4.1.5 Adv Array Manipulation

Common Member Closures

• filter → returns array elements that pass a certain condition
  
• map → returns array elements transformed by a function
  
• reduce → returns the sum of array elements plus an initial value
  
• reverse → in-line reverses the array order
  
• sort → in-line sort of values w/in the array
  
• sorted → returns a copy of a sorted array of values of a known type

Counting Frequencies in an Array

let items = ["a", "b", "a", "c"]
let mappedItems = items.map { ($0, 1) }
let counts = Dictionary(mappedItems, uniquingKeysWith: +)

• use when you want to count how often each item appears
  
• mappedItems →  creates an array of key-value pairs using tuples, where each value is the number 1
  
• counts → dictionary from that tuple array that adds the 1s together

Find the Difference Between Two Arrays

• using an Extension

extension Array where Element: Hashable {
   func difference(from other: [Element]) -> [Element] {
      let thisSet = Set(self)
      let otherSet = Set(other)
      return Array(thisSet.symmetricDifference(otherSet))
   }
}
let names1 = ["John", "Paul", "Ringo"]
let names2 = ["Ringo", "Paul", "George"]
let differenceInArrays = names1.difference(from: names2)

• sets have symmetricDifference function
  
• convert the arrays to Sets, then convert the result back