Blogger code #2 (filter, snippet #2)

blogger_a_more_efficient_filter_2.R

quicker.count = function(data, window.width = default.window.width) {
# ...
  # start and do the counting
  for(i in 2:n.positions) {
    # the value leaving the window is the first item in the
    # previous window, i.e. data[i - 1]
    old.data.idx = x[i - 1] + 1
    frequency[old.data.idx] = frequency[old.data.idx] - 1
    # the value entering the window is the last item in the
    # current window
    new.data.idx = x[i + window.width - 1] + 1
    frequency[new.data.idx] = frequency[new.data.idx] + 1

    ans[i] = count.positives(frequency)
  }
# ...
}