Iterators & Closures

Closures are anonymous functions you can store in variables and pass around. Iterators provide a way to process sequences of elements lazily and efficiently. Together, they enable a functional programming style that is both expressive and zero-cost in Rust.

Closures

A closure is a function without a name, defined inline with |parameters| body:

fn main() {
    let add = |a, b| a + b;
    let greet = |name: &str| format!("Hello, {name}!");

    println!("{}", add(3, 4));       // 7
    println!("{}", greet("Alice"));  // Hello, Alice!
}

Unlike regular functions, closures:

• Can capture variables from their surrounding scope
• Infer parameter and return types (explicit annotations are optional)
• Are each their own unique, anonymous type

Capturing variables

Closures can use variables from the enclosing scope:

fn main() {
    let multiplier = 3;
    let multiply = |x| x * multiplier;

    println!("{}", multiply(5));  // 15
    println!("{}", multiply(10)); // 30
}

multiply captures multiplier by reference. The closure borrows multiplier for as long as it is used.

How closures capture

Closures capture in three ways, from least to most restrictive:

Capture mode	Trait	When used
By reference	Fn	Closure only reads the captured value
By mutable ref	FnMut	Closure modifies the captured value
By value (move)	FnOnce	Closure takes ownership

Rust automatically chooses the least restrictive mode needed:

fn main() {
    let name = String::from("Alice");

    // Captures by reference (Fn) -- only reads name
    let greet = || println!("Hello, {name}!");
    greet();
    greet(); // Can call multiple times
    println!("{name}"); // name is still valid

    // Captures by mutable reference (FnMut)
    let mut count = 0;
    let mut increment = || {
        count += 1;
        count
    };
    println!("{}", increment()); // 1
    println!("{}", increment()); // 2

    // Captures by value with move
    let data = vec![1, 2, 3];
    let owns_data = move || {
        println!("{:?}", data);
    };
    owns_data();
    // println!("{:?}", data); // Error: data was moved into the closure
}

The move keyword forces a closure to take ownership of all captured variables. This is essential for threads (chapter 16) where data must be moved to another thread.

Closures as function parameters

Use the Fn traits to accept closures:

fn apply_twice<F: Fn(i32) -> i32>(f: F, value: i32) -> i32 {
    f(f(value))
}

fn main() {
    let double = |x| x * 2;
    let add_three = |x| x + 3;

    println!("{}", apply_twice(double, 5));    // 20
    println!("{}", apply_twice(add_three, 5)); // 11
}

Trait	Meaning	Calling
Fn	Can be called multiple times, no mutation	Most flexible
FnMut	Can be called multiple times, may mutate	Mutable access
FnOnce	Can be called only once (consumes captured data)	Most restrictive

The hierarchy: every Fn is also FnMut, and every FnMut is also FnOnce.

Tip: Accept Fn when possible, FnMut if the closure needs to mutate, and FnOnce if it needs to take ownership. This gives callers maximum flexibility.

Returning closures

fn make_adder(n: i32) -> impl Fn(i32) -> i32 {
    move |x| x + n
}

fn main() {
    let add_five = make_adder(5);
    println!("{}", add_five(10)); // 15
    println!("{}", add_five(20)); // 25
}

Iterators

An iterator produces a sequence of values one at a time. The Iterator trait has one required method:

trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
}

Each call to next() returns Some(value) or None when the sequence is exhausted.

Creating iterators

fn main() {
    let numbers = vec![1, 2, 3, 4, 5];

    let mut iter = numbers.iter();

    println!("{:?}", iter.next()); // Some(1)
    println!("{:?}", iter.next()); // Some(2)
    println!("{:?}", iter.next()); // Some(3)
    println!("{:?}", iter.next()); // Some(4)
    println!("{:?}", iter.next()); // Some(5)
    println!("{:?}", iter.next()); // None
}

iter vs into_iter vs iter_mut

This is a common source of confusion:

Method	Yields	Consumes collection?	Use when
.iter()	&T	No	You need to read elements
.iter_mut()	&mut T	No	You need to modify elements
.into_iter()	T	Yes	You need owned elements

fn main() {
    let numbers = vec![1, 2, 3];

    // .iter() -- borrows, collection stays valid
    for n in numbers.iter() {
        println!("{n}"); // n is &i32
    }
    println!("Still valid: {:?}", numbers);

    // .iter_mut() -- mutable borrow
    let mut numbers = vec![1, 2, 3];
    for n in numbers.iter_mut() {
        *n *= 10; // n is &mut i32
    }
    println!("Modified: {:?}", numbers);

    // .into_iter() -- takes ownership
    let numbers = vec![1, 2, 3];
    for n in numbers.into_iter() {
        println!("{n}"); // n is i32 (owned)
    }
    // numbers is no longer valid
}

Note: A for loop calls .into_iter() implicitly. Writing for n in numbers is the same as for n in numbers.into_iter(). Use for n in &numbers for .iter() and for n in &mut numbers for .iter_mut().

Iterator adaptors

Adaptors transform an iterator into another iterator. They are lazy -- they do nothing until consumed.

map -- transform each element

fn main() {
    let numbers = vec![1, 2, 3, 4, 5];

    let doubled: Vec<i32> = numbers.iter().map(|n| n * 2).collect();

    println!("{:?}", doubled); // [2, 4, 6, 8, 10]
}

filter -- keep elements matching a predicate

fn main() {
    let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    let evens: Vec<&i32> = numbers.iter().filter(|n| *n % 2 == 0).collect();

    println!("{:?}", evens); // [2, 4, 6, 8, 10]
}

Chaining adaptors

The real power is chaining multiple operations:

fn main() {
    let words = vec!["hello", "world", "foo", "bar", "rust"];

    let result: Vec<String> = words
        .iter()
        .filter(|w| w.len() > 3)
        .map(|w| w.to_uppercase())
        .collect();

    println!("{:?}", result); // ["HELLO", "WORLD", "RUST"]
}

Common adaptors

Adaptor	What it does
map(f)	Transform each element
filter(f)	Keep elements where f returns true
enumerate()	Pair each element with its index
zip(other)	Pair elements from two iterators
take(n)	Take the first n elements
skip(n)	Skip the first n elements
chain(other)	Concatenate two iterators
flatten()	Flatten nested iterators
flat_map(f)	Map then flatten
peekable()	Allow peeking at the next element
inspect(f)	Call f on each element for debugging
rev()	Reverse (requires DoubleEndedIterator)

Consuming iterators

Adaptors are lazy. Consumers drive the iteration and produce a final value.

collect -- gather into a collection

fn main() {
    let chars: Vec<char> = "hello".chars().collect();
    println!("{:?}", chars); // ['h', 'e', 'l', 'l', 'o']

    let csv = vec!["Alice", "Bob", "Charlie"];
    let joined: String = csv.into_iter().collect::<Vec<&str>>().join(", ");
    println!("{joined}"); // Alice, Bob, Charlie
}

collect can produce many types -- Vec, String, HashMap, HashSet, and more.

fold -- reduce to a single value

fn main() {
    let numbers = vec![1, 2, 3, 4, 5];

    let sum = numbers.iter().fold(0, |acc, n| acc + n);
    let product = numbers.iter().fold(1, |acc, n| acc * n);

    println!("Sum: {sum}");       // 15
    println!("Product: {product}"); // 120
}

fold takes an initial accumulator value and a closure that combines the accumulator with each element.

Other consumers

Consumer	What it does
collect()	Gather into a collection
fold(init, f)	Reduce to a single value
sum()	Sum all elements (requires Sum trait)
product()	Multiply all elements
count()	Count elements
any(f)	True if any element satisfies f
all(f)	True if all elements satisfy f
find(f)	First element matching f (returns Option)
position(f)	Index of first match (returns Option<usize>)
min() / max()	Smallest / largest element
for_each(f)	Call f on each element (like a for loop)

fn main() {
    let numbers = vec![1, 2, 3, 4, 5];

    println!("Sum: {}", numbers.iter().sum::<i32>());
    println!("Any even? {}", numbers.iter().any(|n| n % 2 == 0));
    println!("All positive? {}", numbers.iter().all(|n| *n > 0));
    println!("First even: {:?}", numbers.iter().find(|n| *n % 2 == 0));
    println!("Max: {:?}", numbers.iter().max());
}

Lazy evaluation

Adaptors do not execute until a consumer drives the iterator. This means no intermediate collections are created:

fn main() {
    let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    // Nothing happens here -- the chain is lazy
    let iter = numbers.iter().filter(|n| *n % 2 == 0).map(|n| n * 10);

    // Only now does the computation happen
    let result: Vec<i32> = iter.collect();
    println!("{:?}", result); // [20, 40, 60, 80, 100]
}

This is efficient -- Rust processes each element through the entire chain before moving to the next, avoiding intermediate Vec allocations.

A complete example

Processing a list of students:

#[derive(Debug)]
struct Student {
    name: String,
    grade: u32,
}

fn main() {
    let students = vec![
        Student { name: String::from("Alice"), grade: 92 },
        Student { name: String::from("Bob"), grade: 67 },
        Student { name: String::from("Charlie"), grade: 85 },
        Student { name: String::from("Diana"), grade: 91 },
        Student { name: String::from("Eve"), grade: 73 },
    ];

    let honor_roll: Vec<&str> = students
        .iter()
        .filter(|s| s.grade >= 85)
        .map(|s| s.name.as_str())
        .collect();

    let average: f64 = students.iter().map(|s| s.grade as f64).sum::<f64>()
        / students.len() as f64;

    let highest = students.iter().max_by_key(|s| s.grade).unwrap();

    println!("Honor roll: {:?}", honor_roll);
    println!("Average grade: {average:.1}");
    println!("Highest: {} ({})", highest.name, highest.grade);
}

Honor roll: ["Alice", "Charlie", "Diana"]
Average grade: 81.6
Highest: Alice (92)

Summary

• Closures are anonymous functions defined with |params| body
• Closures capture variables from their scope by reference, mutable reference, or ownership (move)
• The Fn, FnMut, and FnOnce traits describe how closures capture and how often they can be called
• Iterators produce sequences via the next() method
• .iter() borrows, .iter_mut() mutably borrows, .into_iter() takes ownership
• Adaptors (map, filter, take, etc.) are lazy -- they build a chain of transformations
• Consumers (collect, fold, sum, find, etc.) drive the iteration and produce results
• Chains of adaptors are zero-cost -- no intermediate collections

Next up: Project: CLI Task Manager -- putting everything together to build a complete command-line application with argument parsing, file storage, and proper error handling.