Common Lisp the Language, 2nd Edition
The function aref is normally used for accessing an element of an array. Other access functions, such as svref, char, and bit, may be more efficient in specialized circumstances.
aref array &rest subscripts
This accesses and returns the element of array specified by the subscripts. The number of subscripts must equal the rank of the array, and each subscript must be a non-negative integer less than the corresponding array dimension.
aref is unusual among the functions that operate on arrays in that it completely ignores fill pointers. aref can access without error any array element, whether active or not. The generic sequence function elt, however, observes the fill pointer; accessing an element beyond the fill pointer with elt is an error.
Note that this remark, predating the design of the Common Lisp Object System, uses the term ``generic'' in a generic sense and not necessarily in the technical sense used by CLOS (see chapter 2).
setf may be used with aref to destructively replace an array element with a new value.
Under some circumstances it is desirable to write code that will extract an element from an array a given a list z of the indices, in such a way that the code works regardless of the rank of the array. This is easy using apply:
(apply #'aref a z)
(The length of the list must of course equal the rank of the array.) This construction may be used with setf to alter the element so selected to some new value w:
(setf (apply #'aref a z) w)
svref simple-vector index
The first argument must be a simple general vector, that is, an object of type simple-vector. The element of the simple-vector specified by the integer index is returned.
The index must be non-negative and less than the length of the vector.
setf may be used with svref to destructively replace a simple-vector element with a new value.
svref is identical to aref except that it requires its first argument to be a simple vector. In some implementations of Common Lisp, svref may be faster than aref in situations where it is applicable. See also schar and sbit.