Question

我对Swift来说还很陌生，所以仍然会修补并找到解决方法。我只是想知道为什么我必须将Switch值chicken.breed, chicken.temper和大写字母Breed.leghorn, Temper.hilarious括在括号内。 删除括号后，看到以下错误

上下文 练习的目的是每当发现有趣的鸡与我想要检查的chickenOfInterestCount和Breed值匹配时就增加Temper。所以我用了一个Switch语句来执行这些操作

Exercise.swift

var chickenOfInterestCount = 0

for chicken in chickens {

    // check the chicken variables breed and temper enums
    switch (chicken.breed, chicken.temper){
        case (Breed.leghorn, Temper.hilarious):
            chickenOfInterestCount += 1
        default:
            print("No match")
    }
}

print(chickenOfInterestCount)

Chicken.swift

public enum Breed {
    case leghorn, rhodeIsland, silkie, plymouthRock
}

public enum Temper {
    case friendly, grumpy, hilarious
}

public struct Chicken {
    public let breed: Breed
    public let temper: Temper
}


public let chickens: [Chicken] = [
    Chicken(breed: .leghorn, temper: .friendly),
    Chicken(breed: .leghorn, temper: .friendly),
    Chicken(breed: .leghorn, temper: .grumpy),
    Chicken(breed: .rhodeIsland, temper: .friendly),
    Chicken(breed: .leghorn, temper: .hilarious),
    Chicken(breed: .rhodeIsland, temper: .friendly),
    Chicken(breed: .rhodeIsland, temper: .grumpy),
    Chicken(breed: .silkie, temper: .friendly),
    Chicken(breed: .rhodeIsland, temper: .grumpy),
    Chicken(breed: .silkie, temper: .grumpy),
    Chicken(breed: .rhodeIsland, temper: .hilarious),
    Chicken(breed: .leghorn, temper: .friendly),
    Chicken(breed: .silkie, temper: .friendly),
    Chicken(breed: .leghorn, temper: .hilarious),
    Chicken(breed: .plymouthRock, temper: .grumpy),
    Chicken(breed: .leghorn, temper: .grumpy),
    Chicken(breed: .silkie, temper: .grumpy),
    Chicken(breed: .plymouthRock, temper: .friendly),
    Chicken(breed: .leghorn, temper: .friendly),
    Chicken(breed: .leghorn, temper: .friendly),
]

删除亲本时出错

error: 19_EnumsAndSwitch.playground:9:25: error: expected '{' after 'switch' subject expression
    switch chicken.breed, chicken.temper{
                        ^

error: 19_EnumsAndSwitch.playground:10:9: error: 'case' label can only appear inside a 'switch' statement
        case (Breed.leghorn, Temper.hilarious):
        ^

error: 19_EnumsAndSwitch.playground:12:9: error: 'default' label can only appear inside a 'switch' statement
        default:
        ^

Answer 1

switch采用一个表达式。通过将两个值放在括号中，您将创建一个元组，并且case语句在该元组上匹配。

没有括号，您试图将两个逗号分隔的表达式放在只能有一个的地方。

在元组上使用switch时，用例可以使元组的各种组合与switch表达式中的组合匹配。

有关更多信息，请参见Swift书中的Tuples and Switch部分。

为什么此Switch语句的“ Switch Value”和“ Compound case”需要括号？

1 个答案: