# Elisp: List

Emacs Lisp's list datatype is known as linked list in computer science.

Here's a high-level tutorial on how to use it.

## Create List

To create a list, write it like this `(list `

.`a` `b` …)

; assign a list to a var (setq mylist (list 1 "b" 3))

; prints a list (message "%S" mylist)

If you do not want the elements evaluated, write it like this:

`'(`

`a` `b` …)

This is equivalent to

`(quote (list `

`a` `b` …))

; assign a list to a var (setq mylist '(a b c)) ; prints a list (message "%S" mylist)

;; create a list of values of variables (let ((x 3) (y 4) (z 5)) (message "%S" (list x y z)) ) ; prints "(3 4 5)"

`(make-list `

→ create a list of length `LENGTH` `INIT`)`LENGTH`, and all elements with value `INIT`.

(make-list 4 0) ;; (0 0 0 0)

## Empty List, nil

In elisp, empty list is equivalent to `nil`

. The following are all equivalent.

`'()`

`(list)`

`nil`

(eq '() (list ) ) ; t (eq '() nil); t (eq (list ) nil ) ; t

## List of Numbers (aka Range Function)

`(number-sequence `

→ returns a list of a range of numbers, from `n` `m` `step`)`n` to `m`, in increment of `step`.

`(number-sequence `

→ default step 1.`n` `m`)

`(number-sequence `

→ returns a list of 1 element of value `n`)`n`.

;; creating a range of numbers ;; just 1 element (number-sequence 5) ; (5) ;; n to m, inclusive (number-sequence 2 5) ; (2 3 4 5) ;; using 3 as step (number-sequence 0 9 3) ; (0 3 6 9) ;; step can be negative (number-sequence 5 0 -1) ; (5 4 3 2 1 0) ;; boundaries can be float but will not include (number-sequence 2.2 5.3) ; (2.2 3.2 4.2 5.2)

## Length

`(length `

→ return number of elements.`list`)

(length '("a" "b" "c") ) ; 3

## Get 1 Element from a List

`(car`

→ first element`list`)`(nth`

→`n``list`)`n`th element`(car (last`

→ last element`list`))

List index count starts from 0.

(car (list "a" "b" "c") ) ; "a"

(nth 1 (list "a" "b" "c") ) ; "b"

(car (last (list "a" "b" "c")) ) ; "c"

## Get Sublist

`(cdr`

→ 2nd to last elements.`list`)`(nthcdr`

→`n``list`)`n`th to last elements.`(butlast`

→ without the last`list``n`)`n`elements.

(cdr (list "a" "b" "c") ) ; ("b" "c")

(butlast (list "a" "b" "c" "d") ) ; ("a" "b" "c") (butlast (list "a" "b" "c" "d") 2) ; ("a" "b")

## Prepend to list

`(cons `

→ Return a new list, with `x` `list`)`x` added to front. (prepend)

(cons "a" (list "c" "d") ) ; ("a" "c" "d") (cons (list "a" "b") (list "c" "d") ) ; (("a" "b") "c" "d")

## Append, Join Lists

`(append `

→ join any sequence types and return a list. Warning: if you want it to return a propert list, the last element must be a list, or `sequence1` `sequence2` …)`nil`.

(equal (list 1 2 3 4) (append (list 1 2) (list 3 4)) ) ;; t

## Modify List Variable

`(push`

→ Add a element to the front variable. Returns the new list.`list`)`(pop`

→ Remove first element from the variable. Returns the removed element.`list`)`(nbutlast`

→ Remove last`list``n`)`n`elements from the variable. Returns the new value of the variable.`(setcar`

→ replaces the first element in`list``x`)`list`with`x`. Returns`x`.`(setcdr`

→ replaces the rest of elements in`list``x`)`list`with`x`. Returns`x`.

(let ((x '(1))) (push 2 x) (equal x '(2 1)) ; true )

(setq mylist '("a" "b" "c")) (pop mylist) ; "a" (print mylist) ; ("b" "c")

The weird names {car, cdr, cons} are like that for historical reasons.

## List to String

Ways to convert a list to string.

;; convert a list to string (mapconcat 'number-to-string '(1 2 3) ",") ;; "1,2,3"

;; convert a list to string (mapconcat 'identity '("a" "b" "c") ",") ;; "a,b,c"

;; convert list to string (format "%s" '(1 "two" 3)) ;; "(1 two 3)" (substring (format "%s" '(1 "two" 3)) 1 -1) ;; "1 two 3"

## Reference

#### Elisp Data Structure

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