Set

This Swift collection allows no duplicate elements. It uses hash codes to test for equality between elements. A set has fast logic tests.

With methods, we perform simple actions like insertion. We can test to see if an element exists. With set operations, we can see if 2 sets intersect.

First program

Here we create an empty set. We then insert() 4 strings into it. The last two strings are the same—only one is retained in the set.

// Create a set and add Strings to it.
var animals = Set<String>()
animals.insert("cat")
animals.insert("bird")
animals.insert("dog")
animals.insert("dog")

// Only unique elements are stored in the set.
print(animals.count)

// The second dog element is not present.
print(animals)
3
["bird", "cat", "dog"]

Remove, contains

We use remove() to erase an element from a set. And with contains() we see whether an element (the argument) exists. It returns true or false.

var knownIds = Set<Int>()
knownIds.insert(100)
knownIds.insert(200)
knownIds.insert(300)

// Remove this Int from the set.
knownIds.remove(200)

// This is contained within the set.
if knownIds.contains(100) {
    print(true)
}

// Not contained.
if !knownIds.contains(200) {
    print(false)
}
true
false

For-in loop

With the for-in loop we can access all the elements in a set. This provides not indexes, but often we do not need indexes.

var languages = Set<String>()
languages.insert("Swift")
languages.insert("Python")
languages.insert("Ruby")

// Loop over all elements in the set.
// ... Ordering is not maintained.
for language in languages {
    print(language)
}
Python
Ruby
Swift

Intersection, subtract

This program first initializes sets with initializer syntax. This is an easier way to create a new set from known element.

var colors1: Set<String> = ["red", "orange", "pink"]
let colors2: Set<String> = ["blue", "red", "pink"]

// Intersection returns a set of the two shared Strings.
// ... Red and pink are shared.
let intersection = colors1.intersection(colors2)
print(intersection)

// Subtract removes shared values.
// ... Red and pink are removed in the set.
// ... The colors1 set is modified in-place.
colors1.subtract(colors2)
print(colors1)
["pink", "red"]
["orange"]

Sets have many uses in programs. They help with simple requirements, like ensuring uniqueness. More advanced things, like set operations, are less often needed.