Association Lists in LISP

Association Lists in Lisp are used for mapping values to respective key elements. They are also known as a-list. The car of the pair represents the key element whereas cdr represents the associated value.

Creating a list:

1. Using list and cons: In this method, we make use of lists and cons for creating our lists.

; Creating a list with name of the student
; as the key and marks as the value using list and 
; cons method

(defparameter *marks* (list (cons 'kishan 96) (cons 'saksham 92)))

2. Using pairlis: Another method for list creation is using Pairlis which eliminates the use of list and cons totally.

;Creating list using pairlis

(pairlis '(kishan saksham) '(96 98))

3. Using acons: A single pair can be added to a list using the acons function.

Example 1:

;Using acons to add pair

(defvar *marks* (pairlis '(Kishan Saksham) '(96 98)))

(defvar *new-marks* (acons 'shubh 91 *marks*))

(write(assoc 'shubh *new-marks*))

Output:

Retrieval Function in Association List:

For the retrieval of the data from the list, we use a function assoc which is used to return the value corresponding to the key element. The parameters it uses are key, association list, and some testing keywords such as (key, test,test-not). Note that the testing keywords we use are totally optional.

Example 2: 

; Lisp code for assoc
(defparameter *marks* (list (cons 'kishan 96) (cons 'saksham 92)))

(write(assoc 'kishan *marks*))

Output:

Note: If the item is not in the list then the assoc function will be returning nil.

Example 3:

; Lisp assoc returning nil in 
; case when value is not present

(defparameter *marks* (list (cons 'kishan 96) (cons 'saksham 92)))

(write(assoc 'sakshi *marks*))

Output:

Updating the values in a list for some key elements:

In Lisp, we make use of the macro setf whenever we want to perform updation. Similarly, we use setf in association lists and also its in-built functionalities for updation of values. Note that setf is used along with cdr since we are updating the value part and not accessing the key.  

Example 4:

; Updating the value associated with key "kishan"

(defparameter *marks* (list (cons 'kishan 96) (cons 'saksham 92)))
(setf (cdr (assoc 'kishan *marks*) )99)
(write(assoc 'kishan *marks*))

Output:

The rassoc function:

In association lists, we can also retrieve the key based on value as a bi-directional map. This we can do with the help of rassoc function which is a reverse assoc function.

Example 5:

; Using the rassoc function

(defparameter *marks* (list (cons 'kishan 99) (cons 'saksham 92)))

(write(rassoc '99 *marks*))

Output: