Programming Algorithms: Arrays

This is a snippet of the "Arrays" chapter of the book.

Arrays are, alongside structs, the most basic data structure and, at the same time, the default choice for implementing algorithms. A one-dimensional array that is also called a "vector" is a contiguous structure consisting of the elements of the same type. One of the ways to create such arrays, in Lisp, is this:

CL-USER> (make-array 3)
#(0 0 0)

The printed result is the literal array representation. It happens that the array is shown to hold 0's, but that's implementation-dependent. Additional specifics can be set during array initialization: for instance, the :element-type, :initial-element, and even full contents:

CL-USER> (make-array 3 :element-type 'list :initial-element nil)
CL-USER> (make-array 3 :initial-contents '(1.0 2.0 3.0))
#(1.0 2.0 3.0)

If you read back such an array you'll get a new copy with the same contents:

CL-USER> #(1.0 2.0 3.0)
#(1.0 2.0 3.0)

It is worth noting that the element type restriction is, in fact, not a limitation the default type is T[1]. In this case, the array will just hold pointers to its elements that can be of arbitrary type. If we specify a more precise type, however, the compiler might be able to optimize storage and access by putting the elements in memory directly in the array space. This is, mainly, useful for numeric arrays, but it makes multiple orders of magnitude difference for them for several reasons, including the existence of vector CPU instructions that operate on such arrays.

The arrays we have created are mutable, i.e. we can change their contents, although we cannot resize them. The main operator to access array elements is aref. You will see it in those pieces of code, in this chapter, where we care about performance.

CL-USER> (let ((vec (make-array 3 :initial-contents '(1.0 2.0 3.0))))
           (print (aref vec 0))
           (print (? vec 1))
           (:= (aref vec 2) 4.0))
           (print (? vec 2))
           (aref vec 3))
; Evaluation aborted on #<SIMPLE-TYPE-ERROR expected-type: (MOD 3) datum: 3>

In Lisp, array access beyond its boundary, as expected, causes an error.

It is also possible to create constant arrays using the literal notation #(). These constants can, actually, be changed in some environments, but don't expect anything nice to come out of such abuse — and the compiler will warn you of that:

CL-USER> (let ((vec #(1.0 2.0 3.0)))
           (:= (aref vec 2) nil)
           (print vec))
; caught WARNING:
;   Destructive function (SETF AREF) called on constant data.
;   See also:
;     The ANSI Standard, Special Operator QUOTE
;     The ANSI Standard, Section
; compilation unit finished
;   caught 1 WARNING condition

#(1.0 2.0 NIL)

RUTILS provides more options to easily create arrays with a shorthand notation:

CL-USER> #v(1 2 3)
#(1 2 3)
CL-USER> (vec 1 2 3)
#(1 2 3)

Although the results seem identical they aren't. The first version creates a mutable analog of #(1 2 3), and the second also makes it adjustable (we'll discuss adjustable or dynamic arrays next).

More details about of the book may be found on its website.

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