Lisp - Adjustable Vectors

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Adjustable Vector is a dynamic array which can change its size during runtime. It is useful feature which allows to work with a collection whose size is not known.

Key Concepts and Functions

Following are the key considerations to understand adjustable vector.

Adjustablity

We can increase/decrease size of an vector as required. This is an important feature where size of an vector is not known at time of creation and can vary as program processes.

Fill Pointer

Fill Pointer is a special index marking the end of the vector. It is a logical flag and can quicky reduce the size requirement of an vector without making change to underlying physical storage. Fill pointer helps in identifying the total number of elements in use.

Allocation

Whenever we need to adjust an array size to a bigger block of memory, Lisp returns the new array while copying existing elements. It is generally an expensive operation and it is important for performance point of view.

Syntax - Creating Adjustable Vector

(make-array <array-size> :adjustable t :fill-pointer t :initial-element <initial-value>)

Where −

• make-array − function to create array or vector.

• array-size − initial size of an array or vector which can be changed later.

• adjustable − flag to mark array or vector as adjustable.

• fill-pointer − a fill pointer.

• initial-element − to specify initial values to the array or vector elements.

• initial-value − initial value of an array or vector element.

Example - Create Adjustable Vector

Create an adjustable vector with initial size of 5, a fill pointer as 0 and all elements are initialized to 0.

main.lisp

; define an adjustable vector
(defvar my-vector(make-array 5 :adjustable t :fill-pointer 0 :initial-element 0))

; print the Vector
(print my-vector)
(terpri)
; get and print the fill-Pointer
(print (fill-pointer my-vector))

Output

When you execute the code, it returns the following result −

#() 
0 

Example - Adding element to Adjustable Vector

vector-push can be used to add element to end of the adjustable vector. It increments the fill-pointer automatically.

main.lisp

; define an adjustable vector
(defvar my-vector(make-array 5 :adjustable t :fill-pointer 0 :initial-element 0))

; add elements to the vector
(vector-push 1 my-vector)
(vector-push 2 my-vector)
(vector-push 3 my-vector)

; Output: #(1 2 3)
(print my-vector)
(terpri)
; prints 3
(print (fill-pointer my-vector))

Output

When you execute the code, it returns the following result −

#(1 2 3) 
3 

Example - Removing element from Adjustable Vector

vector-pop can be used to remove element from the end of the adjustable vector. It decrements the fill-pointer automatically.

main.lisp

; define an adjustable vector
(defvar my-vector(make-array 5 :adjustable t :fill-pointer 0 :initial-element 0))

; add elements to the vector
(vector-push 1 my-vector)
(vector-push 2 my-vector)
(vector-push 3 my-vector)

; Output: #(1 2 3)
(print my-vector)

; remove last element
(vector-pop my-vector)
(terpri)
; Output: #(1 2)
(print my-vector)
(terpri)
; prints 2
(print (fill-pointer my-vector))

Output

When you execute the code, it returns the following result −

#(1 2 3) 
#(1 2) 
2 

Example - Extending Vector

vector-push-extend can be used to extend an adjustable vector. vector-push-extend adds an elements to the vector and increment the size of the vector.

main.lisp

; define an adjustable vector
(defvar my-vector(make-array 3 :adjustable t :fill-pointer 0 :initial-element 0))

; add elements to the vector
(vector-push 1 my-vector)
(vector-push 2 my-vector)
(vector-push 3 my-vector)

; Output: #(1 2 3)
(print my-vector)

; add 10 items to my-vector
(dotimes (i 10)
   (vector-push-extend i my-vector))
(terpri)
; the vector now holds 13 elements, starting with 1,2,3 and then from 0 to 9.
(print my-vector)
(terpri)
; print the fill pointer
(print (fill-pointer my-vector))

Output

When you execute the code, it returns the following result −

#(1 2 3) 
#(1 2 3 0 1 2 3 4 5 6 7 8 9) 
13