In Swift, mutability<\/strong> = \u201ccan this thing be changed after I create it?\u201d<\/p>\n Swift gives you two keywords<\/strong> to control mutability:<\/p>\n Very important mindset shift<\/strong> (this is what separates beginners from good Swift developers):<\/p>\n In modern Swift, the default recommendation<\/strong> is: Use let by default \u2014 everywhere.<\/strong> Use var only when you are sure the value will actually need to change.<\/strong><\/p>\n<\/blockquote>\n This is not<\/strong> just a style rule \u2014 it is one of the strongest safety features of Swift.<\/p>\n Ask yourself this question every single time you declare something:<\/strong><\/p>\n \u201cWill this value ever need to be completely replaced<\/strong> with a different value later in this scope?\u201d<\/p>\n<\/blockquote>\n This single question catches ~80\u201390% of cases correctly.<\/p>\n \u2192 Identity<\/strong> fields \u2192 let \u2192 Editable<\/strong> fields \u2192 var<\/p>\n \u2192 Fixed constants \u2192 let \u2192 User preferences \u2192 var<\/p>\n \u2192 Identity & creation data \u2192 let \u2192 State that changes during gameplay \u2192 var<\/p>\n Golden habit<\/strong>: Inside functions, default to let<\/strong> for every local variable unless you actually reassign it.<\/p>\n When you write:<\/p>\n Ask yourself:<\/p>\n For each line, decide let or var and explain why:<\/p>\n Expected answers<\/strong> (and reasoning):<\/p>\n Paste your decisions here if you want feedback!<\/p>\n What would you like to explore next?<\/p>\n Just tell me \u2014 we\u2019ll continue in the same clear, detailed, teacher-like style \ud83d\ude0a<\/p>\n","protected":false},"excerpt":{"rendered":" 1. The Core Idea \u2013 What \u201cmutability\u201d really means In Swift, mutability = \u201ccan this thing be changed after I create it?\u201d Swift gives you two keywords to control mutability: let = constant \u2192...<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[76],"tags":[],"class_list":["post-2724","post","type-post","status-publish","format-standard","hentry","category-swift"],"_links":{"self":[{"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/posts\/2724","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/comments?post=2724"}],"version-history":[{"count":1,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/posts\/2724\/revisions"}],"predecessor-version":[{"id":2725,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/posts\/2724\/revisions\/2725"}],"wp:attachment":[{"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/media?parent=2724"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/categories?post=2724"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/tags?post=2724"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}\n
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2. The simplest examples \u2013 feel the difference<\/h3>\n
\/\/ let = constant = cannot be reassigned\r\nlet name = \"Priya\"\r\nname = \"Sneha\" \/\/ \u274c Compile error: Cannot assign to value: 'name' is a 'let' constant\r\n\r\n\/\/ var = variable = can be reassigned\r\nvar score = 100\r\nscore = 150 \/\/ \u2705 OK\r\nscore += 50 \/\/ \u2705 OK, now 200\r\nscore = 0 \/\/ \u2705 OK<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\nlet temperature = 32.5\r\n\/\/ temperature = 33.0 \/\/ \u274c cannot change\r\n\r\nvar currentTemperature = 32.5\r\ncurrentTemperature = 33.2 \/\/ \u2705 OK<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n3. The most important rule used by experienced developers<\/h3>\n
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4. Real-life examples \u2013 how good developers actually choose let vs var<\/h3>\n
Example 1 \u2013 User profile (very common)<\/h4>\n
struct User {\r\n let id: UUID \/\/ never changes\r\n let createdAt: Date \/\/ never changes\r\n let email: String \/\/ usually never changes\r\n var displayName: String \/\/ user can change it\r\n var bio: String? \/\/ user can change it\r\n var profilePhotoURL: URL? \/\/ user can change it\r\n var isPremium: Bool \/\/ can change over time\r\n}<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\nExample 2 \u2013 Settings screen<\/h4>\n
struct AppSettings {\r\n let appVersion = \"2.3.1\" \/\/ never changes in this run\r\n let apiBaseURL = URL(string: \"https:\/\/api.example.com\/v2\")!\r\n \r\n var theme: Theme = .system \/\/ user can change\r\n var fontSize: CGFloat = 16 \/\/ user can change\r\n var notificationsEnabled = true \/\/ user can change\r\n}<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\nExample 3 \u2013 Game state (very typical)<\/h4>\n
class GameSession {\r\n let playerID: UUID \/\/ fixed\r\n let startTime: Date \/\/ fixed\r\n \r\n var currentScore = 0 \/\/ changes a lot\r\n var livesRemaining = 3 \/\/ decreases\r\n var level = 1 \/\/ increases\r\n var isGameOver = false \/\/ changes once\r\n}<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\nExample 4 \u2013 Function parameters & local variables<\/h4>\n
func calculateTotalPrice(items: [CartItem], taxRate: Double) -> Double {\r\n let subtotal = items.reduce(0.0) { $0 + $1.price * Double($1.quantity) }\r\n \r\n \/\/ subtotal is never reassigned \u2192 let\r\n let taxAmount = subtotal * taxRate\r\n \r\n \/\/ final total is never reassigned \u2192 let\r\n let finalTotal = subtotal + taxAmount\r\n \r\n return finalTotal\r\n}<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n5. Very Common Beginner Mistakes & Correct Habits<\/h3>\n
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\n \nMistake<\/th>\n Wrong \/ Risky code<\/th>\n Correct \/ Better habit<\/th>\n Why?<\/th>\n<\/tr>\n<\/thead>\n \n Using var everywhere \u201cjust in case\u201d<\/td>\n var name = “Priya”; var age = 24;<\/td>\n Default to let unless you really reassign<\/td>\n let prevents accidental changes + better safety<\/td>\n<\/tr>\n \n Reassigning constants by mistake<\/td>\n let maxUsers = 100; maxUsers = 200<\/td>\n Compiler error \u2014 good!<\/td>\n let protects you from bugs<\/td>\n<\/tr>\n \n Using var for values that never change<\/td>\n var pi = 3.14159<\/td>\n let pi = 3.14159<\/td>\n False expectation \u2014 looks like it may change<\/td>\n<\/tr>\n \n Declaring loop counters as let<\/td>\n for let i in 0..<10 { i += 1 }<\/td>\n for i in 0..<10 { \u2026 } (implicitly mutable)<\/td>\n Loop variable is mutable by default<\/td>\n<\/tr>\n \n Making struct properties let when they should be editable<\/td>\n struct User { let name: String }<\/td>\n var name if user can change it<\/td>\n Immutability is good, but not when it blocks real use<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n 6. Quick Summary \u2013 Decision Checklist (use this every time)<\/h3>\n
let\/var something = \u2026<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n
7. Small Practice \u2013 Choose let or var<\/h3>\n
____ maximumUsers = 1000\r\n____ currentScore = 0\r\n____ apiKey = \"xyz123\"\r\n____ temperature = 32.5\r\n____ playerName = \"Aarav\"\r\n____ playerName = getNewNameFromServer()\r\n____ isDarkMode = traitCollection.userInterfaceStyle == .dark\r\n____ counter = 0; counter += 1 (in loop)<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\nlet maximumUsers = 1000 \/\/ fixed limit, never changes\r\nvar currentScore = 0 \/\/ definitely increases\r\nlet apiKey = \"xyz123\" \/\/ fixed secret\r\nvar temperature = 32.5 \/\/ can change over time\r\nlet playerName = \"Aarav\" \/\/ fixed at creation\r\nvar playerName = getNewNameFromServer() \/\/ value changes\r\nlet isDarkMode = traitCollection.userInterfaceStyle == .dark \/\/ computed once, not reassigned\r\nvar counter = 0 \/\/ counter must increase<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n