energizer.R

df = data.frame(id=rep(c("A","B","C","D"), each=5),n=1:20)
flags = numeric(20)
flags[c(4,5,7)] = 1
df$flag = flags

reduce_me = function(df, flagname)
{
  to_copy = integer(NROW(df)*2) * NA
  ptr = 1
  for (i in seq_along(df[,1])) {
    if(!df[i, flagname]) 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)]
  df[to_copy,]
}