95 lines
4.5 KiB
Markdown
95 lines
4.5 KiB
Markdown
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---
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title: Clojure Hashmaps
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---
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A hashmap is a collection that maps keys to values. They have various names in other languages; Python refers to them as dictionaries, and Javascript's objects essentially work like hashmaps.
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A hashmap can, like many collections, be constructed in two ways. There is the constructor function:
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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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You can also define them using a hashmap literal. This is often more concise and clear. Using commas to separate key/value pairs in hashmaps is recommended, as it can make the boundaries more clear.
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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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## Keywords and retrieving values from hashmaps
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Hold up. What is this? `:a`? `:b`? `:c`? Those look odd. Those, you see, are keywords. They're called _key_-words because they're often used as keys in hashmaps.
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Why are they often used as keys? Well, unlike strings, keywords can be used as functions to extract values from a hashmap; no need for `get` or `nth`!
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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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## Converting other collections to hashmaps
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Converting to a hashmap is tricky. To demonstrate, let's try using it like `vec` or `seq`.
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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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The `hash-map` function thinks that we're trying to create a hashmap with `[:a 1 :b 2 :c 3]` as one of the keys. Watch what happens if we give it the right number of arguments:
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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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To convert a sequence to a hashmap, you'll need to use and understand `apply`. Luckily, this is pretty simple: `apply` essentially destructures a collection before applying a function to it.
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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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This is how you would convert a vector to a hashmap:
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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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 <a href='https://ideone.com/k9cOjo' target='_blank' rel='nofollow'>IDEOne it!</a>
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## Update a hashmap
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You can update values inside a hashmap using `assoc`. This allows you to append new key/value pairs or change old ones.
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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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## When to use a hashmap?
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A hashmap is useful when you want to give names to your variables. If you're ever thinking to yourself, _"What if I used an object..."_ before you snap out of it and realise you're using Clojure, try using a hashmap.
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They are also useful if you want to associate two different values with each other. Take, for example, a ROT13 cipher: you could associate `\A` with `\N`, `\B` with `\M`, etc. (This would be long and boring to write in most languages, but Clojure has some functions that can generate it for you and make it _fun!_)
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| [ Previous](//forum.freecodecamp.com/t/clojure-vectors/18421) | [ Home ](//forum.freecodecamp.com/t/clojure-resources/18422) | Next  |
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| [Vectors](//forum.freecodecamp.com/t/clojure-vectors/18421) | [Table of Contents](//forum.freecodecamp.com/t/clojure-resources/18422) | To Be Added |
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