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* Javascript -> JavaScript (English) * Update technical documentation page for required change * Update use-class-syntax-to-define-a-constructor-function.english.md * Update left-factorials.md
64 lines
3.4 KiB
Markdown
64 lines
3.4 KiB
Markdown
---
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title: Clojure Vectors
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---
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A vector is perhaps the most simple type of collection in Clojure. You can think of it like an array in JavaScript. Let's define a simple vector:
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(def a-vector [1 2 3 4 5])
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;; Alternatively, use the vector function:
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(def another-vector (vector 1 2 3 4 5))
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;; You can use commas to separate items, since Clojure treats them as whitespace.
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(def comma-vector [1, 2, 3, 4, 5])
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You'll see that it uses square brackets, just like an array in JS. Since Clojure, like JS, is dynamically typed, vectors can hold elements of any type, including other vectors.
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(def mixed-type-vector [1 "foo" :bar ["spam" 22] #"^baz$"])
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## Adding items to a vector
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You can append items to a vector using `conj`. You can also prepend to a list using `into`, but note that `into` is intended for merging two vectors, so both its arguments must be vectors, and using `into` is slower than using `conj`.
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(time (conj [1 2] 3))
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; => "Elapsed time: 0.032206 msecs"
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; [1 2 3]
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(time (into [1] [2 3]))
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; => "Elapsed time: 0.078499 msecs"
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; [1 2 3]
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 <a href='https://ideone.com/wBSUEd' target='_blank' rel='nofollow'>IDEOne it!</a>
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## Retrieving items from a vector
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You can retrieve items from a vector using `get`. This is equivalent to using bracket notation to access items in an array in many imperative languages. Items in a vector are 0-indexed, counting from the left.
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var arr = [1, 2, 3, 4, 5];
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arr[0];
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// => 1
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In Clojure, this would be written like so:
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(def a-vector [1 2 3 4 5])
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(get a-vector 0)
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; => 1
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You can also give `get` a default value, if you give it an index that isn't in the array.
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;; the list doesn't have 2147483647 elements, so it'll return a string instead.
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(get a-vector 2147483646 "sorry, not found!")
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; => "sorry, not found!"
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## Converting other collections into vectors
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Non-vector data structures can be converted into vectors using the `vec` function. With hashmaps, this produces a 2D vector containing pairs of keys and values.
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(vec '(1 2 3 4 5))
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; => [1 2 3 4 5]
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(vec {:jack "black" :barry "white"})
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; => [[:jack "black"] [:barry "white"]]
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## When to use a vector?
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A vector should be used in almost all cases if you need a collection, because they have the shortest random-access times, which makes it easy to retrieve items from a vector. Note that vectors are ordered. If order doesn't matter, it may be better to use a set. Also note that vectors are designed for appending items; if you need to prepend items, you might want to use a list.
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