated three times consecutively.

When it is not a clear multiple, though, R may not tell you anything. Let's change the "4" to a "3", where length 8 does not go evenly into 27:

seq(1, (a+b)*q)[c(rep(TRUE,5),rep(FALSE,3))]
#  [1]  1  2  3  4  5  9 10 11 12 13 17 18 19 20 21 25 26 27
#       ^^^^^^^^^^^^^  ------------- ^^^^^^^^^^^^^^ --------
#          len5     gap3    len5   gap3   len5    gap3  len3

So while the 5/3/5/3/... cycling is consistent, it doesn't necessarily end on a 5. If that's okay, then you're good. If not, you should add in some logical to catch this type of mismatch.

Upvotes: 4

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