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
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Unordered collections: sets and dictionaries
Sets and dictionaries are unordered collections of objects. A set contains objects in
no particular order. No matter how many times an object is added to a set, it can
be removed only once.
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:.
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