Similar to my question at Using a sample list as a template for sampling from a larger list without wraparound, how can I know do this allowing for a wrap-around?
Thus, if I have a vector of letters:
> all <- letters
> all
[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t" "u" "v" "w" "x" "y" "z"
and then I define a reference sample from letters as follows:
> refSample <- c("j","l","m","s")
in which the spacing between elements is 2 (1st to 2nd), 1 (2nd to 3rd) and 6 (3rd to 4th), how can I then select n samples from all that have identical, wrap-around spacing between its elements to refSample? For example, "a","c","d","j", "q" "s" "t" "z" and "r" "t" "u" "a" would be valid samples, but "a","c","d","k" would not.
Again, parameterised for a function is best.
I would have left it as an exercise but here goes --
all <- letters
refSample <- c("j","l","m","s")
pick_matches <- function(n, ref, full, wrap = FALSE) {
iref <- match(ref,full)
spaces <- diff(iref)
tot_space <- sum(spaces)
N <- length( full ) - 1
max_start <- N - tot_space*(1-wrap)
starts <- sample(0:max_start, n, replace = TRUE)
return( sapply( starts, function(s) full[ 1 + cumsum(c(s, spaces)) %% (N+1) ] ) )
}
> set.seed(1)
> pick_matches(5, refSample, all, wrap = FALSE)
[,1] [,2] [,3] [,4] [,5]
[1,] "e" "g" "j" "p" "d"
[2,] "g" "i" "l" "r" "f"
[3,] "h" "j" "m" "s" "g"
[4,] "n" "p" "s" "y" "m"
> pick_matches(5, refSample, all, wrap = TRUE)
[,1] [,2] [,3] [,4] [,5]
[1,] "x" "y" "r" "q" "b"
[2,] "z" "a" "t" "s" "d"
[3,] "a" "b" "u" "t" "e"
[4,] "g" "h" "a" "z" "k"