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# Enum.sort または Enum.sort_by で安定ソートさせるために気をつけること

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プログラミングElixir読み途中です。

`Enum.sort/2` の第二引数には、同じ値の場合は `true` を返すようなを関数を渡さないと安定ソートになりません。

``````iex> words = ["some","kind","of","monster"]
["some", "kind", "of", "monster"]
iex> Enum.sort words, &(String.length(&1) < String.length(&2))
["of", "kind", "some", "monster"]
iex> Enum.sort words, &(String.length(&1) <= String.length(&2))
["of", "some", "kind", "monster"]
``````

`Enum.sort_by/3` でも同様です。

``````iex> words = ["some","kind","of","monster"]
["some", "kind", "of", "monster"]
iex> Enum.sort_by words, &String.length/1, &</2
["of", "kind", "some", "monster"]
iex> Enum.sort_by words, &String.length/1, &<=/2
["of", "some", "kind", "monster"]
``````

Enum.html#sort/2 の Example には、このことが書いていますが、sort_byの方は特に何も書いてないので… `sort`の方も見ろやってことですかね(´・ω:;.:...

Enum.sort/2 not stable · Issue #2163 · elixir-lang/elixirのコメントにも書いてますが、自分もつい `>=``<=` より `>``<` を使うことが多いので気をつけます(｀･ω･´)ゞ

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