How to modify a single element in an array with a custom type [duplicate]

Question

I'm trying to learn swift. When I do:

var myIntArray = [Int](count: 3, repeatedValue: 0)
myIntArray[0] = 1
myIntArray[1] = 11
myIntArray[2] = 111

// prints "my array: [1, 11, 111]"
NSLog("my int array: [%d, %d, %d]", myIntArray[0], myIntArray[1], myIntArray[2])

It works as expected, but when I wrap the int in a class like this:

class Foo {
    var bar = 0
}

var myWrappedIntArray = [Foo](count: 3, repeatedValue: Foo())
myWrappedIntArray[0].bar = 1
myWrappedIntArray[1].bar = 11
myWrappedIntArray[2].bar = 111

// prints "my array: [111, 111, 111]"
NSLog("my wrapped int array: [%d, %d, %d]", myWrappedIntArray[0].bar, myWrappedIntArray[1].bar, myWrappedIntArray[2].bar)

It seems to modify the entire array when trying to modify a single element. What am I doing wrong here?

Perhaps my argument for repeatedValue creates a single Foo instance and fills the array with pointers to this instance? But how to fix that? — Kevin
– Kevin, Commented Sep 12, 2016 at 10:27
@RashmiRanjanmallick Because I'm used to using NSLog in iOS development, is that really relevant to the issue at hand? — Kevin
– Kevin, Commented Sep 12, 2016 at 10:30

Oleg Gordiichuk · Accepted Answer · 2016-09-12 10:31:44Z

1

For this case it is better to use Struct:

struct Foo {
    var bar = 0
}

var myWrappedIntArray = [Foo](count: 3, repeatedValue: Foo())
myWrappedIntArray[0].bar = 1
myWrappedIntArray[1].bar = 11
myWrappedIntArray[2].bar = 111

// prints "my array: [1, 11, 111]"
print("my wrapped int array: [%d, %d, %d]", myWrappedIntArray[0].bar, myWrappedIntArray[1].bar, myWrappedIntArray[2].bar)

In you're case you are working with reference type value such as [Foo]. But Struct is value typed and create each element of the array unique.

You are referencing to the same object in myWrappedIntArray[0].bar and myWrappedIntArray[n].bar.

Because of the constructor.

var myWrappedIntArray = [Foo](count: 3, repeatedValue: Foo())

You repeat Foo() reference for each element of the array instead of the creation of the new class for each index of the array.

edited Sep 12, 2016 at 10:31

answered Sep 12, 2016 at 10:27

Oleg Gordiichuk

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4 Comments

Kevin Over a year ago

My use case is different, but this is the smallest example I could write that reproduces my issue

Oleg Gordiichuk Over a year ago

i edit my answer.

Kevin Over a year ago

How can I create the array with a Foo instance for every element? Do I need to create the array empty and append the Foo instances manually or is there an equivalent of init(count: repeatedValue:) that creates a new instance for every element?

Oleg Gordiichuk Over a year ago

One of the ways you describe. Take a look at answer of the MartinR. stackoverflow.com/questions/32921425/… For you if it is possible use Structs instead of classes.

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