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Swift Dictionaries


What is Dictionary ?

Dictionary is simply a collection type used to link keys of same type and values of same type. Keys of same type means all the keys in a dictionary must be of same type i.e if one key is of Int type then all the keys must be of Int type. Same in case of Values. All the values must be of same type.

Note:-

If dictionary is assigned to a variable then that dictionary becomes Mutable and If dictionary is assigned to a 'constant' then that dictionary becomes Immutable .
Create immutable dictionary
let dictionary = ["Item 1": "description", "Item 2": "description"]
Create mutable dictionary
var dictionary = ["Item 1": "description", "Item 2": "description"]

Append new pair to dictionary
dictionary["Item 3"] = "description"

Create an empty Dictionary

Following examples clears the way to create an empty dictionary.

var dict = [String:String]()
var intDict = [Int:Int]()
var mixDict = [Int:String]()

Create Dictionary with key-value pairs

var dict:[String:String] = ["Abc":"PQR" , "XYZ":"aa"]
print(dict["Abc"])

Output :-

Optional("PQR")

As we studied in Swift Optionals about how to

unwrapp

an optional value , you can refer it before you move forward.

var dict:[String:String] = ["Abc":"PQR" , "XYZ":"aa"]
print(dict["Abc"]!)

Output :-

PQR

How to access an Dictionary ?

var someVar = someDict[key]

Example : dict["Abc"] is used to retreive the value of key "ABC"

Example 1 :-

import UIKit
var dict:[String:String] = ["Abc":"PQR" , "XYZ":"aa"]
print(dict["XYZ"]!)

Output :-

aa

Example 2 :-

import UIKit

var dict:[Int:String] = [1:"Codzify" , 2:"Microsoft"]
print(dict[1]!)

Output :-

Codzify

Update Value Property of dictionary

Dictionary’s updateValue(_:forKey:) method to set or update the value for a particular key.

import UIKit
var someDict:[Int:String] = [1:"Codzify", 2:"Microsoft"]

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

var someVar = someDict[1]

print( "Old value of key = 1 is \(oldVal)" )
print( "Value of key = 1 is \(someVar)" )

Output :-

Old value of key = 1 is Optional("Codzify")
Value of key = 1 is Optional("New value of one")

Remove Value For Key method

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.

Example :-

import UIKit

var someDict:[Int:String] = [1:"Codzify", 2:"Microsoft"]

var someVar = someDict.removeValue(forKey: 2)

print( "Value for key = 1 is \(someDict[1]!)" )
print( "Value of key = 2 is \(someDict[2])" )

Iterate over a dictionary

for..in loop is used to iterate over a dictionary of key-value pairs.

import UIKit
var dict:[String:String] = ["Abc":"PQR" , "XYZ":"aa"]

for (key, value) in dict {
    print("Dictionary key \(key) -  Dictionary value \(value)")
}

Output :-

Dictionary key Abc -  Dictionary value PQR
Dictionary key XYZ -  Dictionary value aa

Iteration using enumerated method

enumerated method is used to iterate over a dictionary and in returns the index of the item along with its (key, value) pair.

import UIKit
var dict:[Int:String] = [0:"PQR" , 1:"aa"]

for (key, value) in dict.enumerated() {
    print("Dictionary key \(key) -  Dictionary value \(value)")
}

Output :-

Dictionary key 0 -  Dictionary value (0, "PQR")
Dictionary key 1 -  Dictionary value (1, "aa")

Count Property & isEmpty Property of an Dictionary

count property is used to count the number of elements in an dictionary . isEmpty property is used to check wether dictionary is empty or not.

//: Playground - noun: a place where people can play

import UIKit

var dict :[Int:String] = [1:"ABC",2:"PQR",3:"XYZ"]
print("Count of element = \(dict.count)")
print("Is dictionary empty  = \(dict.isEmpty)")

Output :-

Count of element = 3
Is dictionary empty  = false