Swift for In In循环遍历自定义类的数组不起作用

Question

I am working a project and have come across this error when iterating through a for in loop like this:

class CustomClass {

    var nameNum : Int { didSet { self.name = "CustomClass \(nameNum)" } }
    var name : String

    init() {
        nameNum = 0
        self.name = "CustomClass \(nameNum)"
    }
}

var myArray : [CustomClass] = [CustomClass](repeating: CustomClass(), count: 5)

for _class in myArray.indices {
    myArray[_class].nameNum = _class
}

print("\n")
for _class in myArray.indices {
    print("Item \(_class): \(myArray[_class].name)")
}

I get the following output:

Item 0: CustomClass 4

Item 1: CustomClass 4

Item 2: CustomClass 4

Item 3: CustomClass 4

Item 4: CustomClass 4

This does not make sense to me as I thought I would get the following output instead:

Item 0: CustomClass 0

Item 1: CustomClass 1

Item 2: CustomClass 2

Item 3: CustomClass 3

Item 4: CustomClass 4

Any help as to why this doesn't work or to how to go about fixing it is appreciated, thanks!

Answer 1

You should change your array initialization to

var myArray : [CustomClass] = (0..<5).map { _ in CustomClass() }

from

var myArray : [CustomClass] = [CustomClass](repeating: CustomClass(), count: 5)

Complete Code:

class CustomClass {

    var nameNum : Int { didSet { self.name = "CustomClass \(nameNum)" } }
    var name : String

    init() {
        nameNum = 0
        self.name = "CustomClass \(nameNum)"
    }
}

var myArray : [CustomClass] = (0..<5).map { _ in CustomClass() }

for _class in myArray.indices {
    myArray[_class].nameNum = _class
}

print("\n")
for _class in myArray.indices {
    print("Item \(_class): \(myArray[_class].name)")
}

The reason is that your code, actually creates an instance of CustomClass and adds it at all 5 indexes of array where as you are expecting 5 different instance of CustomClass . If the same instance is added 5 times, then all of them will have the last set value which in your case is 4.

Item 0: CustomClass 0

Item 1: CustomClass 1

Item 2: CustomClass 2

Item 3: CustomClass 3

Item 4: CustomClass 4

Answer 2

Why are you setting nameNum to 0 in the initializer, if you're immediately going to change it? Just combine the operations:

class CustomClass {
    var nameNum: Int
    var name: String { return "CustomClass \(nameNum)" }

    init(nameNum: Int) {
        self.nameNum = nameNum
    }
}

var myArray = (0..<5).map(CustomClass.init(nameNum:))

for (index, element) in myArray.enumerated() {
    print("Item #\(index): \(element.name)")
}

Also, if name always has a value derived from nameNum , then it's better to have it be a computed property whose derived from nameNum , rather than having the responsibility of setting it fall to a side effect of setting nameNum .