回答編集履歴
1
Lists.fold_leftを使った例を追記しました
test
CHANGED
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@@ -127,3 +127,63 @@
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[("melon", 3); ("grape", 3); ("grape", 1); ("banana", 3); ("apple", 2); ("apple", 1)]
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```
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---
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**追記**
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コメント欄で`Lists.fold_left`を使わない方法で完成したとの連絡を受けました。完成してよかったです！
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参考までに`Lists.fold_left`を使った方法を追記します。
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```ocaml
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# let l = [("apple", 1); ("grape", 1) ; ("apple", 2); ("grape", 3); ("melon", 3); ("banana", 3)];;
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val l : (string * int) list =
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[("apple", 1); ("grape", 1); ("apple", 2); ("grape", 3); ("melon", 3); ("banana", 3)]
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# let l' = List.sort (fun x y -> ~- (compare x y)) l;;
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val l' : (string * int) list =
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[("melon", 3); ("grape", 3); ("grape", 1); ("banana", 3); ("apple", 2); ("apple", 1)]
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# let f = fun acc item ->
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match (item, acc) with
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(* 第一項が一緒なのでまとめる *)
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| ((x, y), (x', ys)::rest) when x = x' -> (x, y::ys)::rest
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(* 第一項が異なる。（accが[]のときも含む） *)
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| ((x, y), _) -> (x, [y])::acc;;
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val f : ('a * 'b list) list -> 'a * 'b -> ('a * 'b list) list = <fun>
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# List.fold_left f [] l';;
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- : (string * int list) list =
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[("apple", [1; 2]); ("banana", [3]); ("grape", [1; 3]); ("melon", [3])]
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```
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