[R] Infinite loop

[Hosack, Michael]

R experts,

Hello, I am trying to sample a vector 1:40 without replacement such that no element in the new vector 
is within 7 units of either of its immediate neighbors. This is part of a larger program I am working 
on. The following code works well about 65 % of the time (14/40). The problem I encounter happens when 
the last element remaining to be sampled from the vector STRATA is within 7 digits +- of the last element 
in the vector s1, at which point an infinite loop occurs. At least that's what I think is happening. 
Any help would be greatly appreciated. 

Thank you,

Mike

require(IRanges)
STRATA <- 1:40
s1 <- sample(STRATA, 1)
for (i in seq(from = 1, to = 39, by = 1)){
  repeat{
    tmp <- sample(STRATA, 1)
    if (!any(s1 == tmp) & !any(as.vector(IRanges(s1[length(s1)]-7, s1[length(s1)]+7)) %in% tmp)) break
    }
  s1 <- c(s1,tmp)
}
s1