Rust — If Else

The basics — familiar but stricter

let temperature = 31;

if temperature > 28 {
    println!("It's hot");
} else if temperature > 18 {
    println!("Pleasant");
} else {
    println!("Bring a jacket");
}

Two hard rules:

1. The condition must be a real bool — no truthy integers/strings
2. Braces are mandatory, even for single statements

if temperature > 28
    println!("hot");     // ❌ compile error — braces required

if IS an expression — the big idea

In most languages if is a statement. In Rust it produces a value:

let number = 7;

let label = if number % 2 == 0 { "even" } else { "odd" };
println!("{}", label);      // "odd"

No ternary operator exists because if already works as one.

Rules when capturing values:

• Every branch must yield the same type
• Branch bodies end WITHOUT semicolons (so the expression is the branch's value)
• Missing else + assigning = error (what would the value be otherwise?)

// ❌ type mismatch — i32 vs ()
let x = if flag { 5 };           // no else!

// ✓ both branches produce i32:
let x = if flag { 5 } else { 0 };

let … if inside larger expressions

let price = 100;
let discount_rate = if premium { 0.2 } else { 0.05 };

let final_price = price * (1.0 - discount_rate);

Combining conditions

if age >= 13 && age <= 19 { … }        // range check
if day == "Sat" || day == "Sun" { … }
if !(name.is_empty()) { … }

// Rust's elegant inclusive range syntax for matching comes later;
// for plain conditions, explicit && stays idiomatic.

Nested vs guard-style

// nested pyramid:
if user.exists() {
    if user.active {
        grant();
    }
}

// flatter with early structure in functions:
if !user.exists() || !user.active { return; }
grant();

Same logic; the second reads top-down and scales better.

Gotchas: = instead of == won't even compile (no silent assignment) · conditions need bools, so if count errors helpfully · forgetting that unbraced single lines are illegal.

Mini Practice

1. Grade classifier returning letter via if-expression.
2. FizzBuzz for 1–15 using % and branches.
3. Convert three nested ifs into guard-style returns.
4. Capture a max of two numbers: let max = if a > b { a } else { b };
5. Trigger each of the three gotchas once, on purpose.

Next: match →