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length

length Function

Syntax:

length sequence → n

Arguments and Values:

sequence—a proper sequence.

n—a non-negative integer .

Description:

Returns the number of elements in sequence.

If sequence is a vector with a fill pointer , the active length as specified by the fill pointer is returned.

Examples:

(length "abc") → 3 
(setq str (make-array ’(3) :element-type ’character 
		       :initial-contents "abc" 
		       :fill-pointer t)) → "abc" 
(length str) → 3 
(setf (fill-pointer str) 2) → 2 
(length str) → 2

Exceptional Situations:

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

See Also:

list-length, sequence

reverse, nreverse

Expanded Reference: length​

Basic usage with lists and strings​

length returns the number of elements in any proper sequence -- lists, vectors, and strings.

(length '(a b c d))
=> 4
(length "hello")
=> 5
(length #(1 2 3))
=> 3
(length nil)
=> 0

Empty sequences​

An empty sequence always has length zero, regardless of type.

(length '())
=> 0
(length "")
=> 0
(length #())
=> 0

Length with a fill pointer​

When a vector has a fill pointer, length returns the active length (the fill pointer value), not the total allocated size.

(let ((v (make-array 10 :fill-pointer 3)))
  (length v))
=> 3

Strings are vectors of characters​

Since strings are vectors, length works naturally on them. This includes strings with special characters.

(length "Common Lisp")
=> 11
(length (make-string 5 :initial-element #\x))
=> 5

Practical example: checking for empty sequences​

length is commonly used to test whether a sequence has elements, though for lists null is more idiomatic.

(defun sequence-empty-p (seq)
  (zerop (length seq)))

(sequence-empty-p '())
=> T
(sequence-empty-p "abc")
=> NIL
(sequence-empty-p #())
=> T