Collection mutability tied to let/var in Swift

Introduction

In Swift, whether you can change a collection depends on how you declare it. This helps keep your data safe and clear.

When you want a list of items that should never change after creation.

When you need to update or add items to a list during your program.

When you want to prevent accidental changes to your data.

When you want to clearly show which collections are fixed and which can change.

Syntax

Swift

let collectionName = [item1, item2, item3]
var collectionName = [item1, item2, item3]

let means the collection cannot be changed (immutable).

var means the collection can be changed (mutable).

Examples

This collection is declared with let, so you cannot add or remove items.

Swift

let fruits = ["Apple", "Banana", "Cherry"]
// fruits.append("Date") // Error: Cannot modify because fruits is a let constant

This collection is declared with var, so you can add new items.

Swift

var fruits = ["Apple", "Banana", "Cherry"]
fruits.append("Date")
print(fruits)

Sample Program

We create a mutable array colors and add "Blue" to it. Then we create an immutable array numbers and print it. Trying to add to numbers would cause an error.

Swift

var colors = ["Red", "Green"]
colors.append("Blue")
print(colors)

let numbers = [1, 2, 3]
// numbers.append(4) // This line would cause an error
print(numbers)

Important Notes

Trying to change a collection declared with let will cause a compile-time error.

You can change the contents of a collection declared with var anytime.

This rule helps prevent bugs by making your intentions clear in code.

Summary

Use let to make collections that cannot change.

Use var to make collections that you want to change.

This helps keep your data safe and your code easy to understand.