How can I change the element in an array of struct

Question

I have an array of struct 'Pair' (Data) and I want to change the element in the array if some condition is met. I have followed the documentation https://docs.swift.org/swift-book/LanguageGuide/Methods.html and created a 'setValue' method with mutating

But when I debug the code, I see the value of the pair variable get changed. But the array 'data?.pairs' does not get changed to the new value 'Point(1,2). Can you please tell what am I missing?

public struct Data {
 var pairs: [Pair<Point>]
}
var array = data?.pairs ?? []
for index in 0..<array.count {
    var pair = data?.pairs[index]
                  
    if /* some test */ {
       print ("Found match")
       pair?.setValue(Point(1,2))   // `pair` is updated, but data?.paris[index] is not updated.
    }
}

struct Pair<Value: Comparable>{
    var key: Value
    var value: Value

    init(_ key: Value, _ value: Value) {
        self.key = key
        self.value = value
    }

    mutating func setValue(_ value: Value) {
        self.value = value
    }
}

Answer 1

Pair is a struct, so value semantics apply. This means that here, you made a copy of each item in data.pairs:

var pair = data?.pairs[index]

And you are changing the copies, not the originals.

One way to solve this is to assign the changed copies back to data.pairs:

if /* some test */ {
   print ("Found match")
   pair?.setValue(Point(1,2))
   data?.pairs[index] = pair // here!
}

Or, don't make a copy in the first place:

if /* some test */ {
   print ("Found match")
   data?.pairs[index].setValue(Point(1,2))
}

Or, make Pair a class, so reference semantics apply, and you won't be making copies.

Answer 2

As we know, arrays and structs in Swift are value types, so at the moment

var pair = data?.pairs[index]

the pair is a copy of an array item value, and has no relation to the original array.