Computer Science Hub: Swift - Dictionaries

Swift - Dictionaries

Swift dictionaries are used to store unordered lists of values of the same type. Swift puts strict checking which does not allow you to enter a wrong type in a dictionary even by mistake.

Swift dictionaries use unique identifier known as a key to store a value which later can be referenced and looked up through the same key. Unlike items in an array, items in a dictionary do not have a specified order. You can use adictionary when you need to look up values based on their identifiers.

A dictionary key can be either an integer or a string without a restriction, but it should be unique within a dictionary.

If you assign a created dictionary to a variable, then it is always mutable which means you can change it by adding, removing, or changing its items. But if you assign a dictionary to a constant, then that dictionary is immutable, and its size and contents cannot be changed.

Creating Dictionary

You can create an empty dictionary of a certain type using the following initializer syntax −

var someDict =  [KeyType: ValueType]()

You can use the following simple syntax to create an empty dictionary whose key will be of Int type and the associated values will be strings −

var someDict = [Int: String]()

Here is an example to create a dictionary from a set of given values −

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

Accessing Dictionaries

You can retrieve a value from a dictionary by using subscript syntax, passing the key of the value you want to retrieve within square brackets immediately after the name of the dictionary as follows −

var someVar = someDict[key]

Let's check the following example to create, initialize, and access values from a dictionary −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

var someVar = someDict[1]

println( "Value of key = 1 is \(someVar)" )
println( "Value of key = 2 is \(someDict[2])" )
println( "Value of key = 3 is \(someDict[3])" )

When the above code is compiled and executed, it produces the following result −

Value of key = 1 is Optional("One")
Value of key = 2 is Optional("Two")
Value of key = 3 is Optional("Three")

Modifying Dictionaries

You can use updateValue(forKey:) method to add an existing value to a given key of the dictionary. This method returns an optional value of the dictionary's value type. Here is a simple example −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

var oldVal = someDict.updateValue("New value of one", forKey: 1)

var someVar = someDict[1]

println( "Old value of key = 1 is \(oldVal)" )
println( "Value of key = 1 is \(someVar)" )
println( "Value of key = 2 is \(someDict[2])" )
println( "Value of key = 3 is \(someDict[3])" )

When the above code is compiled and executed, it produces the following result −

Old value of key = 1 is Optional("One")
Value of key = 1 is Optional("New value of one")
Value of key = 2 is Optional("Two")
Value of key = 3 is Optional("Three")

You can modify an existing element of a dictionary by assigning new value at a given key as shown in the following example −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

var oldVal = someDict[1]
someDict[1] = "New value of one"
var someVar = someDict[1]

println( "Old value of key = 1 is \(oldVal)" )
println( "Value of key = 1 is \(someVar)" )
println( "Value of key = 2 is \(someDict[2])" )
println( "Value of key = 3 is \(someDict[3])" )

When the above code is compiled and executed, it produces the following result −

Old value of key = 1 is Optional("One")
Value of key = 1 is Optional("New value of one")
Value of key = 2 is Optional("Two")
Value of key = 3 is Optional("Three")

Remove Key-Value Pairs

You can use removeValueForKey() method to remove a key-value pair from a dictionary. This method removes the key-value pair if it exists and returns the removed value, or returns nil if no value existed. Here is a simple example −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

var removedValue = someDict.removeValueForKey(2)

println( "Value of key = 1 is \(someDict[1])" )
println( "Value of key = 2 is \(someDict[2])" )
println( "Value of key = 3 is \(someDict[3])" )

When the above code is compiled and executed, it produces the following result −

Value of key = 1 is Optional("One")
Value of key = 2 is nil
Value of key = 3 is Optional("Three")

You can also use subscript syntax to remove a key-value pair from a dictionary by assigning a value of nil for that key. Here is a simple example −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

someDict[2] = nil

println( "Value of key = 1 is \(someDict[1])" )
println( "Value of key = 2 is \(someDict[2])" )
println( "Value of key = 3 is \(someDict[3])" )

When the above code is compiled and executed, it produces the following result −

Value of key = 1 is Optional("One")
Value of key = 2 is nil
Value of key = 3 is Optional("Three")

Iterating Over a Dictionary

You can use a for-in loop to iterate over the entire set of key-value pairs in a Dictionary as shown in the following example −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

for (key, value) in someDict {
   println("Dictionary key \(key) -  Dictionary value \(value)")
}

When the above code is compiled and executed, it produces the following result −

Dictionary key 2 -  Dictionary value Two
Dictionary key 3 -  Dictionary value Three
Dictionary key 1 -  Dictionary value One

You can use enumerate() function which returns the index of the item along with its (key, value) pair as shown below in the example −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

for (key, value) in enumerate(someDict) {
   println("Dictionary key \(key) -  Dictionary value \(value)")
}

When the above code is compiled and executed, it produces the following result −

Dictionary key 0 -  Dictionary value (2, Two)
Dictionary key 1 -  Dictionary value (3, Three)
Dictionary key 2 -  Dictionary value (1, One)

Convert to Arrays

You can extract a list of key-value pairs from a given dictionary to build separate arrays for both keys and values. Here is an example −

import Cocoa

var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

let dictKeys = [Int](someDict.keys)
let dictValues = [String](someDict.values)

println("Print Dictionary Keys")

for (key) in dictKeys {
   println("\(key)")
}

println("Print Dictionary Values")

for (value) in dictValues {
   println("\(value)")
}

When the above code is compiled and executed, it produces the following result −

Print Dictionary Keys
2
3
1
Print Dictionary Values
Two
Three
One

The count Property

You can use the read-only count property of a dictionary to find out the number of items in a dictionary as shown below −

import Cocoa

var someDict1:[Int:String] = [1:"One", 2:"Two", 3:"Three"]
var someDict2:[Int:String] = [4:"Four", 5:"Five"]

println("Total items in someDict1 = \(someDict1.count)")
println("Total items in someDict2 = \(someDict2.count)")

When the above code is compiled and executed, it produces the following result −

Total items in someDict1 = 3
Total items in someDict2 = 2

The empty Property

You can use read-only empty property of a dictionary to find out whether a dictionary is empty or not, as shown below −

import Cocoa

var someDict1:[Int:String] = [1:"One", 2:"Two", 3:"Three"]
var someDict2:[Int:String] = [4:"Four", 5:"Five"]
var someDict3:[Int:String] = [Int:String]()

println("someDict1 = \(someDict1.isEmpty)")
println("someDict2 = \(someDict2.isEmpty)")
println("someDict3 = \(someDict3.isEmpty)")

When the above code is compiled and executed, it produces the following result −

someDict1 = false
someDict2 = false
someDict3 = true

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