Dictionaries
1、A dictionary is an unordered collection of pairs,where each pair is comprised ofa keyanda value. The same key can’t appear twice in a dictionary,but different keys may point to the same value.All keys have to be of the same type,and all values have to be of the same type.
2、When you create a dictionary,you can explicitly declare the type of the keys and the type of the values:
let pairs: Dictionary<String,Int>
Swift can also infer the type of a dictionary from the type of the initializer:
let inferredPairs = Dictionary<String,Int>()
Or written with the preferred shorthand:
let alsoInferredPairs = [String: Int]()
Within the square brackets,the type of the keys is followed bya colonand the type of the values. This is the most common way to declare a dictionary.
If you want to declare a dictionary with initial values,you can use a dictionary literal. This is a list of key-value pairs separated by commas,enclosed in square brackets:
let namesAndscores = ["Anna": 2,"Brian": 2,"Craig": 8,"Donna": 6] print(namesAndscores) // > ["Brian": 2,"Anna": 2,"Donna": 6]
In this example,the dictionary has pairs of[String: Int]. When you print the dictionary,you seethere’s no particular order to the pairs.
An empty dictionary literal looks like this:[:].You can use that to empty an existing dictionary like so:
var emptyDictionary: [Int: Int] emptyDictionary = [:]
print(namesAndscores["Anna"]) // > Optional(2)
Notice that the return type is an optional. The dictionary will check if there’s a pair with the key “Anna”,and if there is,return its value. If the dictionary doesn’t find the key,it will return
nil:
print(namesAndscores["Greg"]) // > nil
Properties and methods:
print(namesAndscores.isEmpty) // > false print(namesAndscores.count) // > 4 print(Array(namesAndscores.keys)) // > ["Brian","Anna","Craig","Donna"] print(Array(namesAndscores.values)) // > [2,2,8,6]
4、Take a look at Bob’s info:
var bobData = ["name": "Bob","profession": "Card Player","country": "USA"]
Let’s say you got more information about Bob and you wanted to add it to the dictionary. This is how you’d do it:
bobData.updateValue("CA",forKey: "state")
There’s even a shorter way,using subscripting:
bobData["city"] = "San Francisco"
You want to change Bob’s name from Bob to Bobby:
bobData.updateValue("Bobby",forKey: "name"]
// > Bob
Why does it return the string “Bob”?updateValue(_:forKey:)replaces the value of the given key with the new value andreturns the old value. If the key doesn’t exist,this method willadd a new pairand returnnil.
As with adding,you can change his profession with less code by using subscripting:
bobData["profession"] = "Mailman"
Like the first method,the code updates the value for this key or,if the key doesn’t exist,creates a new pair.
You want to remove all information about his whereabouts:
bobData.removeValueForKey("state")
As you might expect,there’s a shorter way to do this using subscripting:
bobData["city"] = nil
Assigningnilas a key’s associated value removes the pair from the dictionary.
5、Thefor-inloop also works when you want to iterate over a dictionary. But since the items in a dictionary are pairs,you need to use tuple:
for (key,value) in namesAndscores { print("\(key) - \(value)") } // > Brian - 2 // > Anna - 2 // > Craig - 8 // > Donna - 6
It’s also possible to iterate over just the keys:
for key in namesAndscores.keys { print("\(key),",terminator: "") // no newline } print("") // print one final newline // > Brian,Anna,Craig,Donna,
You can iterate over just the values in the same manner with thevaluesproperty on the dictionary.
6、You could usereduce(_:combine:)to replace the prevIoUs code snippet with a single line of code:
let namesString = namesAndscores.reduce("",combine: { $0 + "\($1.0)," }) print(namesString)
In a reduce statement,$0refers to the partially-combined result,and$1refers to the current element. Since the elements in a dictionary are tuples,you need to use$1.0to get thekeyof the pair.
Let’s see how you could usefilter(_:)to find all the players with a score of less than 5:
print(namesAndscores.filter({ $0.1 < 5 })) // > [("Brian",2),("Anna",2)]
Here you use$0.1to access thevalueof the pair.
