Lazy collections for performance in Swift

Introduction

Lazy collections help your program run faster by doing work only when needed. They save time and memory by not processing everything at once.

When you have a big list but only need some results.

When you want to save memory by not creating big temporary lists.

When you want to chain many operations without slowing down your app.

When you want to improve performance in loops or filters.

When you want to delay work until the last moment.

Syntax

Swift

let lazyCollection = collection.lazy

// Example: let lazyNumbers = numbers.lazy.map { $0 * 2 }

Use .lazy on any collection to make it lazy.

Lazy collections only do work when you ask for the results.

Examples

This creates a lazy map that doubles numbers but does not run it yet.

Swift

let numbers = [1, 2, 3, 4, 5]
let lazyNumbers = numbers.lazy.map { $0 * 2 }
print(type(of: lazyNumbers))

This filters even numbers lazily and prints them one by one.

Swift

let numbers = [1, 2, 3, 4, 5]
let lazyFiltered = numbers.lazy.filter { $0 % 2 == 0 }
for num in lazyFiltered {
    print(num)
}

This multiplies numbers by 3 lazily and takes only the first 3 results.

Swift

let numbers = [1, 2, 3, 4, 5]
let firstThree = numbers.lazy.map { $0 * 3 }.prefix(3)
print(Array(firstThree))

Sample Program

This program shows how lazy collections avoid creating big arrays right away. It prints the count of doubled numbers and then prints the first five doubled numbers using lazy evaluation.

Swift

let numbers = Array(1...1_000_000)

// Without lazy: creates a big array immediately
let doubled = numbers.map { $0 * 2 }
print("Doubled count: \(doubled.count)")

// With lazy: does not create big array until needed
let lazyDoubled = numbers.lazy.map { $0 * 2 }

// Take first 5 doubled numbers
let firstFive = lazyDoubled.prefix(5)
print("First five doubled: \(Array(firstFive))")

Important Notes

Lazy collections are great for big data but might be slower for small data because of overhead.

Use Array() to convert lazy results back to a normal array when needed.

Lazy collections work well with chaining multiple operations like map, filter, and prefix.

Summary

Lazy collections delay work until results are needed.

They improve performance and save memory for big data.

Use .lazy on collections to make them lazy.