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[py2rb] itertools.islice

Posted at

はじめに

移植やってます。
( from python 3.7 to ruby 2.7 )

islice (Python)

    def _get_by_slice(self, slc):
        self.reset()
        value = list(islice(self, slc.start, slc.stop, slc.step))
        return value

分かりやすい記事。

どうする (Ruby)

def islice(iterable, *args)
  start = args[0] || 0
  stop = args[1] || Float::INFINITY
  step = args[2] || 1
  cnt = 0
  if iterable.is_a?(String)
    it = iterable.split('').to_enum
  else
    it = iterable.to_enum
  end
  start.times do
    it.next
    cnt += 1
  end
  loop do
    yield it.next
    cnt += 1
    step.pred.times do
      it.next
      cnt += 1
    end
    if stop < cnt
      break
    end
  end
end

enumyieldを利用して、isliceメソッドを作成するのが手っ取り早そう。

exma.rb
s = 0..10
ans = []
enum_for(:islice, s, 0, 3).each do |x|
  ans << x
end
p ans
ans.clear
enum_for(:islice, s, 2, nil, 3).each do |x|
  ans << x
end
p ans
ans.clear
enum_for(:islice, s, nil, nil).each do |x|
  ans << x
end
p ans
s = (0..)  # 無限大
ans.clear
enum_for(:islice, s, nil, nil).each do |x|
  break if x > 8  # ようさそう
  ans << x
end
p ans

s = 'abcdefghij'
ans.clear
enum_for(:islice, s, 0, 3).each do |x|
  ans << x
end
p ans

s = 'a'..'j'
ans.clear
enum_for(:islice, s, 0, 3).each do |x|
  ans << x
end
p ans

# output
[0, 1, 2, 3]
[2, 5, 8]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[0, 1, 2, 3, 4, 5, 6, 7, 8]       
["a", "b", "c", "d"]
["a", "b", "c", "d"]

途中で打ち切る遅延評価パターンが便利そう。
もしかしたら、Enumerable#lazy辺りで実装可能かもしれませんが。

メモ

  • Python の islice を学習した
  • 百里を行く者は九十里を半ばとす
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