Hash Table in LISP

A hash table is a type of collection in Common LISP, that is used to map keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method.  

Types of  Hash Tables in LISP:

There are three types of hashtables in lisp, which are listed below:

1. eq : It is used for comparison when the hashing of the hash table is done on LISP objects.
2. eql:  It is also used for comparison when the hashing of the hash table is done on LISP objects.
3. equal:  It is used for comparison when the hashing of the hash table is done on LISP tree structures.

Creating Hash Table in LISP:

The make-hash-table function is used in Common LISP to create a hash table.

Syntax:

make-hash-table &key :test :size :rehash-size : rehash-threshold

Here,

• key:- It is the key name.
• :test:- It is used to determine how the keys are compared. It takes one of the three types of hash table values mentioned above(ie, eq, eql, equal).
• :size:- It is used to set the initial size of the hash table.
• :rehash-size:- It is used to set the increment in the size of the hash table once it gets full and more data needs to be added.
• :rehash-threshold:- It is used to set the maximum size of the hashtable after which the increment can be done to its size.

Adding & Fetching Data from Hash Table in LISP:

The gethash function is used to fetch data from a hash table in LISP.

Syntax:

gethash key hash-table &optional default

Here,

• key: It is the key name.
• hash-table: It is the name of the hashtable
• default: It is the return type. If not set it returns nil if the value is not found.

The setf function is used with the gethash function to add data to a hash table in LISP.

Syntax:

setf (gethash 'key hash-table-name) '(value)

Here,

• key: It is the key name.
• hash-table-name: It is the name of the hashtable
• value: It is the value to be associated with the key

Example: Creating a hash table, adding data, and fetching data from it.

; call make-hash-table function
(setq makeHashTable (make-hash-table)) 

; 1st entry
(setf (gethash 'gfg makeHashTable) '(Geeksforgeeks))

; 2nd entry
(setf (gethash 'DSA makeHashTable) '(Data Structure & Algorithms)) 

; output 1
(write (gethash 'gfg makeHashTable)) 
(terpri)

; output 2
(write (gethash 'DSA makeHashTable))

Output:

(GEEKSFORGEEKS)
(DATA STRUCTURE & ALGORITHMS)

Removing Entries from Hash Table in LISP:

The remhash function is used to remove key-value entries from a hash table.

Syntax:

remhash key hash-table

Here,

• key: It is the name of the key.
• hash-table: It is the name of the hash table from where the entries are to be removed.

Example: Here we will remove the 1st entry done in the previous example and print out the result.

; call make-hash-table function
(setq makeHashTable (make-hash-table)) 

; 1st entry
(setf (gethash 'gfg makeHashTable) '(Geeksforgeeks))

; 2nd entry
(setf (gethash 'DSA makeHashTable) '(Data Structure & Algorithms)) 

; output 1
(write (gethash 'gfg makeHashTable)) 
(terpri)

; output 2
(write (gethash 'DSA makeHashTable))

; remove 1st entry
(remhash 'gfg makeHashTable)
(terpri)

; output the first entry
(write (gethash 'gfg makeHashTable))  

Output:

(GEEKSFORGEEKS)
(DATA STRUCTURE & ALGORITHMS)
NIL