Swift 3 Extracting similar objects - arrays

Question

I have an array of arrays containg int, so let pairs:[[Int]].
I am looking for an elegant way to extract similar elements.
For example my pairs variable could contain something like: [ [1,2], [4,6], [1,2] ]
I would like to extract any array that occurs more than once like [1,2].
In the example [ [1,2], [4,6], [1,2], [3,7], [4,6] ] I would like to extract both [1,2] and [4,6].
This seemed trivial at first, but every go I had at it became very cumbersome with many "helper arrays" and nested "for loops". There surely is a simpler way in Swift, right?
Thanks

Answer 1

Here the way using just one helper Dictionary and one loop:
let pairs = [[1,2], [4,6], [1,2]] // The original array
var pairCount = [NSArray : Int]() // This is helper dictionary. The key is the array, the value is - how much time it appears in the source array. I use NSArray because swift array does not conform to Hashable protocol.
var newPairs = [[Int]]()
for pair in pairs { // Iterate over our pairs
var count: Int = pairCount[pair as NSArray] ?? 0 // If we already have a pair in our dictionary get count of it in array otherwise het 0
count += 1 // increase counter
pairCount[pair as NSArray] = count // save in dictionary
}
let pairsWithCountMoreThanOne = pairCount.flatMap({$1 > 1 ? $0 : nil}) // here we get the result
for pair in pairsWithCountMoreThanOne { // Just print it
print("\(pair)")
}
This code may not be memory efficient for large arrays or large objects but it is really trivial.

Answer 2

Please check the below :
let pairs = [ [1,2], [4,6], [1,2], [3,7], [4,6], [3,5], [4,6] ]
var repeats = [[Int]]()
pairs.forEach { (i) in
let count = (pairs.filter{ return $0 == i }).count
if count > 1 {
if !repeats.contains(where: { (pair) -> Bool in
return pair == i
}) {
repeats.append(i)
}
}
}
print(repeats) // Output is : [[1, 2], [4, 6]]