July 2018
Intermediate to advanced
596 pages
13h 35m
Japanese
Content preview from Pythonによるデータ分析入門 第2版 ―NumPy、pandasを使ったデータ処理
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A.4
ufunc
の使い方：応用編
507
A.4
ufunc
の使い方：応用編
多くの人にとってユニバーサル関数（
ufunc
）を使う場面は限られていると思います。要素単位での高
速な操作くらいではないでしょうか。しかし、ユニバーサル関数には他にもたくさんの機能があります
ので、ここでそれらの使い方の一部を紹介します。これらの機能は、ループを使わずに簡潔なコードを
書く手助けになることでしょう。
A.4.1
ufunc
のインスタンスメソッド
NumPy
の二項
ufunc
には、特殊なベクトル化演算のための特殊メソッドが用意されています。これ
らは表A-2にまとめられていますが、どのように動作するか具体例をいくつかお見せしましょう。
reduce
は配列を
1
つ引数に取り、軸が指定されている場合はその軸に沿って、配列に対して一連の
2
項演算を行って値を集計します。例えば、ある配列内の要素の合計を求めるのに、順に足していく代
わり
に、
np.add.reduce
を使って計算することができます。
In [115]: arr = np.arange(10)
In [116]: np.add.reduce(arr)
Out[116]: 45
In [117]: arr.sum()
Out[117]: 45
初期値は
ufunc
によって異なります（上の例の
add
の場合は
0
）。また、引数で軸が指定されている場
合、集計はその軸に沿って行われます。うまくこの機能を使うことで、コードが簡潔になる場合があ
ります。もう少し実用的な例として、
np.
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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.