Find an item and change value in custom object array - Swift

array = array.map { $0.eventID == id ? newValue : $0 }

Since you are using a class, use filter and first to find the value:

array.filter({$0.eventID == id}).first?.added = value

In this you:

1. filter the array down to elements that match the event ID
2. pick the first result, if any
3. then set the value

This works since classes are pass by reference. When you edit the return value from array.filter({$0.eventID == id}).first?, you edit the underlying value. You'll need to see the answers below if you are using a struct

EDIT: In Swift 3 you can save yourself a couple of characters

array.first({$0.eventID == id})?.added = value

EDIT: Swift 4.2:

array.first(where: { $0.eventID == id })?.added = value
array.filter {$0.eventID == id}.first?.added = value

For me, the above answer did not work. So, what I did was first find the index of the object that I want to replace then using the index replace it with the new value

if let row = self.upcoming.index(where: {$0.eventID == id}) {
       array[row] = newValue
}

In Swift 5.0:

if let row = self.upcoming.firstIndex(where: {$0.eventID == id}) {
       array[row] = newValue
}

The filter operator is not the best in this case, it works for some of you because classes are passed by reference.

Explanation: (You can copy the following code in a playground if you want to verify it).

class Book {
    let id: Int
    var title = "default"

    init (id: Int) {
        self.id = id
    }
}
var arrayBook = [Book]()
arrayBook.append(Book(id: 0))
arrayBook.append(Book(id:1))
arrayBook.forEach { book in
    print(book.title)
}

arrayBook.filter{ $0.id == 1 }.first?.title = "modified"

arrayBook.forEach { book in
    print(book.title)
}

Arrays are copied by value not reference, so when you are using filter you are creating a new array (different than the initial), but when you modify the new one, the initial one gets modified too because both are pointing to the same class (classed are passed by reference), so after the filter your array will have changed and the new one gets deallocated. So in this case it will print "default", "default" and then "default, "modified".

What happens if you change class for struct, the value will be passed by value not reference so you will have 2 arrays in memory with different values, so if you go through arrayBooks again it will print before the filter "default","default", and then "default", "default" again. Because when you are using the filter you are creating and modifying a new array that will get deallocated if you do not store it).

The solution is using map, creating a new array with all the values but with the modified items or fields that we want and then replace our array with the new one. This will print "default", "default" before the map, and then "default", "modified"

This will work with structs, classes and everything that you want :).

struct Book {
    let id: Int
    var title = "default"

    init (id: Int) {
        self.id = id
    }
}
var arrayBook = [Book]()
arrayBook.append(Book(id: 0))
arrayBook.append(Book(id:1))
arrayBook.forEach { book in
    print(book.title)
}

arrayBook = arrayBook.map{
    var mutableBook = $0
    if $0.id == 1 {
        mutableBook.title = "modified"
    }
    return mutableBook
}

arrayBook.forEach { book in
    print(book.title)
}