Comparing objects in an Array extension causing error in 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
    }

}

Are you trying to compare the objects themselves or just check if they point to the same object? — Connor
– Connor, Commented Jun 4, 2014 at 6:12
Is this an exercise? You should be able to use Cocoa/NSArray's indexOfObject: on an Array. — jscs
– jscs, Commented Jun 4, 2014 at 6:13
@JoshCaswell -- The indexOfObject isn't available on Array type variables, only on NSArray type variables. I'd like to add an extension that provides the functionality w/ native swift w/o the need to bridge to objective-c -- mostly just because I'm experimenting. — Logan
– Logan, Commented Jun 4, 2014 at 6:19

matt · Accepted Answer · 2014-06-06 01:25:33Z

7

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

answered Jun 6, 2014 at 1:25

matt

540k97 gold badges945 silver badges1.3k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

matt Over a year ago

It came as a surprise to me too.

Logan Over a year ago

Any chance you could point me to where you found this? Possibly a place where others like it exist?

mythz · Accepted Answer · 2014-06-04 06:47:52Z

6

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
    }
}

edited Jun 4, 2014 at 6:47

answered Jun 4, 2014 at 6:22

mythz

144k31 gold badges253 silver badges400 bronze badges

3 Comments

Logan Over a year ago

Is there much of a cost involved in the bridging? I guess my thinking was that keeping it native would ultimately be better in the long run.

mythz Over a year ago

@Logan I imagine calling Obj-C code from Swift would be highly optimized

Logan Over a year ago

Any thoughts on how to compare objects like this in general? I'd perhaps like to implement similar functionality in an extension.

Gregory Higley · Accepted Answer · 2014-06-04 06:13:32Z

2

My guess is that you have to do something like this:

func indexOfObject<T: Equatable>(object: T) -> Int? {

and so on.

answered Jun 4, 2014 at 6:13

Gregory Higley

16.7k9 gold badges71 silver badges102 bronze badges

4 Comments

Logan Over a year ago

I'm sorry if this is painfully obvious, but would you mind elaborating just a touch more. I'm getting the same error with this.

Cezary Wojcik Over a year ago

We can't know that there exists a == operator for any two AnyObjects. The <> syntax specifies a generic. They're like templates in C++. The Equatable keyword basically means that a comparison operator for T == T exists.

Gregory Higley Over a year ago

I, too, get the same error, but the docs say that Equatable objects must define the == operator, yet it still doesn't seem to compile. I suspect it's because Swift is not yet ready for prime time.

Gregory Higley Over a year ago

Actually, if I'd spent two seconds reading mythz's answer, I would have seen why.

Cezary Wojcik · Accepted Answer · 2014-06-04 06:50:58Z

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.

shucao · Accepted Answer · 2014-06-04 16:56:51Z

0

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}

edited Jun 4, 2014 at 16:56

answered Jun 4, 2014 at 6:52

shucao

2,2321 gold badge13 silver badges7 bronze badges

Comments

Jean Le Moignan · Accepted Answer · 2014-06-05 01:46:48Z

0

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.

answered Jun 5, 2014 at 1:46

Jean Le Moignan

22.2k3 gold badges33 silver badges37 bronze badges

Collectives™ on Stack Overflow

Comparing objects in an Array extension causing error in Swift

Question

Example

6 Answers 6

2 Comments

3 Comments

4 Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

Question

Example

6 Answers 6

2 Comments

3 Comments

4 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related