Collection - Vector

A vector is a very flexible and easy to use collection type, so it is very common and often the default choice for holding information.

We have seen how to define a vector using the square brackets [ ] notation. You can also create a vector using the function vector as well as convert another collection to a vector using the vec function.

(vector 1 2 3)
;; (vec (list 1 2 3))

Getting values from a vector

Our friends first, second, and nth work here too; but unlike lists, nth is fast on vectors. That’s because internally, vectors are represented as a very broad tree of elements, where each part of the tree branches into 32 smaller trees. Even very large vectors are only a few layers deep, which means getting to elements only takes a few hops.

Get a value from a vector by its position

Use the functions first, second, last and rest to get values from a vector collection. Each function takes a collection as an argument.

examples

Looking up values in a specific position

A vector is indexed, meaning that each of the values are assigned a number in the order they are in the vector. Indexes start at zero, rather than one.

The nth function takes a vector and a position in the index and returns the value at that position

(nth ["Jane Doe" 162 "Clojure"] 0)

(def jane ["Jane Doe" "age 21" "height 163" "loves Clojure"])

Get a value from the vector called Jane using nth

Counting the number of values in a vector

The function count will return the number of values in a collection.

(count [1 2 3])

Adding values to a vector

The conj function will join new values to a vector. conj is short for conjoin and when adding vales to a vector they are placed at the end of the collection.

Are we changing the vector? Try the following code and see…

(conj [1 2 3] 4)
(conj ["Can" "we" "join"] ["two" "collections"])
(conj ["Can" "we" "join"] "multiple" "values")

(def favourite-ice-cream-flavors ["Mint" "Vanilla"])
(conj favourite-ice-cream-flavors "Chocolate")
favourite-ice-cream-flavors

Hint: The conj function works with all collection types (list, vector, map, set).

Vectors are equal when values are equal

Finally, note that vectors containing the same elements are considered equal in Clojure.

Take a guess as to if the following are equal

(= [1 2 3] [1 2 3])
(= [1 2 3] [4 5 6])
(= ["Hello" "World"] ["Hello" "World" "Wide" "Web"])
(= '(1 2 3) [1 2 3])

If any line above is equal, it will return true when pasted into the code box below