Array map vs. forEach in Swift

Question

In Swift arrays you can do:

var myArray = [1,2,3,4]

myArray.forEach() { print($0 * 2) }

myArray.map() { print($0 * 2) }

They both do the same thing. The only difference is .map also returns an array of voids as well, [(),(),(),()], which gets unused. Does that mean .map performs worse than .forEach when it's not assigning to anything?

Answer 1

In Swift as per Apple's definition,

map is used for returning an array containing the results of mapping the given closure over the sequence’s elements

whereas,  

forEach calls the given closure on each element in the sequence in the same order as a for-in loop.

Both got two different purposes in Swift. Even though in your example map works fine, it doesn't mean that you should be using map in this case.

map eg:-

let values = [1.0, 2.0, 3.0, 4.0]
let squares = values.map {$0 * $0}

[1.0, 4.0, 9.0, 16.0] //squares has this array now, use it somewhere

forEach eg:-

let values = [1.0, 2.0, 3.0, 4.0]
values.forEach() { print($0 * 2) }

prints below numbers. There are no arrays returned this time.

2.0

4.0

6.0

8.0

In short to answer your questions, yes the array generated from map is wasted and hence forEach is what you should use in this case.

Update:

OP has commented that when he tested, the performance was better for map compared to forEach. Here is what I tried in a playground and found. For me forEach performed better than map as shown in image. forEach took 51.31 seconds where as map took 51.59 seconds which is 0.28 seconds difference. I don't claim that forEach is better based on this, but both has similar performance attributes and which one to use, depends on the particular use case.