Swift: declare an empty dictionary

I am beginning to learn swift by following the iBook-The Swift Programming Language on Swift provided by Apple. The book says to create an empty dictionary one should use [:] same as while declaring array as []:

I declared an empty array as follows :

let emptyArr = [] // or String[]()

But on declaring empty dictionary, I get syntax error:

let emptyDict = [:]

How do I declare an empty dictionary?

var emptyDictionary = [String: String]()

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var populatedDictionary = ["key1": "value1", "key2": "value2"]

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Note: if you're planning to change the contents of the dictionary over time then declare it as a variable (var). You can declare an empty dictionary as a constant (let) but it would be pointless if you have the intention of changing it because constant values can't be changed after initialization.

You can't use [:] unless type information is available.

You need to provide it explicitly in this case:

var dict = Dictionary<String, String>()

var means it's mutable, so you can add entries to it. Conversely, if you make it a let then you cannot further modify it (let means constant).

You can use the [:] shorthand notation if the type information can be inferred, for instance

var dict = ["key": "value"]

// stuff

dict = [:] // ok, I'm done with it

In the last example the dictionary is known to have a type Dictionary<String, String> by the first line. Note that you didn't have to specify it explicitly, but it has been inferred.

The Swift documentation recommends the following way to initialize an empty Dictionary:

var emptyDict = [String: String]()

I was a little confused when I first came across this question because different answers showed different ways to initialize an empty Dictionary. It turns out that there are actually a lot of ways you can do it, though some are a little redundant or overly verbose given Swift's ability to infer the type.

var emptyDict = [String: String]()
var emptyDict = Dictionary<String, String>()
var emptyDict: [String: String] = [:]
var emptyDict: [String: String] = [String: String]()
var emptyDict: [String: String] = Dictionary<String, String>()
var emptyDict: Dictionary = [String: String]()
var emptyDict: Dictionary = Dictionary<String, String>()
var emptyDict: Dictionary<String, String> = [:]
var emptyDict: Dictionary<String, String> = [String: String]()
var emptyDict: Dictionary<String, String> = Dictionary<String, String>()

After you have an empty Dictionary you can add a key-value pair like this:

emptyDict["some key"] = "some value"

If you want to empty your dictionary again, you can do the following:

emptyDict = [:]

The types are still <String, String> because that is how it was initialized.

Use this will work.

var emptyDict = [String: String]()

You can simply declare it like this:

var emptyDict:NSMutableDictionary = [:]

You have to give the dictionary a type

// empty dict with Ints as keys and Strings as values
var namesOfIntegers = Dictionary<Int, String>()

If the compiler can infer the type, you can use the shorter syntax

namesOfIntegers[16] = "sixteen"
// namesOfIntegers now contains 1 key-value pair
namesOfIntegers = [:]
// namesOfIntegers is once again an empty dictionary of type Int, String

#Declaring & Initializing Dictionaries in Swift ###Dictionary of String

var stringDict: [String: String] = [String: String]()

OR

var stringDict: Dictionary<String, String> = Dictionary<String, String>()

###Dictionary of Int

var stringDict: [String: Int] = [String: Int]()

OR

var stringDict: Dictionary<String, Int> = Dictionary<String, Int>()

###Dictionary of AnyObject

var stringDict: [String: AnyObject] = [String: AnyObject]()

OR

var stringDict: Dictionary<String, AnyObject> = Dictionary<String, AnyObject>()

###Dictionary of Array of String

var stringDict: [String: [String]] = [String: [String]]()

OR

var stringDict: Dictionary<String, Array<String>> = Dictionary<String, Array<String>>()

###Array of Dictionaries of String

var stringDict: [[String: String]] = [[String: String]]()

OR

var stringDict: Array<Dictionary<String, String>> = Array<Dictionary<String, String>>()

Swift:

var myDictionary = Dictionary<String, AnyObject>()

I'm playing with this too. It seems strange that you can just declare an empty dictionary and then add a key/value pair to it like so :

var emptyDictionary = Dictionary<String, Float>()
var flexDictionary = [:]
emptyDictionary["brian"] = 4.5
flexDictionary["key"] = "value" // ERROR : cannot assign to the result of this expression

But you can create a Dictionary that accepts different value types by using the "Any" type like so :

var emptyDictionary = Dictionary<String, Any>()
emptyDictionary["brian"] = 4.5
emptyDictionary["mike"] = "hello"

You need to explicitly tell the data type or the type can be inferred when you declare anything in Swift.

Swift 3

The sample below declare a dictionary with key as a Int type and the value as a String type.

Method 1: Initializer

let dic = Dictionary<Int, String>()

Method 2: Shorthand Syntax

let dic = [Int:String]()

Method 3: Dictionary Literal

var dic = [1: "Sample"]
// dic has NOT to be a constant
dic.removeAll()

If you want to create a generic dictionary with any type

var dictionaryData = [AnyHashable:Any]()

Swift 4

let dicc = NSDictionary()

//MARK: - This is empty dictionary
let dic = ["":""]

//MARK:- This is variable dic means if you want to put variable 
let dic2 = ["":"", "":"", "":""]

//MARK:- Variable example
let dic3 = ["name":"Shakeel Ahmed", "imageurl":"https://abc?abc.abc/etc", "address":"Rawalpindi Pakistan"]

//MARK: - This is 2nd Variable Example dictionary
let dic4 = ["name": variablename, "city": variablecity, "zip": variablezip]

//MARK:- Dictionary String with Any Object
var dic5a = [String: String]()
//MARK:- Put values in dic
var dic5a = ["key1": "value", "key2":"value2", "key3":"value3"]

var dic5b = [String:AnyObject]()
dic5b = ["name": fullname, "imageurl": imgurl, "language": imgurl] as [String : AnyObject]
     
or
//MARK:- Dictionary String with Any Object
let dic5 = ["name": fullname, "imageurl": imgurl, "language": imgurl] as [String : AnyObject]

//MARK:- More Easy Way
let dic6a = NSDictionary()
let dic6b = NSMutalbeDictionary()

I'm usually using

var dictionary:[String:String] = [:]
dictionary.removeAll()

To create an empty dictionary with the [:] aka the empty dictionary literal, you actually need to provide the context first as in the type of both the key and the value. The correct way to use the [:] to create an empty dictionary is:

var dict: [String: Int] = [:]

You can declare it as nil with the following:

var assoc : [String:String]

Then nice thing is you've already typeset (notice I used var and not let, think of these as mutable and immutable). Then you can fill it later:

assoc = ["key1" : "things", "key2" : "stuff"]

You can use the following code:

var d1 = Dictionary<Int, Int>()
var d2 = [Int: Int]()
var d3: Dictionary<Int, Int> = [Int : Int]()
var d4: [Int : Int] = [:]

var dictList = String:String for dictionary in swift var arrSectionTitle = String for array in swift

var parking = [Dictionary < String, Double >()]

^ this adds a dictionary for a [string:double] input

It is very handy for finding your way

var dict:Dictionary = [:]