svref

svref Accessor

Syntax:

svref simple-vector index → element

(setf (svref simple-vector index**)** new-element**)**

Arguments and Values:

simple-vector—a simple vector .

index—a valid array index for the simple-vector.

element, new-element—an object (whose type is a subtype of the array element type of the simple-vector).

Description:

Accesses the element of simple-vector specified by index.

Examples:

(simple-vector-p (setq v (vector 1 2 ’sirens))) → true 
(svref v 0) → 1 
(svref v 2) → SIRENS 
(setf (svref v 1) ’newcomer) → NEWCOMER 
v → #(1 NEWCOMER SIRENS) 

See Also:

aref, sbit, schar, vector, Section 3.2.1 (Compiler Terminology)

Notes:

svref is identical to aref except that it requires its first argument to be a simple vector . (svref v i) ≡ (aref (the simple-vector v) i)

Expanded Reference: svref

Basic Element Access

svref accesses elements of a simple vector by index. It is identical to aref but requires its first argument to be a simple vector, which may allow optimized access.

(let ((v (vector 'a 'b 'c 'd)))
  (list (svref v 0) (svref v 2)))
=> (A C)

Setting Elements with SETF

svref works with setf to modify elements in place.

(let ((v (vector 1 2 3 4 5)))
  (setf (svref v 2) 99)
  v)
=> #(1 2 99 4 5)

Only Works on Simple Vectors

svref requires a simple vector -- a one-dimensional array with element-type t, no fill pointer, not adjustable, and not displaced. Strings, bit vectors, and vectors with fill pointers are not valid arguments.

;; This works: vector creates a simple vector
(svref (vector 'x 'y 'z) 1)
=> Y

;; simple-vector-p can verify the argument type
(let ((v (make-array 3 :initial-element 0)))
  (list (simple-vector-p v)
        (svref v 0)))
=> (T 0)

Performance Consideration

When you know your array is a simple vector, using svref communicates this to both the reader and the compiler, potentially enabling faster element access.

(let ((v (vector 10 20 30 40 50)))
  ;; svref and aref return the same value
  (= (svref v 3) (aref v 3)))
=> T