October 2014
Intermediate to advanced
288 pages
6h 27m
Japanese
Content preview from 実践 機械学習システム
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10
1
章
Python
ではじめる機械学習
ここで興味深い結果が、二つ現れました。一つ目はデータを格納するのにNumPyを使用した場合
（「Naive NumPy」に該当）、Pythonのリストを用いたときよりも3.5倍も時間がかかっていることで
す。これには驚いた人がいるかもしれません。Cで拡張されたライブラリを使用したのだから、早くな
ることを期待するのはもっともです。処理速度が遅い理由の一つは、Pythonから（NumPyのメソッド
を通じて）配列要素にアクセスするのに、通常より時間を要しているためです。処理速度を向上させる
ためには、最適化されたライブラリ内で処理を完結させる必要があります。上の例では、NumPyの関
数である
dot()
を使用したときが、それに該当します。この場合、25 倍以上も処理速度が向上してい
ます。以上をまとめると、実装しようとしているアルゴリズムについて、自前で書いたループ処理を、
最適化されたNumPyやSciPy
側に移すことはできないか、常に点検すべきということです。
ここで注意すべき点があります。それは、「速さには代償がつきものである」ということです。
NumPyの配列には、Pythonで使われるリストのような柔軟性はありません。Pythonのリストはどの
ようなデータタイプでも、まとめて格納することができますが、NumPyの場合、データタイプは一種
類だけしか格納できません。
>>>
a = np.array([1,2,3])
>>>
a.dtype
dtype('int64') ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.