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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516
付録
A
NumPy
：応用編
('Jones', 'Bob'),
('Walters', 'Barbara')]
初めて
lexsort
を使ったときに戸惑うであろうポイントは、最後に引数に渡された配列から順に、配
列内のキーが並べ替えに使われていくところです
＊
1
。上の例では、
first_name
を使う前に
last_name
を
使って並べ替えられています
＊
2
。
シリーズやデータフレームの
sort_values
メソッドなど、
pandas
のソート用のメソッドは、
これらの関数の変種を用いて実装されています（これらは、欠損値を含むケースを考慮に入
れた実装となっています）。
A.6.2
使用可能な他のソートアルゴリズム
安定ソートアルゴリズムとは、同等な要素の位置関係をソート後も保存するものです。これは、相対
順序が意味を持つ間接ソートでは特に大切になることがあります。
In [189]: values = np.array(['2:first', '2:second', '1:first', '1:second',
.....: '1:third'])
In [190]: key = np.array([2, 2, 1, 1, 1])
In [191]: indexer = key.argsort(kind='merges
ort')
In [192]: indexer
Out[192]: array([2, 3, 4, 0, 1])
In [193]: values.take(indexer)
Out[193]: ...
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