filter

The way filter works is quite simple. It iterates through a collection, applying a test to each element. Each element that passes the test is added to a new collection, which is returned:

let primes = [2,3,5,7,11,13,17,19,23] 
let singleDigitPrimes = primes.filter({$0 <10}) 

The preceding code returns an array of single digit primes, leaving the primes array unchanged.

let intStrings = [1:"1",2:"4", 3:"3"] 
let results = intStrings.filter { key, str in"\(key)"== str } 

The preceding code returns a dictionary that does not contain the entry 2: "4", since it failed the test in the closure.