What is a lazy sequence?
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 lazyDoubled = 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…
- What is function composition?
- How to fix the error “protocol can only be used as a generic constraint because it has Self or associated type requirements”
- How to compare dates
- How to use reduce() to condense an array into a single value
- What is trailing closure syntax?
About the Swift Knowledge Base
This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.
