Unordered collections: sets and dictionaries

• A set contains an object only once.

• A dictionary is an arbitrary mapping.

• Both implemented using hash tables. Set elements and dictionary keys must be comparable using = and return an integer hash value when sent hash. defaultBehavior defines hash as identityHash , a VM hash function.

Examples

s: set copy.
s add: ©x©. s add: 23. s add: nil.
s size.			"3"
s remove: nil.
s includes: ©x©.	"true"
d: dictionary copy.
d at: ©x© Put: 42.
d at: nil Put: ().
d includesKey: nil.	"true"
d includes: nil.	"false"
d removeKey: nil

Unordered collections: sets and dictionaries

A dictionary is a mapping from objects to objects. The keys of the dictionary form a set (i.e., no two keys are equal).

Both sets and dictionaries are implemented using hash tables. The elements of a set and the keys of a dictionary must be comparable using = and able to return an integer hash value when sent hash (this value must not vary while the object is in the collection). In defaultBehavior is a method, identityHash, which uses a primitive hashing function supplied by the Virtual Machine.

Objects are added to sets using add:, and removed using remove: or remove:IfAbsent: (which takes a block as second argument). Set membership can be tested with includes:.

Dictionaries can be accessed using at: (which takes a key and returns the corresponding mapped object), or at:IfAbsent: (which sends value: to the second argument if the key is not in the dictionary), and updated using at:Put:. Membership can be tested with includes: and includesKey:.