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elt

elt Accessor

Syntax:

elt sequence index → object

(setf (elt sequence index**)** new-object**)**

Arguments and Values:

sequence—a proper sequence.

index—a valid sequence index for sequence.

object—an object.

new-object—an object.

Description:

Accesses the element of sequence specified by index.

Examples:

(setq str (copy-seq "0123456789")) → "0123456789" 
(elt str 6) → #\6 
(setf (elt str 0) #\#) → #\# 
str → "#123456789"

Exceptional Situations:

Should be prepared to signal an error of type type-error if sequence is not a proper sequence. Should signal an error of type type-error if index is not a valid sequence index for sequence.

See Also:

aref, nth, Section 3.2.1 (Compiler Terminology)

Notes:

aref may be used to access vector elements that are beyond the vector ’s fill pointer .

Expanded Reference: elt​

Basic element access​

elt accesses an element of any sequence by its zero-based index. It works uniformly on lists, vectors, and strings.

(elt '(a b c d) 2)
=> C
(elt "hello" 0)
=> #\h
(elt #(10 20 30) 1)
=> 20

Using setf with elt​

elt is an accessor, so you can use setf to modify elements in place.

(let ((lst (list 1 2 3 4 5)))
  (setf (elt lst 2) 99)
  lst)
=> (1 2 99 4 5)

Modifying string characters​

Since strings are sequences, elt can read and write individual characters.

(let ((str (copy-seq "hello")))
  (setf (elt str 0) #\H)
  str)
=> "Hello"

elt vs. nth and aref​

elt is the generic sequence accessor. For lists, nth may be more idiomatic; for vectors, aref or svref is often preferred. However, elt works on any sequence type, making it useful for polymorphic code.

(defun second-element (seq)
  (elt seq 1))

(second-element '(a b c))
=> B
(second-element "abc")
=> #\b
(second-element #(10 20 30))
=> 20

Out-of-bounds access signals an error​

Accessing an index beyond the sequence length signals a type-error.

;; (elt '(a b c) 5) ; would signal a TYPE-ERROR
;; (elt #() 0)      ; would signal a TYPE-ERROR