Help us understand the problem. What is going on with this article?

【F#】Seq.pairwise

More than 5 years have passed since last update.

F# の Seq.pairwise は良いですね。
pairwise は zip を使って次のように実装することもできますが、

let pairwiseByZip x = Seq.zip x (Seq.skip 1 x)

これだと、シーケンスによっては扱いが面倒になる場合があります。たとえば、ファイルを読み込んでシーケンスにするような場合を考えます:

let lines =
    seq {
        use r = new StreamReader(path)
        printfn "read: %s" path
        while r.Peek() >= 0 do
            yield r.ReadLine()
    }

printfn "%d" (lines |> pairwiseByZip |> Seq.length)

これに対し、10行のファイル sample.txt のパスをpathとして与えて実行すると、次のようになりました。

read: sample.txt
read: sample.txt
9

2回ファイルを読み込んでいました。ここで、pairwiseByZipをSeq.pairwiseに変える:

printfn "%d" (lines |> Seq.pairwise |> Seq.length)

と結果は

read: sample.txt
9

となり、ファイルの読み込みは1回です。

シーケンスを扱う場合はこういうことにも気をつけないと(自戒)。

Why not register and get more from Qiita?
  1. We will deliver articles that match you
    By following users and tags, you can catch up information on technical fields that you are interested in as a whole
  2. you can read useful information later efficiently
    By "stocking" the articles you like, you can search right away
Comments
No comments
Sign up for free and join this conversation.
If you already have a Qiita account
Why do not you register as a user and use Qiita more conveniently?
You need to log in to use this function. Qiita can be used more conveniently after logging in.
You seem to be reading articles frequently this month. Qiita can be used more conveniently after logging in.
  1. We will deliver articles that match you
    By following users and tags, you can catch up information on technical fields that you are interested in as a whole
  2. you can read useful information later efficiently
    By "stocking" the articles you like, you can search right away
ユーザーは見つかりませんでした