Introduction
Hash Tables
Hash tables in Common Lisp are data structures for storing associative data.
They map keys (which can be any object) to values (which can be any object).
Other languages may refer to this concept as maps, dictionaries or hashes.
Creating hash tables
Hash tables can be created with the make-hash-table function.
(make-hash-table) ; => #<HASH-TABLE :TEST EQL :COUNT 0 {10024AD2B3}>
(Note the printed presentation of a hash table is implementation dependent so it may appear differently in your REPL.)
Retrieving values by keys
To get a value by a key from the hash table you use the gethash function:
*h* ; => #<HASH-TABLE :TEST EQL :COUNT 3 {10024AD2B3}>`
(gethash :foo *h*) ; => :bar
; T
gethash returns multiple values which will be explained in another concept.
Inserting values
To insert a value into a hash table you use setf with gethash like this:
(setf (gethash :foo *hash-table*) :bar) : => :bar
This will modify the value for the key :foo in the hash table *hash-table* to be :bar.
It returns the value.
The use of setf will be explained in another concept.
Instructions
Lamont is a Lisp alien that wants to write their own video game. One task they have asked for your help with is keeping track of a high score list.
1. Creating the high score table
Define a function make-high-score-table which does not take any arguments are evaluates to a new empty hash table:
(make-high-score-table) ; => #<HASH-TABLE :TEST EQL :COUNT 0 {1005A90103}>
2. Adding a player to the table
To add a player to the table define a function add-player which takes two arguments:
- the high score table
- the player's name as a keyword
Set the player's initial score to zero.
(add-player hash-table :louis) ; => 0
(add-player hash-table :lilly) ; => 0
3. Setting a player's score
To set the score for a player define a function set-score which takes three arguments:
- the high score table
- the player's name as a keyword
- the player's score as an integer
(set-score hash-table :louis 1234) ; => 1234
4. Getting a score from the table
To get a score for a player from the table define a function get-score which takes two arguments:
- the high score table
- the player's name as a keyword
This function should evaluate to the score stored for the player.
If there is no score stored for that player, or that player is not yet in the table then evaluate to 0.
(get-score hash-table :louis) ; => 1234
(get-score hash-table :lilly) ; => 0
(get-score hash-table :not-a-character-in-this-story) ; => 0
5. Removing a player from the table
To remove a player from the high score table define a function remove-player which takes two arguments:
- the high score table
- the player's name as a keyword
This function should gracefully handle cases where the player was not yet added to the table.
Obviously after a player is removed if one gets their score from the table it will be zero.
(set-score hash-table :louis 1234) ; => 1234
(remove-player hash-table :louis) ; => T
(get-score hash-table :louis) ; => 0
(remove-player hash-table :not-in-the-table) ; => NIL
Learning Exercise
