reduce.r

df = data.frame(id=1:20)
flags = numeric(20)
flags[c(5,7)] = 1
df$flag = flags

to_copy = numeric(20) * NA
ptr = 1
for (i in seq_along(df$id)) {
  if(!df[i, "flag"]) next
  candidates = c((i-5):(i-1), (i+1):(i+5))
  candidates = candidates[candidates >= 1 & candidates <= NROW(df)]
  to_copy[ptr:(ptr+length(candidates)-1)] = candidates
  ptr = ptr + length(candidates)
}

to_copy = to_copy[!is.na(to_copy)]
reduced = df[to_copy,]