library(magrittr)

# The data
sal_dat <- c(25, 15, 20, 25, 18, 12, 24, 30, 15, 20, 10, 10, 11, 14, 22, 16)
salary <- sal_dat * 1000

# Function for systematic sampling
sys_sample <- function (df, r, n) {
  k <- (df %>% length) / n

  b <- numeric(); a <- r
  b[1] <- a

  for (i in 2:n) {
    a <- a + k
    if (a > (df %>% length)) {
      a <- a - (df %>% length)
    }
    b[i] <- a
  }

  return (list(Data = df[b], Index = b, K = k))
}

# Do the sampling for random start,
# r = 2, and number of sample, n = 4
sys_sample(salary1, r = 2, n = 8)

# OUTPUT
$Data
[1] 15000 25000 12000 30000 20000 10000 14000 16000

$Index
[1]  2  4  6  8 10 12 14 16

$K
[1] 2