Swift 笔记（八）

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Arrays

1、You can declare an array either explicitly or taking advantage of Swift's type inference. Here's an example of an explicit declaration:

let numbers: Array<Int>

let inferredNumbers = Array<Int>()

let alsoInferredNumbers = [Int]() 

This isthe most common declaration formfor an array.

When you declare an array,you might want to give it initial values. You can do that usingarray literals,which is a concise way to provide array values. An array literal is a list of values separated by commas and surrounded by square brackets:

let evenNumbers = [2,4,6,8]

let allZeros = [Int](count: 5,repeatedValue: 0)  

var players = ["Alice","Bob","Cindy","Dan"]  
print(players.isEmpty)
// > false  

if players.count < 2 {
    print("We need at least two players!")
} else {
    print("Let's start!")
}
// > Let's start!

var currentPlayer = players.first
print(currentPlayer)
// > Optional("Alice")
print(players.last)
// > Optional("Dan")

currentPlayer = players.minElement()
print(currentPlayer)
// > Optional("Alice")

print([2,3,1].first)
// > Optional(2)
print([2,1].minElement())
// > Optional(1)

As you might have guessed,arrays also have amaxElement()method.

Now that you've figured out how to get the first player,you'll announce who that player is:

if let currentPlayer = currentPlayer {
    print("(currentPlayer) will start")
}
// > Alice will start

let upcomingPlayers = players[1...2]
print(upcomingPlayers)
// >["Bob","Cindy"]

As you can see,the constantupcomingPlayersis actually an array of the same type as the original array. You can use any index here as long as the start value is smaller than the end value and they're both within the bounds of the array.

4、You can check if there's at least one occurrence of a specific element in an array by usingcontains(_:),which returnstrueif it finds the element in the array,andfalseif it doesn't.

func isPlayerEliminated(playerName: String) -> Bool {
    if players.contains(playerName) {
        return false
    } else {
        return true
    }
}

You could even test for the existence of an element in a specific range:

players[1...3].contains("Bob")
// > true

players.append("Eli")

You can also append elements using the+=operator:

players += ["Gina"]

The right-hand side of this expression is an array with a single element,the string "Gina".
Here,you added a single element to the array but you see how easy it would be to appendmultiple items by adding more namesafter Gina's.

6、You want to add Frank to the list between Eli and Gina. To do that,you can useinsert(_:atIndex:):

players.insert("Frank",atIndex: 5)  

TheatIndexargument defines where you want to add the element. Remember that the array is zero-indexed.

7、You can remove the last one in the players list easily withremoveLast():

var removedPlayer = players.removeLast()
print("(removedPlayer) was removed")
// > Gina was removed

This method does two things: It removes the last element andthen it returns it,in case you need to print it or store it somewhere else.

To remove Cindy from the game,you need to know the exact index where her name is stored. Looking at the list of players,you see that she's third in the list,so her index is 2.

removedPlayer = players.removeAtIndex(2)
print("(removedPlayer) was removed")
// > Cindy was removed

print(players)
// > ["Alice","Eli","Frank"]
players[3] = "Franklin"
print(players)
// > ["Alice","Franklin"]  

players[0...1] = ["Donna","Craig","Brian","Anna"]
print(players)
// > ["Donna","Anna","Franklin"]

players = players.sort()
print(players)
// > ["Anna","Donna",monospace; font-size:1em; border:0px; outline:0px; vertical-align:baseline; background:transparent">sort()does exactly what you expect it to do: It returnsa sorted copyof the array.If you'd like to sort the array in place instead of returning a sorted copy,monospace; font-size:1em; border:0px; outline:0px; vertical-align:baseline; background:transparent">sortInPlace().
 
 
 10、If you also need the index of each element,you can iterate over the return value of the array'senumerate()method,which returns a tuple with the index and value of each element in the array.
 
 
for (index,playerName) in players.enumerate() {
    print("(index + 1). (playerName)")
}
// > 1. Anna
// > 2. Brian
// > 3. Craig
// > 4. Donna
// > 5. Eli
// > 6. Franklin
 
 
 
 11、 
 reduce(_:combine:) 
 take an initial value as the first argument and accumulates every value in the array in turn,using the combine operation. 
 
 

let scores = [2,2,8,1,2]
let sum = scores.reduce(0,combine: +)
print(sum)
// > 21

print(scores.filter({ $0 > 5 }))
// > [8,6]

print(scores)
// > [2,2]
let newScores = scores.map({ $0 * 2 })
print(newScores)
// > [4,16,12,4]

（编辑：李大同）

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