Swift For In Loop Iterating Through an Array of Custom Classes Doesn't Work

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!

_class is a misleading name in the second loop. It's actually an index — Alexander
– Alexander, Commented Jul 28, 2017 at 0:29

Community · Accepted Answer · 2020-06-20 09:12:55Z

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

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 28, 2017 at 0:08

adev

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1 Comment

adev Over a year ago

Good to hear that.

Alexander · Accepted Answer · 2017-07-28 00:30:31Z

1

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.

answered Jul 28, 2017 at 0:30

Alexander

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1 Comment

grahamcracker1234 Over a year ago

Thank you +1 for the combined operations

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