clojure Collections and Sequences Vectors

Example

A vector is denoted by square brackets:

[]
;;=> []

[:foo]
;;=> [:foo]

[:foo :bar]
;;=> [:foo :bar]

[1 (+ 1 1) 3]
;;=> [1 2 3]

In addition using to the literal syntax, you can also use the vector function to construct a vector:

(vector)
;;=> []

(vector :foo)
;;=> [:foo]

(vector :foo :bar)
;;=> [:foo :bar]

(vector 1 (+ 1 1) 3)
;;=> [1 2 3]

You can test whether something is a vector using the vector? predicate:

(vector? [])
;;=> true

(vector? [:foo :bar])
;;=> true

(vector? nil)
;;=> false

(vector? 42)
;;=> false

(vector? :foo)
;;=> false

conj adds elements to the end of a vector:

(conj [] :foo)
;;=> [:foo]

(conj (conj [] :foo) :bar)
;;=> [:foo :bar]

(conj [] :foo :bar)
;;=> [:foo :bar]

count returns the number of items, in constant time:

(count [])
;;=> 0

(count (conj [] :foo))
;;=> 1

(count [:foo :bar])
;;=> 2

You can get the last element of a vector using peek:

(peek [])
;;=> nil

(peek [:foo])
;;=> :foo

(peek [:foo :bar])
;;=> :bar

You can get a new vector without the last element using pop:

(pop [:foo])
;;=> []

(pop [:foo :bar])
;;=> [:foo]

Note that if you try to pop an empty vector, you'll get an IllegalStateException:

(pop [])
;; java.lang.IllegalStateException: Can't pop empty vector

Unlike lists, vectors are indexed. You can get an element of a vector by index in "constant" time using get:

(get [:foo :bar] 0)
;;=> :foo

(get [:foo :bar] 1)
;;=> :bar

(get [:foo :bar] -1)
;;=> nil

(get [:foo :bar] 2)
;;=> nil

In addition, vectors themselves are functions that take an index and return the element at that index:

([:foo :bar] 0)
;;=> :foo

([:foo :bar] 1)
;;=> :bar

However, if you call a vector with an invalid index, you'll get an IndexOutOfBoundsException instead of nil:

([:foo :bar] -1)
;; java.lang.IndexOutOfBoundsException:

([:foo :bar] 2)
;; java.lang.IndexOutOfBoundsException:

You can get a new vector with a different value at a particular index using assoc:

(assoc [:foo :bar] 0 42)
;;=> [42 :bar]

(assoc [:foo :bar] 1 42)
;;=> [:foo 42]

If you pass an index equal to the count of the vector, Clojure will add the element as if you had used conj. However, if you pass an index that is negative or greater than the count, you'll get an IndexOutOfBoundsException:

(assoc [:foo :bar] 2 42)
;;=> [:foo :bar 42]

(assoc [:foo :bar] -1 42)
;; java.lang.IndexOutOfBoundsException:

(assoc [:foo :bar] 3 42)
;; java.lang.IndexOutOfBoundsException:

You can get a sequence of the items in a vector using seq:

(seq [])
;;=> nil

(seq [:foo])
;;=> (:foo)

(seq [:foo :bar])
;;=> (:foo :bar)

Since vectors are indexed, you can also get a reversed sequence of a vector's items using rseq:

(rseq [])
;;=> nil

(rseq [:foo])
;;=> (:foo)

(rseq [:foo :bar])
;;=> (:bar :foo)

Note that, although all lists are sequences, and sequences are displayed in the same way as lists, not all sequences are lists!

'(:foo :bar)
;;=> (:foo :bar)

(seq [:foo :bar])
;;=> (:foo :bar)

(list? '(:foo :bar))
;;=> true

(list? (seq [:foo :bar]))
;;=> false

(list? (rseq [:foo :bar]))
;;=> false