Set.insert() / remove() / contains()

Since:		Swift 1.0(2014)

A Swift Set is a collection of unique elements. Order is not guaranteed, and elements must conform to the Hashable protocol.

Syntax

// Declaring and initializing a Set
var set: Set<String> = ["item_a", "item_b", "item_c"]
var emptySet = Set<Int>() // Empty Set
var set2: Set = ["a", "b", "c"] // Type inference

// Checking if an element exists
set.contains("item_a") // true

Method List

Method / Property	Description
set.insert(_:)	Adds an element. Returns a tuple indicating whether the insertion succeeded and the inserted or existing element.
set.remove(_:)	Removes the specified element and returns it as an Optional.
set.removeAll()	Removes all elements.
set.contains(_:)	Returns a Bool indicating whether the element is in the set.
set.count	Returns the number of elements.
set.isEmpty	Returns a Bool indicating whether the set is empty.
set.update(with:)	Inserts or replaces an element, and returns the old element as an Optional if one was replaced.

Sample Code

sample_set_insert_remove.swift

// Creating a Set
var members: Set<String> = ["item_a", "item_b", "item_c"]
print("Initial: \(members)")

// insert: Add an element (check result with the return value)
let (inserted, member) = members.insert("item_d")
print("Insert item_d: \(inserted ? "succeeded" : "already exists") - \(member)")

// Attempting to insert a duplicate
let (inserted2, _) = members.insert("item_a")
print("Insert item_a again: \(inserted2 ? "succeeded" : "already exists")")

// contains: Check if an element exists
print("item_b exists: \(members.contains("item_b"))")
print("item_e exists: \(members.contains("item_e"))")

// remove: Remove an element
if let removed = members.remove("item_b") {
    print("Removed: \(removed)")
}
print("After removal: \(members)")

// count / isEmpty
print("Count: \(members.count)")
print("Empty: \(members.isEmpty)")

// Iterating with for-in (order is not guaranteed)
print("List:")
for member in members.sorted() {
    print("  \(member)")
}

Running the above produces the following output:

swift sample_set_insert_remove.swift
Initial: ["item_a", "item_b", "item_c"]
Insert item_d: succeeded - item_d
Insert item_a again: already exists
item_b exists: true
item_e exists: false
Removed: item_b
After removal: ["item_a", "item_c", "item_d"]
Count: 3
Empty: false
List:
  item_a
  item_c
  item_d

Because a Set does not guarantee element order, the output of print("Initial: \(members)") and print("After removal: \(members)") may appear in a different order on each run.

Notes

A Set automatically eliminates duplicates. Even if an array contains duplicates, converting it with Set(array) removes them.

The return value of insert(_:) is a tuple of type (inserted: Bool, memberAfterInsert: Element). If the element already exists, the existing element is returned. Elements in a Set must conform to Hashable. If you use a custom type, you must add Hashable conformance.

For set operations between sets, see set.union() / intersection() / subtracting().

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