How to create a sequence of numbers by skipping cerain ranges in between?

[Nile]

Hi,

I'd like to create a sequence of numbers such that it'll run for three consecutive numbers and skip the next three and so on within a given range. For instance, if I define a range of 12 to 1000, then the series should like this:
12, 13, 14, (skipped 15, 16, 17) 18, 19, 20, (skipped 21, 22, 23) 24, 25..........1000.

I found some examples where they used functions like i++, but I didn't understand that as there were no explanations. Please let me know if this is possible to do.

Thank you in advance.

[FJCC]

Here is one method using integer division and modulus calculations. There is probably a more elegant solution.

X <- seq(12,1000,1)
IntDiv <- X %/% 3
Mod2 <- IntDiv %% 2
X2 <- X[Mod2 == 0]
X2[1:9]
#> [1] 12 13 14 18 19 20 24 25 26
X2[487:495]
#> [1] 984 985 986 990 991 992 996 997 998

^{Created on 2022-08-28 by the reprex package (v2.0.1)}