Control Flow

Q: Can for loop over a reference?

Yes. for x in &vec borrows elements. for x in &mut vec mutably borrows.

Q: How do I loop with an index?

for (i, v) in items.iter().enumerate() is idiomatic. Raw 0..len works when neighbors are needed.

Q: What does break with a value do?

In loop, break 42 makes the loop expression evaluate to 42.

Q: Can while return a value?

No. Only loop supports break value. Use loop or assign inside while.

Q: How do guards work in match?

match x { n if n > 0 => ..., _ => ... } adds a boolean condition to a pattern.

Q: Is switch available?

No. Use match for exhaustive pattern matching.

Q: Can I match on ranges?

Yes for integers and chars: 1..=5, 'a'..='z'.

Q: What is loop else?

Not in Rust. Use break flags or Option return values instead.

Q: How do I iterate a HashMap?

for (k, v) in &map or map.iter(). Order is arbitrary unless you use BTreeMap.

Rust provides if/else, three loop forms (loop, while, for), and exhaustive match. Loops and branches can produce values when used as expressions.

Recipe

fn sum_evens(nums: &[i32]) -> i32 {
    let mut total = 0;
    for &n in nums {
        if n % 2 == 0 {
            total += n;
        }
    }
    total
}
 
fn first_negative(nums: &[i32]) -> Option<i32> {
    for &n in nums {
        if n < 0 {
            return Some(n);
        }
    }
    None
}

When to reach for this: Any branching or iteration. Prefer for over while with manual index when iterating collections.

Working Example

fn find_pair(target: i32, nums: &[i32]) -> Option<(usize, usize)> {
    'outer: for i in 0..nums.len() {
        for j in (i + 1)..nums.len() {
            if nums[i] + nums[j] == target {
                break 'outer (i, j);
            }
        }
    }
}
 
fn retry<F: FnMut() -> bool>(mut f: F, max: u32) -> bool {
    let mut attempts = 0;
    loop {
        if f() {
            break true;
        }
        attempts += 1;
        if attempts >= max {
            break false;
        }
    }
}
 
fn main() {
    let nums = [2, 7, 11, 15];
    println!("{:?}", find_pair(9, &nums));
    println!("{}", retry(|| false, 3));
}

What this demonstrates:

Labeled break 'outer exits nested loops with a tuple value
loop with conditional break implements retry logic
Range 0..nums.len() for index-based access when needed
Option for absent results instead of sentinel values

Deep Dive

Loop Forms

Construct	Use
`for x in iter`	Iterate collections, ranges, adapters
`while cond`	Loop until condition is false
`loop`	Infinite loop until `break` (can return value)

let squares: Vec<i32> = (0..5).map(|n| n * n).collect();

`match` Exhaustiveness

enum Color { Red, Green, Blue }
 
fn hex(c: Color) -> &'static str {
    match c {
        Color::Red => "#f00",
        Color::Green => "#0f0",
        Color::Blue => "#00f",
    }
}

Compiler errors if a variant is missing. Use _ only when you intentionally catch remaining cases.

`if let` and `while let`

let Some(x) = opt else { return };
while let Some(line) = lines.next() {
    println!("{line}");
}

Ergonomic partial matching without full match.

Gotchas

if condition not bool - if 1 { } fails. Fix: Use explicit comparisons.
Off-by-one ranges - 0..n excludes n. Fix: Use 0..=n for inclusive end when needed.
Borrowing in for loops - for item in &vec borrows; for item in vec moves. Fix: Choose iter(), iter_mut(), or into_iter() deliberately.
Non-exhaustive match - Adding enum variants breaks compiles. Fix: Treat as a feature; update all matches.
break value type mismatch - All break in a loop must return the same type. Fix: Annotate or unify types.

Alternatives

Alternative	Use When	Don't Use When
Iterator adapters	Transform/filter without explicit loop	Side effects need sequential clarity
`if let` chain	Single-pattern checks	Many variants need `match`
`matches!` macro	Boolean predicate on pattern	You need bound values
Recursion	Tree/graph structure	Deep stacks risk overflow

FAQs

Can `for` loop over a reference?

Yes. for x in &vec borrows elements. for x in &mut vec mutably borrows.

How do I loop with an index?

for (i, v) in items.iter().enumerate() is idiomatic. Raw 0..len works when neighbors are needed.

What does `break` with a value do?

In loop, break 42 makes the loop expression evaluate to 42.

Can `while` return a value?

No. Only loop supports break value. Use loop or assign inside while.

What is `continue 'label`?

Skips to the next iteration of the labeled loop, like labeled break.

How do guards work in `match`?

match x { n if n > 0 => ..., _ => ... } adds a boolean condition to a pattern.

Is `switch` available?

No. Use match for exhaustive pattern matching.

Can I match on ranges?

Yes for integers and chars: 1..=5, 'a'..='z'.

What is loop `else`?

Not in Rust. Use break flags or Option return values instead.

How do I iterate a `HashMap`?

for (k, v) in &map or map.iter(). Order is arbitrary unless you use BTreeMap.

Functions & Expressions - expression-oriented blocks
Pattern Matching with match - deep match patterns
if let, let else & while let - partial matches
Iterator Adapters - loop-free transforms

The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide (ownership, borrowing, traits, etc.).
Rust By Example - Runnable examples covering language features and standard library.
Rust Playground - Online REPL for quick experiments.
The Rust Programming Language, 3rd Edition (O'Reilly) - Klabnik, Nichols, Krycho.
Rust in Action (Amazon) - Tim McNamara on Amazon.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Recipe

fn sum_evens(nums: &[i32]) -> i32 {
    let mut total = 0;
    for &n in nums {
        if n % 2 == 0 {
            total += n;
        }
    }
    total
}
 
fn first_negative(nums: &[i32]) -> Option<i32> {
    for &n in nums {
        if n < 0 {
            return Some(n);
        }
    }
    None
}

When to reach for this: Any branching or iteration. Prefer for over while with manual index when iterating collections.

Working Example

fn find_pair(target: i32, nums: &[i32]) -> Option<(usize, usize)> {
    'outer: for i in 0..nums.len() {
        for j in (i + 1)..nums.len() {
            if nums[i] + nums[j] == target {
                break 'outer (i, j);
            }
        }
    }
}
 
fn retry<F: FnMut() -> bool>(mut f: F, max: u32) -> bool {
    let mut attempts = 0;
    loop {
        if f() {
            break true;
        }
        attempts += 1;
        if attempts >= max {
            break false;
        }
    }
}
 
fn main() {
    let nums = [2, 7, 11, 15];
    println!("{:?}", find_pair(9, &nums));
    println!("{}", retry(|| false, 3));
}

What this demonstrates:

Labeled break 'outer exits nested loops with a tuple value
loop with conditional break implements retry logic
Range 0..nums.len() for index-based access when needed
Option for absent results instead of sentinel values

Deep Dive

Loop Forms

Construct	Use
`for x in iter`	Iterate collections, ranges, adapters
`while cond`	Loop until condition is false
`loop`	Infinite loop until `break` (can return value)

let squares: Vec<i32> = (0..5).map(|n| n * n).collect();

`match` Exhaustiveness

enum Color { Red, Green, Blue }
 
fn hex(c: Color) -> &'static str {
    match c {
        Color::Red => "#f00",
        Color::Green => "#0f0",
        Color::Blue => "#00f",
    }
}

Compiler errors if a variant is missing. Use _ only when you intentionally catch remaining cases.

`if let` and `while let`

let Some(x) = opt else { return };
while let Some(line) = lines.next() {
    println!("{line}");
}

Ergonomic partial matching without full match.

Gotchas

if condition not bool - if 1 { } fails. Fix: Use explicit comparisons.
Off-by-one ranges - 0..n excludes n. Fix: Use 0..=n for inclusive end when needed.
Borrowing in for loops - for item in &vec borrows; for item in vec moves. Fix: Choose iter(), iter_mut(), or into_iter() deliberately.
Non-exhaustive match - Adding enum variants breaks compiles. Fix: Treat as a feature; update all matches.
break value type mismatch - All break in a loop must return the same type. Fix: Annotate or unify types.

Alternatives

Alternative	Use When	Don't Use When
Iterator adapters	Transform/filter without explicit loop	Side effects need sequential clarity
`if let` chain	Single-pattern checks	Many variants need `match`
`matches!` macro	Boolean predicate on pattern	You need bound values
Recursion	Tree/graph structure	Deep stacks risk overflow

FAQs

Can `for` loop over a reference?

Yes. for x in &vec borrows elements. for x in &mut vec mutably borrows.

How do I loop with an index?

for (i, v) in items.iter().enumerate() is idiomatic. Raw 0..len works when neighbors are needed.

What does `break` with a value do?

In loop, break 42 makes the loop expression evaluate to 42.

Can `while` return a value?

No. Only loop supports break value. Use loop or assign inside while.

What is `continue 'label`?

Skips to the next iteration of the labeled loop, like labeled break.

How do guards work in `match`?

match x { n if n > 0 => ..., _ => ... } adds a boolean condition to a pattern.

Is `switch` available?

No. Use match for exhaustive pattern matching.

Can I match on ranges?

Yes for integers and chars: 1..=5, 'a'..='z'.

What is loop `else`?

Not in Rust. Use break flags or Option return values instead.

How do I iterate a `HashMap`?

for (k, v) in &map or map.iter(). Order is arbitrary unless you use BTreeMap.

Functions & Expressions - expression-oriented blocks
Pattern Matching with match - deep match patterns
if let, let else & while let - partial matches
Iterator Adapters - loop-free transforms

The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide (ownership, borrowing, traits, etc.).
Rust By Example - Runnable examples covering language features and standard library.
Rust Playground - Online REPL for quick experiments.
The Rust Programming Language, 3rd Edition (O'Reilly) - Klabnik, Nichols, Krycho.
Rust in Action (Amazon) - Tim McNamara on Amazon.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Control Flow

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Control Flow