map(), flatMap() in Swift, Its So Fantastic

Array

I have a class:

class Person {
    var name: String?
    var age = 0

    init(name: String?, age: Int) {
        self.name = name
        self.age = age
    }
}

and I have a lot of people and all of them I stored in an array:

let A = Person(name: "A", age: 20)
let B = Person(name: "B", age: 25)
let C = Person(name: "C", age: 27)
let D = Person(name: "D", age: 32)
let people = [A, B, C, D]

Question: How to join all names of people in the array?

Normal way:

var names = ""
for person in people {
  names += person.name! + ","
}
print(names)

Shorter way:

let arrayNames = people.map { person in
  return person.name
}
print(arrayNames.joinWithSeparator(","))

or shorter :]

let arrayNames = people.map { $0.name }
print(arrayNames.joinWithSeparator(","))

So how about if I using .flatMap???

Now I need to change a little bit here:

A.name = nil
C.name = nil

Using .map:

let arrayNames = people.map { person in
    guard let name = person.name else { return nil }
    return person.name
}

Output:

[nil, “B”, nil, “D”]

Using .flatMap:

let arrayNames = people.flatMap { person in guard let name = person.name else { return nil } return name }

Output:

[“B”, “D”]

-> The nil values won’t be added into returned array

Difference:

`.map`	`.flatMap`
Add nil values into returned array	Doesn’t add nil values into returned array