New array method: partition

Array+partition.swift

// Here, I’ve added a function to Array called `partition`. 
// In short, the function will partition the Array into a Dictionary based on a function 
// which takes an Element of the Array and returns its corresponding Key into the Dictionary.

extension Array {
  func partition<Key>(key: (Element)->Key) -> [Key: [Element]] {
  var groups = [Key: [Element]]()
    for element in self {
      let key = key(element)
      var group = groups[key] ?? [Element]()
      group.append(element)
      groups[key] = group
    }
    return groups
  }
}

let nums = [1,2,3,4,5,6,7,8,9,10]

nums.partition { n in
	n % 2 == 0 ? "even" : "odd"
}
// => ["odd": [1, 3, 5, 7, 9], "even": [2, 4, 6, 8, 10]]