NEW: Master Swift design patterns with my latest book! >>

What is a lazy sequence?

Written by Paul Hudson    @twostraws

Lazy sequences are regular sequences where each item is computed on demand rather than up front. For example, consider this array of numbers:

let numbers = Array(1...100000)

That will hold 100,000 numbers. Now, if we wanted to double all those numbers, we’d write something like this:

let doubled = numbers.map { $0 * 2 }

That will cause Swift to double all 100,000 numbers, and sometimes that’s exactly what you want. However, if you know you intend to use only a handful of them, you can make the calculation lazy instead, like this:

let doubled = numbers.lazy.map { $0 * 2 }

Now that map() call won’t do any work up front – it just stores the original array (numbers 1 to 100,000) alongside the transformation closure (double each number). So, when you request item 5,000 it can calculate just that one for you and return it in a split second – a significant time saving.

Available from iOS 8.0 – learn more in my book Pro Swift

Did this solution work for you? Please pass it on!

Other people are reading…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Go from iOS to macOS the easy way!

If you like Hacking with Swift, you'll love Hacking with macOS – learn to build macOS apps today, using 18 real-world projects!

Click here to visit the Hacking with Swift store >>