
Data Structure

This illustrates a data structure, a means of storing data within a program.

You may see other such structures in the Data Structures category.
A   set  is a collection of elements, without duplicates and without order.



Task

Show each of these set operations:

 Set creation
 Test m ∈ S -- "m is an element in set S"
 A ∪ B -- union; a set of all elements either in set A or in set B.
 A ∩ B -- intersection; a set of all elements in both set A and set B.
 A ∖ B -- difference; a set of all elements in set A, except those in set B.
 A ⊆ B -- subset; true if every element in set A is also in set B.
 A = B -- equality; true if every element of set A is in set B and vice versa.


As an option, show some other set operations.

(If A ⊆ B, but A ≠ B, then A is called a true or proper subset of B, written A ⊂ B or A ⊊ B.)

As another option, show how to modify a mutable set.



One might implement a set using an associative array (with set elements as array keys and some dummy value as the values).

One might also implement a set with a binary search tree, or with a hash table, or with an ordered array of binary bits (operated on with bit-wise binary operators).

The basic test, m ∈ S, is O(n) with a sequential list of elements, O(log n) with a balanced binary search tree, or (O(1) average-case, O(n) worst case) with a hash table.



See also

 Array
 Associative array: Creation, Iteration
 Collections
 Compound data type
 Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
 Linked list
 Queue: Definition, Usage
 Set
 Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
 Stack


 
public class MySetController{
    public Set<String> strSet {get; private set; }
    public Set<Id> idSet {get; private set; }
 
    public MySetController(){
        //Initialize to an already known collection.  Results in a set of abc,def.
        this.strSet = new Set<String>{'abc','abc','def'};
 
        //Initialize to empty set and add in entries.
        this.strSet = new Set<String>();
        this.strSet.add('abc');
        this.strSet.add('def');
        this.strSet.add('abc');
        //Results in {'abc','def'}
 
        //You can also get a set from a map in Apex. In this case, the account ids are fetched from a SOQL query.
        Map<Id,Account> accountMap = new Map<Id,Account>([Select Id,Name From Account Limit 10]);
        Set<Id> accountIds = accountMap.keySet();
 
        //If you have a set, you can also use it with the bind variable syntax in SOQL:
        List<Account> accounts = [Select Name From Account Where Id in :accountIds];
 
        //Like other collections in Apex, you can use a for loop to iterate over sets:
        for(Id accountId : accountIds){
            Account a = accountMap.get(accountId);
            //Do account stuffs here.
        }
    }
}
 