Книга: Practical Common Lisp

Vectors As Sequences

Vectors As Sequences

As mentioned earlier, vectors and lists are the two concrete subtypes of the abstract type sequence. All the functions I'll discuss in the next few sections are sequence functions; in addition to being applicable to vectors—both general and specialized—they can also be used with lists.

The two most basic sequence functions are LENGTH, which returns the length of a sequence, and ELT, which allows you to access individual elements via an integer index. LENGTH takes a sequence as its only argument and returns the number of elements it contains. For vectors with a fill pointer, this will be the value of the fill pointer. ELT, short for element, takes a sequence and an integer index between zero (inclusive) and the length of the sequence (exclusive) and returns the corresponding element. ELT will signal an error if the index is out of bounds. Like LENGTH, ELT treats a vector with a fill pointer as having the length specified by the fill pointer.

(defparameter *x* (vector 1 2 3))
(length *x*) ==> 3
(elt *x* 0) ==> 1
(elt *x* 1) ==> 2
(elt *x* 2) ==> 3
(elt *x* 3) ==> error
ELT is also a SETFable place, so you can set the value of a particular element like this:

(setf (elt *x* 0) 10)
*x* ==> #(10 2 3)

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