104 lines
4.3 KiB
Markdown
104 lines
4.3 KiB
Markdown
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---
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title: Clojure Hashmaps
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localeTitle: Clojure Hashmaps
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---
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散列映射是将键映射到值的集合。他们有其他语言的各种名字; Python将它们称为字典，而Javascript的对象基本上像hashmaps一样工作。
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与许多集合一样，hashmap可以以两种方式构造。有构造函数：
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```
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;; Note that each argument is *prepended* to the hashmap, not appended.
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(def a-hashmap (hash-map :a 1 :b 2 :c 3))
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a-hashmap
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; => {:c 3, :b 2, :a 1}
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```
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您还可以使用hashmap文字定义它们。这通常更简洁明了。建议使用逗号分隔散列图中的键/值对，因为它可以使边界更清晰。
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```
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;; This hashmap is actually in the right order, unlike the one above.
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(def another-hashmap {:a 1, :b 2, :c 3})
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another-hashmap
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; => {:a 1, :b 2, :c 3}
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```
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## 关键字和从哈希映射中检索值
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耽误。这是什么？ `:a` ？ `:b` ？ `:c` ？那看起来很奇怪。你看，那些是关键词。它们被称为_关键词，_因为它们经常被用作哈希映射中的键。
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为什么他们经常被用作钥匙？好吧，与字符串不同，关键字可以用作从散列映射中提取值的函数;无需`get`或`nth` ！
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```
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(def string-hashmap {"a" 1, "b" 2, "c" 3})
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("a" string-hashmap)
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; => ClassCastException java.lang.String cannot be cast to clojure.lang.IFn
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(def keyword-hashmap {:a 1, :b 2, :c 3})
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(:a keyword-hashmap)
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; => 1
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;; You can also pass a keyword a default value in case it's not found, just like get.
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(:not-in-the-hashmap keyword-hashmap "not found!")
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; => "not found!"
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```
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## 将其他集合转换为哈希映射
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转换为hashmap非常棘手。为了演示，让我们尝试使用它像`vec`或`seq` 。
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```
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(hash-map [:a 1 :b 2 :c 3])
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; => IllegalArgumentException No value supplied for key: [:a 1 :b 2 :c 3]
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```
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`hash-map`函数认为我们正在尝试使用`[:a 1 :b 2 :c 3]`创建一个hashmap作为其中一个键。观察如果我们给它正确数量的参数会发生什么：
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```
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(hash-map [:a 1 :b 2 :c 3] "foo")
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; => {[:a 1 :b 2 :c 3] "foo"}
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```
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要将序列转换为hashmap，您需要使用并理解`apply` 。幸运的是，这是非常简单的： `apply`基本应用功能之前destructures的集合。
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```
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;; These two expressions are exactly the same.
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(+ 1 2 3)
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; => 6
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(apply + [1 2 3])
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; => 6
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```
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这是您将矢量转换为hashmap的方法：
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```
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(apply hash-map [:a 1 :b 2 :c 3])
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; => {:c 3, :b 2, :a 1}
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;; This is the same as:
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(hash-map :a 1 :b 2 :c 3)
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; => {:c 3, :b 2, :a 1}
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```
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 [IDEOne吧！](https://ideone.com/k9cOjo)
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## 更新哈希映射
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您可以使用`assoc`更新hashmap内的值。这允许您添加新的键/值对或更改旧的键/值对。
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```
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(def outdated-hashmap {:a 1, :b 2, :c 3})
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(def newer-hashmap (assoc outdated-hashmap :d 4))
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newer-hashmap
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; => {:a 1, :b 2, :c 3, :d 4}
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(def newest-hashmap (assoc newer-hashmap :a 22))
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newest-hashmap
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; => {:a 22, :b 2, :c 3, :d 4}
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;; Note that outdated-hashmap has not been mutated by any of this.
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;; Assoc is pure and functional.
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outdated-hashmap
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; => {:a 1, :b 2, :c 3}
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```
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## 何时使用hashmap？
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想要为变量命名时，散列图非常有用。如果你曾经想过自己， _“如果我使用了一个对象......”，_在你突然发现它并意识到你正在使用Clojure之前，请尝试使用一个hashmap。
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如果要将两个不同的值相互关联，它们也很有用。例如，使用一个ROT13密码：你可以将`\A`与`\N` ， `\B`与`\M`等关联起来。（在大多数语言中写这将是漫长而无聊的，但Clojure有一些可以为你生成它的函数让它变得_有趣！_ ）
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