How can I compare two Arrays in Swift and mutate one of the arrays if two Items are the same

Question

I want to compare two Arrays with each other, that means each single item of them.

I need to run some code if two items in this arrays are the same. I've done that so far with two For-Loops, but someone in the comments says that's not that good (because I get an Error too.

Has anybody an Idea which Code I can use to reach that?

Btw: That's the code I used before:

   var index = 0
   
   for Item1 in Array1 {
       for Item2 in Array2 {
           if (Item1 == Item2) {
               
               // Here I want to put some code if the items are the same
   
           }
       }
    
       // Index is in this case the counter which counts on which place in the Array1 I am.
       index += 1
   }

Okay I'll try again to describe what I mean: I want to compare two Arrays. If there are some Items the same, I want to delete that Item from Array A / Array 1. Okay if that is working, I want to add a few more statements that alow me to delete a Item only if an parameter of this item has a special worth, but I think I can do this step alone.

Answer 1

If you want to compare items from different array you need to add Equatable protocol for your Item

---

For example:

struct Item: Equatable {
    let name: String
    
    static func ==(l: Item, r: Item) -> Bool {
        return l.name == r.name
    }
}

You need to decide by which attributes you want to compare your Item . In my case I compare by name .

---

let array1: [Item] = [
    .init(name: "John"),
    .init(name: "Jack"),
    .init(name: "Soer"),
    .init(name: "Kate")
]

let array2: [Item] = [
    .init(name: "John"),
]

for item1 in array1 {
    if array2.contains(item1) {
        // Array 2 contains item from the array1 and you can perform your code.
    }
}

---

If you want to support this

Okay I'll try again to describe what I mean: I want to compare two Arrays. If there are some Items the same, I want to delete that Item from Array A / Array 1. Okay if that is working, I want to add a few more statements that alow me to delete a Item only if an parameter of this item has a special worth, but I think I can do this step alone.

I guess it can fit for you

You need to make your array1 mutable

var array1: [Item] =...

Filter the array1 like this

let filteredArray1 = array1.filter { (item) -> Bool in
    return !array2.contains(item)
}

And redefine your array1 with filtered array.

array1 = filteredArray1

---

Let me know it it works for you.

Answer 2

var itemsToRemove = array1.filter { array2.contains($0) }

for item in itemsToRemove {
    if let i = array1.firstIndex(of: item) {
        // run your conditional code here.
        array1.remove(at: i)
    }
}

Answer 3

Update

The question has been restated that elements are to be removed from one of the arrays.

You don't provide the actual code where you get the index out of bounds error, but it's almost certainly because you don't account for having removed elements when using the index, so you're probably indexing into a region that was valid at the start, but isn't anymore.

My first advice is don't do the actual removal inside the search loop. There is a solution to achieve the same result, which I'll give, but apart from having to be very careful about indexing into the shortened array, there is also a performance issue: Every deletion requires Array to shift all the later elements down one. That means that every deletion is an O( n ) operation, which makes an the overall algorithim O( n ^3).

So what do you do instead? The simplest method is to create a new array containing only the elements you wish to keep. For example, let's say you want to remove from array1 all elements that are also in array2 :

array1 = array1.filter { !array2.contains($0) }

I should note that one of the reasons I kept my original answer below is because you can use those methods to replace array2.contains($0) to achieve better performance in some cases, and the original answer describes those cases and how to achieve it.

Also even though the closure is used to determine whether or not to keep the element, so it has to return a Bool , there is nothing that prevents you from putting additional code in it to do any other work you might want to:

array1 = array1.filter 
{
    if array2.contains($0) 
    {
        doSomething(with: $0)
        return false // don't keep this element
    }
    return true
}

The same applies to all of the closures below.

You could just use the removeAll(where:) method of Array .

array1.removeAll { array2.contains($0) }

In this case, if the closure returns true , it means "remove this element" which is the opposite sense of the closure used in filter , so you have to take that into account if you do additional work in it.

I haven't looked up how the Swift library implements removeAll(where:) but I'm assuming it does its job the efficient way rather than the naive way. If you find the performance isn't all that good you could roll your own version.

extension Array where Element: Equatable
{
    mutating func efficientRemoveAll(where condition: (Element) -> Bool)
    {
        guard count > 0 else { return }
        
        var i = startIndex
        var iEnd = endIndex
        
        while i < iEnd
        {
            if condition(self[i])
            {
                iEnd -= 1
                swapAt(i, iEnd)
                continue // note: skips incrementing i,iEnd has been decremented
            }
            
            i += 1
        }
        
        removeLast(endIndex - iEnd)
    }
}

array1.efficientRemoveAll { array2.contains($0) }

Instead of actually removing the elements inside the loop, this works by swapping them with the end element, and handling when to increment appropriately. This collects the elements to be removed at the end, where they can be removed in one go after the loop finishes. So the deletion is just one O( n ) pass at the end, which avoids increasing the algorithmic complexity that removing inside the loop would entail.

Because of the swapping with the current "end" element, this method doesn't preserve the relative order of the elements in the mutating array.

If you want to preserve the order you can do it like this:

extension Array where Element: Equatable
{
    mutating func orderPreservingRemoveAll(where condition: (Element) -> Bool)
    {
        guard count > 0 else { return }
        
        var i = startIndex
        var j = i
        
        repeat
        {
            swapAt(i, j)
            if !condition(self[i]) { i += 1 }
            j += 1
        } while j != endIndex
        
        removeLast(endIndex - i)
    }
}

array1.orderPreservingRemoveAll { array2.contains($0) }

If I had to make a bet, this would be very close to how standard Swift's removeAll(where:) for Array is implemented. It keeps two indices, one for the current last element to be kept, i , and one for the next element to be examined, j . This has the effect of accumulating elements to be removed at the end (those past i ).

Original answer

The previous solutions are fine for small (yet still surprisingly large) arrays, but they are O( n ^2) solutions.

If you're not mutating the arrays, they can be expressed more succinctly

array1.filter { array2.contains($0) }.forEach { doSomething($0) }

where doSomething doesn't actually have to be a function call - just do whatever you want to do with $0 (the common element).

You'll normally get better performance by putting the smaller array inside the filter .

Special cases for sorted arrays

If one of your arrays is sorted, then you might get better performance in a binary search instead of contains , though that will require that your elements conform to Comparable . There isn't a binary search in the Swift Standard Library, and I also couldn't find one in the new swift-algorithms package, so you'd need to implement it yourself:

extension RandomAccessCollection where Element: Comparable, Index == Int
{
    func sortedContains(_ element: Element) -> Bool
    {
        var range = self[...]
        while range.count > 0
        {
            let midPoint = (range.startIndex + range.endIndex) / 2
            if range[midPoint] == element { return true }
            range = range[midPoint] > element
                ? self[..<midPoint]
                : self[index(after: midPoint)...]
        }
        return false
    }
}

unsortedArray.filter { sortedArray.sortedContains($0) }.forEach { doSomething($0) }

This will give O( n log n ) performance. However, you should test it for your actual use case if you really need performance, because binary search is not especially friendly for the CPU's branch predictor, and it doesn't take that many mispredictions to result in slower performance than just doing a linear search.

If both arrays are sorted, you can get even better performance by exploiting that, though again you have to implement the algorithm because it's not supplied by the Swift Standard Library.

extension Array where Element: Comparable
{
    // Assumes no duplicates
    func sortedIntersection(_ sortedOther: Self) -> Self
    {
        guard self.count > 0, sortedOther.count > 0 else { return [] }
        
        var common = Self()
        common.reserveCapacity(Swift.min(count, sortedOther.count))
        
        var i = self.startIndex
        var j = sortedOther.startIndex
        
        var selfValue = self[i]
        var otherValue = sortedOther[j]
        
        while true
        {
            if selfValue == otherValue
            {
                common.append(selfValue)
                i += 1
                j += 1
                if i == self.endIndex || j == sortedOther.endIndex { break }
                selfValue = self[i]
                otherValue = sortedOther[j]
                continue
            }
            
            if selfValue < otherValue
            {
                i += 1
                if i == self.endIndex { break }
                selfValue = self[i]
                continue
            }
            
            j += 1
            if j == sortedOther.endIndex { break }
            otherValue = sortedOther[j]
        }
        
        return common
    }
}

array1.sortedIntersection(array2).forEach { doSomething($0) }

This is O( n ) and is the most efficient of any of the solutions I'm including, because it makes exactly one pass through both arrays. That's only possible because both of the arrays are sorted.

Special case for large array of Hashable elements

However, if your arrays meet some criteria, you can get O( n ) performance by going through Set . Specifically if

1. The arrays are large
2. The array elements conform to Hashable
3. You're not mutating the arrays

Make the larger of the two arrays a Set :

let largeSet = Set(largeArray)
smallArray.filter { largeSet.contains($0) }.forEach { doSomething($0) }

For the sake of analysis if we assume both arrays have n elements, the initializer for the Set will be O( n ). The intersection will involve testing each element of the smallArray 's elements for membership in largeSet . Each one of those tests is O(1) because Swift implements Set as a hash table. There will be n of those tests, so that's O( n ). Then the worst case for forEach will be O( n ). So O( n ) overall.

There is, of course, overhead in creating the Set , the worst of which is the memory allocation involved, so it's not worth it for small arrays.