{"id":2598,"date":"2026-02-05T07:13:35","date_gmt":"2026-02-05T07:13:35","guid":{"rendered":"https:\/\/demo.materiamedica.net\/demo6\/?p=2598"},"modified":"2026-02-05T07:13:35","modified_gmt":"2026-02-05T07:13:35","slug":"chapter-9-swift-variables","status":"publish","type":"post","link":"https:\/\/demo.materiamedica.net\/demo6\/chapter-9-swift-variables\/","title":{"rendered":"Chapter 9: Swift Variables"},"content":{"rendered":"

1. What is a variable in Swift? (the big picture)<\/h3>\n

A variable is a named storage location<\/strong> in memory that holds a value.<\/p>\n

In Swift you have two kinds<\/strong> of these storage locations:<\/p>\n

\n
\n\n\n\n\n\n\n
Keyword<\/th>\nMeaning<\/th>\nCan you change the value later?<\/th>\nReal-life analogy<\/th>\n<\/tr>\n<\/thead>\n
let<\/td>\nconstant \u2013 immutable<\/td>\nNo<\/strong><\/td>\nYour birth date, your Aadhaar number<\/td>\n<\/tr>\n
var<\/td>\nvariable \u2013 mutable<\/td>\nYes<\/strong><\/td>\nYour bank balance, current temperature<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
<\/div>\n<\/div>\n<\/div>\n

Golden rule that almost every good Swift developer follows (2025 style):<\/strong><\/p>\n

\n

Use let<\/strong> by default Use var<\/strong> only<\/strong> when you are sure<\/strong> the value will actually change<\/p>\n<\/blockquote>\n

This is probably the most important habit<\/strong> to develop early.<\/p>\n

2. Basic syntax \u2013 declaring variables<\/h3>\n
\n
\n
\n
Swift<\/div>\n
\n
\/\/ Constant (preferred in most cases)\r\nlet pi = 3.14159\r\nlet maximumUsers = 1000\r\nlet appName = \"MyTodoApp\"\r\n\r\n\/\/ Variable (only when it will change)\r\nvar score = 0\r\nvar currentTemperature = 28.6\r\nvar username = \"guest123\"<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
After declaration:<\/p>\n
\n
\n
\n
Swift<\/div>\n
\n
\/\/ This is allowed\r\nscore = 10\r\nscore += 50\r\ncurrentTemperature = 29.2\r\nusername = \"player_456\"\r\n\r\n\/\/ This will NOT compile \u2013 good!\r\npi = 3.14           \/\/ Error: Cannot assign to value: 'pi' is a 'let' constant<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
3. Type inference \u2013 Swift usually knows the type<\/h3>\n
Swift is very smart \u2014 you rarely<\/strong> need to write the type.<\/p>\n
\n
\n
\n
Swift<\/div>\n
\n
let age = 25                    \/\/ Swift knows \u2192 Int\r\nlet height = 1.68               \/\/ \u2192 Double\r\nlet isStudent = true            \/\/ \u2192 Bool\r\nlet name = \"Aarav\"              \/\/ \u2192 String\r\nlet emoji = \"\ud83d\udd25\"                 \/\/ \u2192 String (not Character!)<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
But you can<\/strong> write the type when it makes code clearer:<\/p>\n
\n
\n
\n
Swift<\/div>\n
\n
let maxFileSize: Int64 = 5_000_000_000\r\nlet taxRate: Double = 0.18\r\nlet userID: UUID = UUID()\r\nlet status: HTTPStatus = .ok<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
When should you write the type explicitly?<\/strong><\/p>\n
    \n
  • Working with very specific number types (Int32, UInt64, Float, etc.)<\/li>\n
  • When the inferred type might be surprising<\/li>\n
  • In function parameters \/ return types (very common)<\/li>\n
  • When reading code is easier with explicit types<\/li>\n<\/ul>\n
    4. Side-by-side comparison with other languages<\/h3>\n
    \n
    \n\n\n\n\n\n\n\n\n
    Language<\/th>\nConstant style<\/th>\nVariable style<\/th>\nSwift style (modern)<\/th>\n<\/tr>\n<\/thead>\n
    C \/ C++<\/td>\nconst int max = 100;<\/td>\nint count = 0;<\/td>\nlet max = 100 \/ var count = 0<\/td>\n<\/tr>\n
    Java<\/td>\nfinal int MAX = 100;<\/td>\nint count = 0;<\/td>\nlet \/ var<\/td>\n<\/tr>\n
    Python<\/td>\n(no real constant)<\/td>\ncount = 0<\/td>\nlet \/ var<\/td>\n<\/tr>\n
    Swift<\/td>\nlet max = 100<\/td>\nvar count = 0<\/td>\nprefer let<\/strong><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    <\/div>\n<\/div>\n<\/div>\n
    Swift encourages immutability<\/strong> much more strongly than most languages.<\/p>\n
    5. Very common beginner mistakes (and correct way)<\/h3>\n
    \n
    \n\n\n\n\n\n\n\n\n\n
    Wrong \/ bad habit<\/th>\nWhy it’s bad<\/th>\nBetter way<\/th>\n<\/tr>\n<\/thead>\n
    var name = “Rahul” when name never changes<\/td>\nCreates false expectation that it will change<\/td>\nlet name = “Rahul”<\/td>\n<\/tr>\n
    var age: Int = 25 everywhere<\/td>\nUnnecessary noise<\/td>\nlet age = 25 (type inference)<\/td>\n<\/tr>\n
    var counter = 0 and then never changing it<\/td>\nMisleading<\/td>\nlet initialCount = 0<\/td>\n<\/tr>\n
    Force-unwrapping optionals inside variables<\/td>\nvar name = optionalName! \u2192 crashes easily<\/td>\nvar name = optionalName ?? “Unknown”<\/td>\n<\/tr>\n
    Declaring everything as var “just in case”<\/td>\nLoses safety & clarity<\/td>\nStart with let, change to var only if needed<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    <\/div>\n<\/div>\n<\/div>\n
    6. Practical real-world examples<\/h3>\n
    Example 1 \u2013 Game score<\/h4>\n
    \n
    \n
    \n
    Swift<\/div>\n
    \n
    let maxLives = 3                \/\/ never changes\r\nvar currentLives = maxLives     \/\/ changes during game\r\nvar score = 0                   \/\/ changes a lot\r\n\r\n\/\/ later in code...\r\nscore += 100\r\ncurrentLives -= 1\r\n\r\nif currentLives == 0 {\r\n    print(\"Game Over! Final score: \\(score)\")\r\n}<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
    Example 2 \u2013 User profile<\/h4>\n
    \n
    \n
    \n
    Swift<\/div>\n
    \n
    let userID = UUID()                 \/\/ never changes\r\nlet registeredOn = Date()           \/\/ fixed\r\n\r\nvar displayName = \"guest_123\"\r\nvar profilePictureURL: URL?         \/\/ can be nil or change\r\nvar isPremium = false\r\n\r\n\/\/ later...\r\ndisplayName = \"Aarav_007\"\r\nisPremium = true<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
    Example 3 \u2013 Temperature converter (shows let vs var clearly)<\/h4>\n
    \n
    \n
    \n
    Swift<\/div>\n
    \n
    func convertToCelsius(fahrenheit: Double) -> Double {\r\n    let adjusted = fahrenheit - 32              \/\/ never changes inside function\r\n    let multiplier = 5.0 \/ 9.0                  \/\/ constant\r\n    return adjusted * multiplier\r\n}\r\n\r\nvar currentTempF = 98.6\r\nvar currentTempC = convertToCelsius(fahrenheit: currentTempF)\r\n\r\n\/\/ later...\r\ncurrentTempF = 104.0\r\ncurrentTempC = convertToCelsius(fahrenheit: currentTempF)<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
    7. Naming conventions \u2013 what real Swift developers do<\/h3>\n
    \n
    \n\n\n\n\n\n\n\n\n
    Kind of name<\/th>\nConvention<\/th>\nExample<\/th>\n<\/tr>\n<\/thead>\n
    Constants (let)<\/td>\ncamelCase or UPPER_CASE for global<\/td>\nmaxRetryCount, API_KEY<\/td>\n<\/tr>\n
    Variables (var)<\/td>\ncamelCase<\/td>\ncurrentUser, totalPrice<\/td>\n<\/tr>\n
    Private properties<\/td>\nOften private var<\/td>\nprivate var internalCache<\/td>\n<\/tr>\n
    Global constants<\/td>\nUPPER_CASE_WITH_UNDERSCORES (rare)<\/td>\nMAX_UPLOAD_SIZE_BYTES<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    <\/div>\n<\/div>\n<\/div>\n
    Very common pattern in modern SwiftUI \/ UIKit:<\/strong><\/p>\n
    \n
    \n
    \n
    Swift<\/div>\n
    \n
    private let cellReuseIdentifier = \"UserCell\"\r\nprivate var users: [User] = []<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
    8. Quick cheat sheet \u2013 when to choose let vs var<\/h3>\n
    \n
    \n\n\n\n\n\n\n\n\n\n\n
    Situation<\/th>\nUse let or var?<\/th>\nWhy?<\/th>\n<\/tr>\n<\/thead>\n
    Value will never change<\/td>\nlet<\/td>\nSafety + clarity<\/td>\n<\/tr>\n
    Value changes in one small scope<\/td>\nvar<\/td>\nNormal<\/td>\n<\/tr>\n
    Loop variable (for-in)<\/td>\nlet (default)<\/td>\nYou rarely change it<\/td>\n<\/tr>\n
    Property that gets set once (like in init)<\/td>\nlet<\/td>\nExpresses intent<\/td>\n<\/tr>\n
    Counter, score, position, status<\/td>\nvar<\/td>\nObviously changes<\/td>\n<\/tr>\n
    Model \/ struct property that should not change<\/td>\nlet<\/td>\nMakes data safer<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
    <\/div>\n<\/div>\n<\/div>\n
    9. Mini practice \u2013 try to fix these<\/h3>\n
    \n
    \n
    \n
    Swift<\/div>\n
    \n
    \/\/ Try to improve these declarations\r\n\r\nvar firstName = \"Sneha\"             \/\/ never changes\r\nvar age = 24                        \/\/ never changes later\r\nvar totalPrice = 1499.99            \/\/ calculated once\r\nvar counter = 0                     \/\/ used in for loop but never reassigned<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
    Expected better versions:<\/strong><\/p>\n
    \n
    \n
    \n
    Swift<\/div>\n
    \n
    let firstName = \"Sneha\"\r\nlet age = 24\r\nlet totalPrice = 1499.99\r\n\/\/ counter \u2192 usually let inside for-in<\/code><\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
    Would you like to continue with:<\/p>\n
      \n
    • Optionals<\/strong> (String?, Int?, etc.) \u2013 very important next step<\/li>\n
    • Computed properties<\/strong> (variables that calculate their value)<\/li>\n
    • Variables inside functions vs properties in structs\/classes<\/strong><\/li>\n
    • Type aliases<\/strong> (typealias UserID = Int64)<\/li>\n
    • Or move to another topic (constants in detail, printing variables, etc.)<\/li>\n<\/ul>\n
      Just tell me what you want next \u2014 I’ll continue in the same detailed, teacher-like way \ud83d\ude0a<\/p>\n","protected":false},"excerpt":{"rendered":"
      1. What is a variable in Swift? (the big picture) A variable is a named storage location in memory that holds a value. In Swift you have two kinds of these storage locations: Keyword...<\/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-2598","post","type-post","status-publish","format-standard","hentry","category-swift"],"_links":{"self":[{"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/posts\/2598","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=2598"}],"version-history":[{"count":1,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/posts\/2598\/revisions"}],"predecessor-version":[{"id":2599,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/posts\/2598\/revisions\/2599"}],"wp:attachment":[{"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/media?parent=2598"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/categories?post=2598"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/demo.materiamedica.net\/demo6\/wp-json\/wp\/v2\/tags?post=2598"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}