Dictionary in Swift

Question

I really confuse with the way we create dictionary in swift. So could you please tell me what is the different between

var myDic3 = [String : AnyObject]()

and

var myDic2 = Dictionary <Int,AnyObject>()

and

var myDic4 = [ : ]

When i declare like myDic4 I cannot add key and value for it:

myDic4["001"] = "ABC"

And the error is "Cannot assign to the result of this expression"

Answer 1

In Swift, you can declare and initialize an empty Dictionary with type String for keys and type Any for values in 4 different ways:

1. var myDic1 = [String : Any]()
2. var myDic2 = Dictionary<String, Any>()
3. var myDic3: [String : Any] = [:]
4. var myDic4: Dictionary<String, Any> = [:]

These will all give you the same result which is an empty Dictionary with Strings as the keys and Anys as the values.

[String : Any] is just shorthand for Dictionary<String, Any>. They mean the same thing but the shorthand notation is preferred.

In cases 1 and 2 above, the types of the variables are inferred by Swift from the values being assigned to them. In cases 3 and 4 above, the types are explicitly assigned to the variables, and then they are initialized with an empty dictionary [:].

When you create a dictionary like this:

var myDic5 = [:]

Swift has nothing to go on, and it gives the error:

Empty collection literal requires an explicit type

Historical Note: In older versions of Swift, it inferred [:] to be of type NSDictionary. The problem was that NSDictionary is an immutable type (you can't change it). The mutable equivalent is an NSMutableDictionary, so these would work:

var myDic6: NSMutableDictionary = [:]

or

var myDic7 = NSMutableDictionary()

but you should prefer using cases 1 or 3 above since NSMutableDictionary isn't a Swift type but instead comes from the Foundation framework. In fact, the only reason you were ever able to do var myDic = [:] is because you had imported the Foundation framework (with import UIKit, import Cocoa, or import Foundation). Without importing Foundation, this was an error.

Answer 2

I believe the following declarations are all identical ways of declaring an empty Dictionary of type < String, Int > in Swift:

var dictionary1 = [String:Int]()
var dictionary2: [String:Int] = [:]
var dictionary3 = Dictionary<String, Int>()
var dictionary4: Dictionary<String, Int> = [:]

And these are identical ways of declaring an empty NSMutableDictionary in Swift:

var dictionaryA = [NSObject:AnyObject]()
var dictionaryB: [NSObject:AnyObject] = [:]
var dictionaryC = Dictionary<NSObject, AnyObject>()
var dictionaryD: Dictionary<NSObject, AnyObject> = [:]
var dictionaryE = NSMutableDictionary()
var dictionaryF: NSMutableDictionary = [:]

So many choices! I put both lists in my order of preference due to readability, but I would think "NSMutableDictionary()" would be prefered method if mixing with ObjectiveC or using with methods that take NSMutableDictionary parameters.