Variables and Data Types

By the end of this lesson you'll store text, numbers, and true/false values in Swift, choose let or var correctly, and handle missing values safely with optionals — the bedrock of every Swift program.

Learn Variables and Data Types in our free Swift course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick…

Part of the free Swift course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

Text always uses "double quotes" . A ? after any type (like Int? ) makes it optional — it can also hold nil .

1️⃣ var vs let — Mutable or Constant

Every value you name is either a variable or a constant . Use var when the value will change, and let when it won't. The golden rule in Swift is prefer let : it documents that the value is fixed, prevents accidental changes, and helps the compiler optimise. Read this worked example, run it, then you'll write your own.

2️⃣ Types: Inference vs Annotations

Swift is statically typed — every value has one fixed type the compiler knows in advance. Usually you don't write the type: type inference reads the value and picks it for you. When there's no starting value, or you want a specific type, add an explicit annotation with : Type . Remember that Int and Double never mix automatically — convert on purpose with Double(...) or Int(...) .

Your turn. The program below is almost complete — fill in the three blanks marked ___ using the hints, then run it and check the output.

3️⃣ String Interpolation

String interpolation lets you drop a value straight into text using \\(...) — the backslash, then your value in parentheses. It reads far better than joining pieces with + , and you can even run a small calculation inside the parentheses.

4️⃣ Optionals — Handling Missing Values

Optionals are Swift's standout safety feature. An optional type (written with ? , e.g. Int? ) holds either a value or nil — Swift's word for "nothing here". Because "might be missing" is part of the type, the compiler forces you to deal with the empty case, which eliminates the null-crash bugs that plague other languages. You must unwrap an optional before using it: the safe ways are optional binding ( if let ) and nil-coalescing ( ?? ). Force-unwrapping with ! skips the check and crashes on nil , so use it rarely.

Now you try. Fill in the two blanks to handle values that might be missing — one with ?? , one with if let .

📋 Quick Reference

No blanks this time — just a brief and an outline. Build it, run it, and check your output against the example in the comments. This is exactly the kind of small program real apps are made of.

Practice quiz

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