A Clojure vector is an immutable, ordered collection with zero-based indexes. Use one when you need direct access to positions, efficient appends, or removal from the tail. Write a vector by placing its elements in square brackets: `[:a :b :c]`. section 12.1
Common operations preserve the vector's indexed role:
- `(get [:a :b :c] 2)` and `(nth [:a :b :c] 2)` return `:c`. A vector can itself be called with an index, but an out-of-range index then throws `IndexOutOfBoundsException`. section 12.1 section 12.2
- `(assoc [:a :b :c] 2 :d)` returns `[:a :b :d]`; `(conj [:a :b :c] :d)` appends `:d`. `peek` reads the last element, and `pop` removes it. For a portion away from the tail, use `subvec` rather than treating deletion as a vector strength. section 12.1 section 12.2
An index is not a value
`(contains? [1 2 :a :b] 3)` is `true`: position 3 exists. `(contains? [1 2 :a :b] :a)` is `false`, even though `:a` is an element. To test for a value instead, the underlying Java `.contains` method is one option. section 12.1
Vectors work with sequence operations, but are not themselves sequence types: `seq?` returns false. Sequence operations implicitly turn them into sequences. Choose `mapv` or `filterv` when the result should be a vector, and keep that collection choice consistent when designing functions. section 12.1 section 12.11
`vec` builds a persistent vector from a collection, iterable, Java array, or `nil`. It preserves order when the input has an order; sets and unsorted maps provide no specific resulting order. It cannot convert transients or structs. Take care with Java arrays of reference types containing 32 elements or fewer: the resulting vector can alias the array, so later array mutation can change its apparent value. section 12.5
