Array extension to remove object by value

Question

extension Array {
    func removeObject<T where T : Equatable>(object: T) {
        var index = find(self, object)
        self.removeAtIndex(index)
    }
}

However, I get an error on var index = find(self, object)

'T' is not convertible to 'T'

I also tried with this method signature: func removeObject(object: AnyObject), however, I get the same error:

'AnyObject' is not convertible to 'T'

What is the proper way to do this?

Answer 1

Your problem is T is not related to the type of your array in anyway for example you could have

var array = [1,2,3,4,5,6]

array.removeObject(object:"four")

"six" is Equatable, but its not a type that can be compared to Integer, if you change it to

var array = [1,2,3,4,5,6]

extension Array where Element : Equatable {
    mutating func removeObject(object: Element) {
        filter { $0 != object }
    }
}

array.removeObject(object:"four")

it now produces an error on calling removeObject for the obvious reason its not an array of strings, to remove 4 you can just

array.removeObject(object:4)

Other problem you have is its a self modifying struct so the method has to be labeled as so and your reference to it at the top has to be a var

Answer 2

As of Swift 2, this can be achieved with a protocol extension method. removeObject() is defined as a method on all types conforming to RangeReplaceableCollectionType (in particular on Array) if the elements of the collection are Equatable:

extension RangeReplaceableCollectionType where Generator.Element : Equatable {

    // Remove first collection element that is equal to the given `object`:
    mutating func removeObject(object : Generator.Element) {
        if let index = self.indexOf(object) {
            self.removeAtIndex(index)
        }
    }
}

Example:

var ar = [1, 2, 3, 2]
ar.removeObject(2)
print(ar) // [1, 3, 2]

Update for Swift 2 / Xcode 7 beta 2: As Airspeed Velocity noticed in the comments, it is now actually possible to write a method on a generic type that is more restrictive on the template, so the method could now actually be defined as an extension of Array:

extension Array where Element : Equatable {
    
    // ... same method as above ...
}

The protocol extension still has the advantage of being applicable to a larger set of types.

Update for Swift 3:

extension Array where Element: Equatable {
    
    // Remove first collection element that is equal to the given `object`:
    mutating func remove(object: Element) {
        if let index = index(of: object) {
            remove(at: index)
        }
    }
}

Update for Swift 5:

extension Array where Element: Equatable {
    
    /// Remove first collection element that is equal to the given `object` or `element`:
    mutating func remove(element: Element) {
        if let index = firstIndex(of: element) {
            remove(at: index)
        }
    }
}

Answer 3

No need to extend:

var ra = [7, 2, 5, 5, 4, 5, 3, 4, 2]

print(ra)                           // [7, 2, 5, 5, 4, 5, 3, 4, 2]

ra.removeAll(where: { $0 == 5 })

print(ra)                           // [7, 2, 4, 3, 4, 2]

if let i = ra.firstIndex(of: 4) {
    ra.remove(at: i)
}

print(ra)                           // [7, 2, 3, 4, 2]

if let j = ra.lastIndex(of: 2) {
    ra.remove(at: j)
}

print(ra)                           // [7, 2, 3, 4]

Answer 4

Maybe I didn't understand the question.

Why wouldn't this work?

import Foundation
extension Array where Element: Equatable {
    mutating func removeObject(object: Element) {
        if let index = self.firstIndex(of: object) {
            self.remove(at: index)
        }
    }
}

var testArray = [1,2,3,4,5,6,7,8,9,0]
testArray.removeObject(object: 6)
let newArray = testArray

var testArray2 = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0"]
testArray2.removeObject(object: "6")
let newArray2 = testArray2

Answer 5

After reading all the above, to my mind the best answer is:

func arrayRemovingObject<U: Equatable>(object: U, # fromArray:[U]) -> [U] {
  return fromArray.filter { return $0 != object }
}

Sample:

var myArray = ["Dog", "Cat", "Ant", "Fish", "Cat"]
myArray = arrayRemovingObject("Cat", fromArray:myArray )

Swift 2 (xcode 7b4) array extension:

extension Array where Element: Equatable {  
  func arrayRemovingObject(object: Element) -> [Element] {  
    return filter { $0 != object }  
  }  
}  

Sample:

var myArray = ["Dog", "Cat", "Ant", "Fish", "Cat"]
myArray = myArray.arrayRemovingObject("Cat" )

Swift 3.1 update

Came back to this now that Swift 3.1 is out. Below is an extension which provides exhaustive, fast, mutating and creating variants.

extension Array where Element:Equatable {
    public mutating func remove(_ item:Element ) {
        var index = 0
        while index < self.count {
            if self[index] == item {
                self.remove(at: index)
            } else {
                index += 1
            }
        }
    }

    public func array( removing item:Element ) -> [Element] {
        var result = self
        result.remove( item )
        return result
    }
}

Samples:

// Mutation...
      var array1 = ["Cat", "Dog", "Turtle", "Cat", "Fish", "Cat"]
      array1.remove("Cat")
      print(array1) //  ["Dog", "Turtle", "Socks"]

// Creation...
      let array2 = ["Cat", "Dog", "Turtle", "Cat", "Fish", "Cat"]
      let array3 = array2.array(removing:"Cat")
      print(array3) // ["Dog", "Turtle", "Fish"]

Answer 6

With protocol extensions you can do this,

extension Array where Element: Equatable {
    mutating func remove(object: Element) {
        if let index = indexOf({ $0 == object }) {
            removeAtIndex(index)
        }
    }
}

Same functionality for classes,

Swift 2

extension Array where Element: AnyObject {
    mutating func remove(object: Element) {
        if let index = indexOf({ $0 === object }) {
            removeAtIndex(index)
        }
    }
}

Swift 3

extension Array where Element: AnyObject {
    mutating func remove(object: Element) {
        if let index = index(where: { $0 === object }) {
             remove(at: index)
        }
    }
}

But if a class implements Equatable it becomes ambiguous and the compiler gives an throws an error.

Answer 7

what about to use filtering? the following works quite well even with [AnyObject].

import Foundation
extension Array {
    mutating func removeObject<T where T : Equatable>(obj: T) {
        self = self.filter({$0 as? T != obj})
    }

}

Answer 8

Implementation in Swift 2:

extension Array {
  mutating func removeObject<T: Equatable>(object: T) -> Bool {
    var index: Int?
    for (idx, objectToCompare) in self.enumerate() {
      if let toCompare = objectToCompare as? T {
        if toCompare == object {
          index = idx
          break
        }
      }
    }
    if(index != nil) {
      self.removeAtIndex(index!)
      return true
    } else {
      return false
    }
  }
}

Answer 9

I finally ended up with following code.

extension Array where Element: Equatable {

    mutating func remove<Element: Equatable>(item: Element) -> Array {
        self = self.filter { $0 as? Element != item }
        return self
    }

}

Answer 10

You cannot write a method on a generic type that is more restrictive on the template.

NOTE: as of Swift 2.0, you can now write methods that are more restrictive on the template. If you have upgraded your code to 2.0, see other answers further down for new options to implement this using extensions.

The reason you get the error 'T' is not convertible to 'T' is that you are actually defining a new T in your method that is not related at all to the original T. If you wanted to use T in your method, you can do so without specifying it on your method.

The reason that you get the second error 'AnyObject' is not convertible to 'T' is that all possible values for T are not all classes. For an instance to be converted to AnyObject, it must be a class (it cannot be a struct, enum, etc.).

Your best bet is to make it a function that accepts the array as an argument:

func removeObject<T : Equatable>(object: T, inout fromArray array: [T]) {
}

Or instead of modifying the original array, you can make your method more thread safe and reusable by returning a copy:

func arrayRemovingObject<T : Equatable>(object: T, fromArray array: [T]) -> [T] {
}

As an alternative that I don't recommend, you can have your method fail silently if the type stored in the array cannot be converted to the the methods template (that is equatable). (For clarity, I am using U instead of T for the method's template):

extension Array {
    mutating func removeObject<U: Equatable>(object: U) {
        var index: Int?
        for (idx, objectToCompare) in enumerate(self) {
            if let to = objectToCompare as? U {
                if object == to {
                    index = idx
                }
            }
        }

        if(index != nil) {
            self.removeAtIndex(index!)
        }
    }
}

var list = [1,2,3]
list.removeObject(2) // Successfully removes 2 because types matched
list.removeObject("3") // fails silently to remove anything because the types don't match
list // [1, 3]

Edit To overcome the silent failure you can return the success as a bool:

extension Array {
  mutating func removeObject<U: Equatable>(object: U) -> Bool {
    for (idx, objectToCompare) in self.enumerate() {  //in old swift use enumerate(self) 
      if let to = objectToCompare as? U {
        if object == to {
          self.removeAtIndex(idx)
          return true
        }
      }
    }
    return false
  }
}
var list = [1,2,3,2]
list.removeObject(2)
list
list.removeObject(2)
list

Answer 11

Using indexOf instead of a for or enumerate:

extension Array where Element: Equatable {

   mutating func removeElement(element: Element) -> Element? {
      if let index = indexOf(element) {
         return removeAtIndex(index)
      }
      return nil
   }

   mutating func removeAllOccurrencesOfElement(element: Element) -> Int {
       var occurrences = 0
       while true {
          if let index = indexOf(element) {
             removeAtIndex(index)
             occurrences++
          } else {
             return occurrences
          }
       }
   }   
}

Answer 12

With using protocol extensions in swift 2.0

extension _ArrayType where Generator.Element : Equatable{
    mutating func removeObject(object : Self.Generator.Element) {
        while let index = self.indexOf(object){
            self.removeAtIndex(index)
        }
    }
}

Answer 13

There is another possibility of removing an item from an array without having possible unsafe usage, as the generic type of the object to remove cannot be the same as the type of the array. Using optionals is also not the perfect way to go as they are very slow. You could therefore use a closure like it is already used when sorting an array for example.

//removes the first item that is equal to the specified element
mutating func removeFirst(element: Element, equality: (Element, Element) -> Bool) -> Bool {
    for (index, item) in enumerate(self) {
        if equality(item, element) {
            self.removeAtIndex(index)
            return true
        }
    }
    return false
}

When you extend the Array class with this function you can remove elements by doing the following:

var array = ["Apple", "Banana", "Strawberry"]
array.removeFirst("Banana") { $0 == $1 } //Banana is now removed

However you could even remove an element only if it has the same memory address (only for classes conforming to AnyObject protocol, of course):

let date1 = NSDate()
let date2 = NSDate()
var array = [date1, date2]
array.removeFirst(NSDate()) { $0 === $1 } //won't do anything
array.removeFirst(date1) { $0 === $1 } //array now contains only 'date2'

The good thing is, that you can specify the parameter to compare. For example when you have an array of arrays, you can specify the equality closure as { $0.count == $1.count } and the first array having the same size as the one to remove is removed from the array.

You could even shorten the function call by having the function as mutating func removeFirst(equality: (Element) -> Bool) -> Bool, then replace the if-evaluation with equality(item) and call the function by array.removeFirst({ $0 == "Banana" }) for example.

Answer 14

briefly and concisely:

func removeObject<T : Equatable>(object: T, inout fromArray array: [T]) 
{
    var index = find(array, object)
    array.removeAtIndex(index!)
}

Answer 15

I was able to get it working with:

extension Array {
    mutating func removeObject<T: Equatable>(object: T) {
        var index: Int?
        for (idx, objectToCompare) in enumerate(self) {
            let to = objectToCompare as T
            if object == to {
                index = idx
            }
        }

        if(index) {
            self.removeAtIndex(index!)
        }
    }
}