Comparing objects in an Array extension causing error in Swift - swift

Question

I'm trying to build an extension that adds some of the convenience functionality of NSArray/NSMutableArray to the Swift Array class, and I'm trying to add this function:
func indexOfObject(object:AnyObject) -> Int? {
if self.count > 0 {
for (idx, objectToCompare) in enumerate(self) {
if object == objectToCompare {
return idx
}
}
}
return nil
}
But unfortunately, this line:
if object == objectToCompare {
Is giving the error:
could not find an overload for '==' that accepts the supplied arguments
Question
What am I doing wrong to cause this error?
Example
extension Array {
func indexOfObject(object:AnyObject) -> Int? {
if self.count > 0 {
for (idx, objectToCompare) in enumerate(self) {
if object == objectToCompare {
return idx
}
}
}
return nil
}
}

Answer 1

Actually there is no need to implement indexOfObject:; there is a global function find(array, element) already.

Answer 2

You can always create an extension that uses NSArray's indexOfObject, e.g:
extension Array {
func indexOfObject(object:AnyObject) -> Int? {
return (self as NSArray).indexOfObject(object)
}
}
You can specify that your array items can be compared with the <T : Equatable> constraint, then you can cast your object into T and compare them, e.g:
extension Array {
func indexOfObject<T : Equatable>(o:T) -> Int? {
if self.count > 0 {
for (idx, objectToCompare) in enumerate(self) {
let to = objectToCompare as T
if o == to {
return idx
}
}
}
return nil
}
}

Answer 3

My guess is that you have to do something like this:
func indexOfObject<T: Equatable>(object: T) -> Int? {
and so on.

Answer 4

Here's a relevant example from Apple's "The Swift Programming Language" in the "Generics" section:
func findIndex<T: Equatable>(array: T[], valueToFind: T) -> Int? {
for (index, value) in enumerate(array) {
if value == valueToFind {
return index
}
}
return nil
}
The key idea here is that both value and valueToFind must of a type that is guaranteed to have the == operator implemented/overloaded. The <T: Equatable> is a generic that allows only objects of a type that are, well, equatable.
In your case, we would need to ensure that the array itself is composed only of objects that are equatable. The Array is declared as a struct with a generic <T> that does not require it to be equatable, however. I don't know whether it is possible to use extensions to change what kind of types an array can be composed of. I've tried some variations on the syntax and haven't found a way.

Answer 5

You can extract the compare part to another helper function, for example
extension Array {
func indexOfObject(object: T, equal: (T, T) -> Bool) -> Int? {
if self.count > 0 {
for (idx, objectToCompare) in enumerate(self) {
if equal(object, objectToCompare) {
return idx
}
}
}
return nil
}
}
let arr = [1, 2, 3]
arr.indexOfObject(3, ==) // which returns {Some 2}

Answer 6

You were close. Here's a working extension:
extension Array {
func indexOfObject<T: Equatable>(object:T) -> Int? {
if self.count > 0 {
for (idx, objectToCompare) in enumerate(self) {
if object == objectToCompare as T {
return idx
}
}
}
return nil
}
}
Swift had no way of knowing if object or objectToCompare were equatable. By adding generic information to the method, we're then in business.