Vectors and Lists

Create a vector literal:

[1 2 3]

Create a vector literal with elements of different types:

[42 13.5 "hello" true nil]

Create a nested vector:

[1 [2 3 [4 5] 6] 7]

Create a vector using a function:

(vector 1 2 3) ; [1 2 3]

Get the first element of a vector:

(first [1 2 3]) ; 1
(first [1]) ; 1
(first []) ; nil

Get all elements except the first of a vector:

(rest [1 2 3]) ; (2 3)
(rest [1]) ; ()
(rest []) ; ()

A sequence rather than a vector is returned.

Tet the third element of a vector:

(first (rest (rest [1 2 3 4 5]))) ; 3

To get to the nth element requires n-1 calls of rest, followed by a single call of first. The nth function is more convenient (and faster):

(nth [1 2 3 4 5] 2) ; 3

The index is zero-based. The same can be achieved by using the vector like a function:

([1 2 3 4 5] 2) ; 3

Add an element to the end of a vector:

(def songs ["Pale Fire" "The Eleventh Hour" "One"])
(conj songs "Monument")
;; ["Pale Fire" "The Eleventh Hour" "One" "Monument"]

The original vector is not modified, but a new, larger vector is created, sharing the common data with the original vector.

Add an element to the front of a vector:

(def amigos ["Dave" "Janick"])
(cons "Adrian" amigos) ; ("Adrian" "Dave" "Janick")

Again, a sequence is returned, and the original vector is not modified.

Create a list literal:

'(1 2 3)

Create a list using a function:

(list 1 2 3) ; (1 2 3)

The leading ' can be left away for empty lists and within list literals:

()
'(1 2 ("hello" true (1.25 "what" 3) 2.5 5.0))

Apply additional operations to a list:

(def years (list 1987 1992 2003 2007 2012))
(count years) ; 5
(first years) ; 1987
(rest years) ; (1992 2003 2007 2012)
(nth year 2) ; 2003

Extend a list:

(def days (list "Tuesday" "Wednesday" "Thursday"))
(conj days "Monday") ; ("Monday" "Tuesday" "Wednesday" "Thursday")
(cons "Monday" days) ; ("Monday" "Tuesday" "Wednesday" "Thursday")

Applied to a list, both conj and cons add new elements to the front for better efficiency.

Exercises

Accessing Elements

Given the following vector:

(def numbers [1 2 [3 [4 5 6 [7 8] 9]]])

Write an expression that returns the element 8.

Hint: Use the first and rest functions.

Test: The element 8 is returned.

(first (rest (first (rest (rest (rest (first (rest (first (rest (rest numbers)))))))))))

Extending a List

Given the following list:

(def numbers '(3 4 5 6 7 8 9))

Write an expression that appends the elements 0, 1, and 2 to the front of the list.

Hint: Use the cons function.

Test: The sequence (0 1 2 3 4 5 6 7 8 9) is returned.

(cons 0 (cons 1 (cons 2 numbers)))

Extending a Vector

Given the following vector:

(def numbers [3 4 5 6 7 8])

Write an expression that appends the elements 0, 1, and 2 to the front, and the element 9 to the end.

Hint: Use the cons and conj functions. Unlike cons, conj returns a vector.

Test: The sequence (0 1 2 3 4 5 6 7 8 9) is returned.

(cons 0 (cons 1 (cons 2 (conj numbers 9))))