Rust array type

What you’ll learn: Rust’s core data structures — arrays, tuples, slices, strings, structs, Vec, and HashMap. This is a dense chapter; focus on understanding String vs &str and how structs work. You’ll revisit references and borrowing in depth in chapter 7.

• Arrays contain a fixed number of elements of the same type
  • Like all other Rust types, arrays are immutable by default (unless mut is used)
  • Arrays are indexed using [] and are bounds checked. The len() method can be used to obtain the length of the array

    fn get_index(y : usize) -> usize {
        y+1        
    }
    
    fn main() {
        // Initializes an array of 10 elements and sets all to 42
        let a : [u8; 3] = [42; 3];
        // Alternative syntax
        // let a = [42u8, 42u8, 42u8];
        for x in a {
            println!("{x}");
        }
        let y = get_index(a.len());
        // Commenting out the below will cause a panic
        //println!("{}", a[y]);
    }

Rust array type continued

• Arrays can be nested
  • Rust has several built-in formatters for printing. In the below, the :? is the debug print formatter. The :#? formatter can be used for pretty print. These formatters can be customized per type (more on this later)

    fn main() {
        let a = [
            [40, 0], // Define a nested array
            [41, 0],
            [42, 1],
        ];
        for x in a {
            println!("{x:?}");
        }
    }

Rust tuples

• Tuples have a fixed size and can group arbitrary types into a single compound type
  • The constituent types can be indexed by their relative location (.0, .1, .2, …). An empty tuple, i.e., () is called the unit value and is the equivalent of a void return value
  • Rust supports tuple destructuring to make it easy to bind variables to individual elements

fn get_tuple() -> (u32, bool) {
    (42, true)        
}

fn main() {
   let t : (u8, bool) = (42, true);
   let u : (u32, bool) = (43, false);
   println!("{}, {}", t.0, t.1);
   println!("{}, {}", u.0, u.1);
   let (num, flag) = get_tuple(); // Tuple destructuring
   println!("{num}, {flag}");
}

Rust references

• References in Rust are roughly equivalent to pointers in C with some key differences
  • It is legal to have any number of read-only (immutable) references to a variable at any point of time. A reference cannot outlive the variable scope (this is a key concept called lifetime; discussed in detail later)
  • Only a single writable (mutable) reference to a mutable variable is permitted and it must no overlap with any other reference.

fn main() {
    let mut a = 42;
    {
        let b = &a;
        let c = b;
        println!("{} {}", *b, *c); // The compiler automatically dereferences *c
        // Illegal because b and still are still in scope
        // let d = &mut a;
    }
    let d = &mut a; // Ok: b and c are not in scope
    *d = 43;
}

Rust slices

• Rust references can be used create subsets of arrays
  • Unlike arrays, which have a static fixed length determined at compile time, slices can be of arbitrary size. Internally, slices are implemented as a “fat-pointer” that contains the length of the slice and a pointer to the starting element in the original array

fn main() {
    let a = [40, 41, 42, 43];
    let b = &a[1..a.len()]; // A slice starting with the second element in the original
    let c = &a[1..]; // Same as the above
    let d = &a[..]; // Same as &a[0..] or &a[0..a.len()]
    println!("{b:?} {c:?} {d:?}");
}

Rust constants and statics

• The const keyword can be used to define a constant value. Constant values are evaluated at compile time and are inlined into the program
• The static keyword is used to define the equivalent of global variables in languages like C/C++ Static variables have an addressable memory location and are created once and last the entire lifetime of the program

const SECRET_OF_LIFE: u32 = 42;
static GLOBAL_VARIABLE : u32 = 2;
fn main() {
    println!("The secret of life is {}", SECRET_OF_LIFE);
    println!("Value of global variable is {GLOBAL_VARIABLE}")
}

Rust strings: String vs &str

• Rust has two string types that serve different purposes
  • String — owned, heap-allocated, growable (like C’s malloc’d buffer, or C++’s std::string)
  • &str — borrowed, lightweight reference (like C’s const char* with length, or C++’s std::string_view — but &str is lifetime-checked so it can never dangle)
  • Unlike C’s null-terminated strings, Rust strings track their length and are guaranteed valid UTF-8

For C++ developers: String ≈ std::string, &str ≈ std::string_view. Unlike std::string_view, a &str is guaranteed valid for its entire lifetime by the borrow checker.

String vs &str: Owned vs Borrowed

Production patterns: See JSON handling: nlohmann::json → serde for how string handling works with serde in production code.

fn main() {
    // &str - string slice (borrowed, immutable, usually a string literal)
    let greeting: &str = "Hello";  // Points to read-only memory

    // String - owned, heap-allocated, growable
    let mut owned = String::from(greeting);  // Copies data to heap
    owned.push_str(", World!");        // Grow the string
    owned.push('!');                   // Append a single character

    // Converting between String and &str
    let slice: &str = &owned;          // String -> &str (free, just a borrow)
    let owned2: String = slice.to_string();  // &str -> String (allocates)
    let owned3: String = String::from(slice); // Same as above

    // String concatenation (note: + consumes the left operand)
    let hello = String::from("Hello");
    let world = String::from(", World!");
    let combined = hello + &world;  // hello is moved (consumed), world is borrowed
    // println!("{hello}");  // Won't compile: hello was moved

    // Use format! to avoid move issues
    let a = String::from("Hello");
    let b = String::from("World");
    let combined = format!("{a}, {b}!");  // Neither a nor b is consumed

    println!("{combined}");
}

Why You Cannot Index Strings with []

fn main() {
    let s = String::from("hello");
    // let c = s[0];  // Won't compile! Rust strings are UTF-8, not byte arrays

    // Safe alternatives:
    let first_char = s.chars().next();           // Option<char>: Some('h')
    let as_bytes = s.as_bytes();                 // &[u8]: raw UTF-8 bytes
    let substring = &s[0..1];                    // &str: "h" (byte range, must be valid UTF-8 boundary)

    println!("First char: {:?}", first_char);
    println!("Bytes: {:?}", &as_bytes[..5]);
}

Exercise: String manipulation

🟢 Starter

• Write a function fn count_words(text: &str) -> usize that counts the number of whitespace-separated words in a string
• Write a function fn longest_word(text: &str) -> &str that returns the longest word (hint: you’ll need to think about lifetimes – why does the return type need to be &str and not String?)

fn count_words(text: &str) -> usize {
    text.split_whitespace().count()
}

fn longest_word(text: &str) -> &str {
    text.split_whitespace()
        .max_by_key(|word| word.len())
        .unwrap_or("")
}

fn main() {
    let text = "the quick brown fox jumps over the lazy dog";
    println!("Word count: {}", count_words(text));       // 9
    println!("Longest word: {}", longest_word(text));     // "jumps"
}

Rust structs

• The struct keyword declares a user-defined struct type
  • struct members can either be named, or anonymous (tuple structs)
• Unlike languages like C++, there’s no notion of “data inheritance” in Rust

fn main() {
    struct MyStruct {
        num: u32,
        is_secret_of_life: bool,
    }
    let x = MyStruct {
        num: 42,
        is_secret_of_life: true,
    };
    let y = MyStruct {
        num: x.num,
        is_secret_of_life: x.is_secret_of_life,
    };
    let z = MyStruct { num: x.num, ..x }; // The .. means copy remaining
    println!("{} {} {}", x.num, y.is_secret_of_life, z.num);
}

Rust tuple structs

• Rust tuple structs are similar to tuples and individual fields don’t have names
  • Like tuples, individual elements are accessed using .0, .1, .2, …. A common use case for tuple structs is to wrap primitive types to create custom types. This can useful to avoid mixing differing values of the same type

struct WeightInGrams(u32);
struct WeightInMilligrams(u32);
fn to_weight_in_grams(kilograms: u32) -> WeightInGrams {
    WeightInGrams(kilograms * 1000)
}

fn to_weight_in_milligrams(w : WeightInGrams) -> WeightInMilligrams  {
    WeightInMilligrams(w.0 * 1000)
}

fn main() {
    let x = to_weight_in_grams(42);
    let y = to_weight_in_milligrams(x);
    // let z : WeightInGrams = x;  // Won't compile: x was moved into to_weight_in_milligrams()
    // let a : WeightInGrams = y;   // Won't compile: type mismatch (WeightInMilligrams vs WeightInGrams)
}

Note: The #[derive(...)] attribute automatically generates common trait implementations for structs and enums. You’ll see this used throughout the course:

#[derive(Debug, Clone, PartialEq)]
struct Point { x: i32, y: i32 }

fn main() {
    let p = Point { x: 1, y: 2 };
    println!("{:?}", p);           // Debug: works because of #[derive(Debug)]
    let p2 = p.clone();           // Clone: works because of #[derive(Clone)]
    assert_eq!(p, p2);            // PartialEq: works because of #[derive(PartialEq)]
}

We’ll cover the trait system in depth later, but #[derive(Debug)] is so useful that you should add it to nearly every struct and enum you create.

Rust Vec type

• The Vec<T> type implements a dynamic heap allocated buffer (similar to manually managed malloc/realloc arrays in C, or C++’s std::vector)
  • Unlike arrays with fixed size, Vec can grow and shrink at runtime
  • Vec owns its data and automatically manages memory allocation/deallocation
• Common operations: push(), pop(), insert(), remove(), len(), capacity()

fn main() {
    let mut v = Vec::new();    // Empty vector, type inferred from usage
    v.push(42);                // Add element to end - Vec<i32>
    v.push(43);                
    
    // Safe iteration (preferred)
    for x in &v {              // Borrow elements, don't consume vector
        println!("{x}");
    }
    
    // Initialization shortcuts
    let mut v2 = vec![1, 2, 3, 4, 5];           // Macro for initialization
    let v3 = vec![0; 10];                       // 10 zeros
    
    // Safe access methods (preferred over indexing)
    match v2.get(0) {
        Some(first) => println!("First: {first}"),
        None => println!("Empty vector"),
    }
    
    // Useful methods
    println!("Length: {}, Capacity: {}", v2.len(), v2.capacity());
    if let Some(last) = v2.pop() {             // Remove and return last element
        println!("Popped: {last}");
    }
    
    // Dangerous: direct indexing (can panic!)
    // println!("{}", v2[100]);  // Would panic at runtime
}

Production patterns: See Avoiding unchecked indexing for safe .get() patterns from production Rust code.

Rust HashMap type

• HashMap implements generic key -> value lookups (a.k.a. dictionary or map)

fn main() {
    use std::collections::HashMap;  // Need explicit import, unlike Vec
    let mut map = HashMap::new();       // Allocate an empty HashMap
    map.insert(40, false);  // Type is inferred as int -> bool
    map.insert(41, false);
    map.insert(42, true);
    for (key, value) in map {
        println!("{key} {value}");
    }
    let map = HashMap::from([(40, false), (41, false), (42, true)]);
    if let Some(x) = map.get(&43) {
        println!("43 was mapped to {x:?}");
    } else {
        println!("No mapping was found for 43");
    }
    let x = map.get(&43).or(Some(&false));  // Default value if key isn't found
    println!("{x:?}"); 
}

Exercise: Vec and HashMap

🟢 Starter

• Create a HashMap<u32, bool> with a few entries (make sure that some values are true and others are false). Loop over all elements in the hashmap and put the keys into one Vec and the values into another

use std::collections::HashMap;

fn main() {
    let map = HashMap::from([(1, true), (2, false), (3, true), (4, false)]);
    let mut keys = Vec::new();
    let mut values = Vec::new();
    for (k, v) in &map {
        keys.push(*k);
        values.push(*v);
    }
    println!("Keys:   {keys:?}");
    println!("Values: {values:?}");

    // Alternative: use iterators with unzip()
    let (keys2, values2): (Vec<u32>, Vec<bool>) = map.into_iter().unzip();
    println!("Keys (unzip):   {keys2:?}");
    println!("Values (unzip): {values2:?}");
}

Deep Dive: C++ References vs Rust References

For C++ developers: C++ programmers often assume Rust &T works like C++ T&. While superficially similar, there are fundamental differences that cause confusion. C developers can skip this section — Rust references are covered in Ownership and Borrowing.

1. No Rvalue References or Universal References

In C++, && has two meanings depending on context:

// C++: && means different things:
int&& rref = 42;           // Rvalue reference — binds to temporaries
void process(Widget&& w);   // Rvalue reference — caller must std::move

// Universal (forwarding) reference — deduced template context:
template<typename T>
void forward(T&& arg) {     // NOT an rvalue ref! Deduced as T& or T&&
    inner(std::forward<T>(arg));  // Perfect forwarding
}

In Rust: none of this exists. && is simply the logical AND operator.

#![allow(unused)]
fn main() {
// Rust: && is just boolean AND
let a = true && false; // false

// Rust has NO rvalue references, no universal references, no perfect forwarding.
// Instead:
//   - Move is the default for non-Copy types (no std::move needed)
//   - Generics + trait bounds replace universal references
//   - No temporary-binding distinction — values are values

fn process(w: Widget) { }      // Takes ownership (like C++ value param + implicit move)
fn process_ref(w: &Widget) { } // Borrows immutably (like C++ const T&)
fn process_mut(w: &mut Widget) { } // Borrows mutably (like C++ T&, but exclusive)
}

2. Moves Are Bitwise — No Move Constructors

In C++, moving is a user-defined operation (move constructor / move assignment). In Rust, moving is always a bitwise memcpy of the value, and the source is invalidated:

#![allow(unused)]
fn main() {
// Rust move = memcpy the bytes, mark source as invalid
let s1 = String::from("hello");
let s2 = s1; // Bytes of s1 are copied to s2's stack slot
              // s1 is now invalid — compiler enforces this
// println!("{s1}"); // ❌ Compile error: value used after move
}

// C++ move = call the move constructor (user-defined!)
std::string s1 = "hello";
std::string s2 = std::move(s1); // Calls string's move ctor
// s1 is now a "valid but unspecified state" zombie
std::cout << s1; // Compiles! Prints... something (empty string, usually)

Consequences:

• Rust has no Rule of Five (no copy ctor, move ctor, copy=, move=, destructor to define)
• No moved-from “zombie” state — the compiler simply prevents access
• No noexcept considerations for moves — bitwise copy can’t throw

3. Auto-Deref: The Compiler Sees Through Indirection

Rust automatically dereferences through multiple layers of pointers/wrappers via the Deref trait. This has no C++ equivalent:

#![allow(unused)]
fn main() {
use std::sync::{Arc, Mutex};

// Nested wrapping: Arc<Mutex<Vec<String>>>
let data = Arc::new(Mutex::new(vec!["hello".to_string()]));

// In C++, you'd need explicit unlocking and manual dereferencing at each layer.
// In Rust, the compiler auto-derefs through Arc → Mutex → MutexGuard → Vec:
let guard = data.lock().unwrap(); // Arc auto-derefs to Mutex
let first: &str = &guard[0];      // MutexGuard→Vec (Deref), Vec[0] (Index),
                                   // &String→&str (Deref coercion)
println!("First: {first}");

// Method calls also auto-deref:
let boxed_string = Box::new(String::from("hello"));
println!("Length: {}", boxed_string.len());  // Box→String, then String::len()
// No need for (*boxed_string).len() or boxed_string->len()
}

Deref coercion also applies to function arguments — the compiler inserts dereferences to make types match:

fn greet(name: &str) {
    println!("Hello, {name}");
}

fn main() {
    let owned = String::from("Alice");
    let boxed = Box::new(String::from("Bob"));
    let arced = std::sync::Arc::new(String::from("Carol"));

    greet(&owned);  // &String → &str  (1 deref coercion)
    greet(&boxed);  // &Box<String> → &String → &str  (2 deref coercions)
    greet(&arced);  // &Arc<String> → &String → &str  (2 deref coercions)
    greet("Dave");  // &str already — no coercion needed
}
// In C++ you'd need .c_str() or explicit conversions for each case.

The Deref chain: When you call x.method(), Rust’s method resolution tries the receiver type T, then &T, then &mut T. If no match, it dereferences via the Deref trait and repeats with the target type. This continues through multiple layers — which is why Box<Vec<T>> “just works” like a Vec<T>. Deref coercion (for function arguments) is a separate but related mechanism that automatically converts &Box<String> to &str by chaining Deref impls.

4. No Null References, No Optional References

// C++: references can't be null, but pointers can, and the distinction is blurry
Widget& ref = *ptr;  // If ptr is null → UB
Widget* opt = nullptr;  // "optional" reference via pointer

#![allow(unused)]
fn main() {
// Rust: references are ALWAYS valid — guaranteed by the borrow checker
// No way to create a null or dangling reference in safe code
let r: &i32 = &42; // Always valid

// "Optional reference" is explicit:
let opt: Option<&Widget> = None; // Clear intent, no null pointer
if let Some(w) = opt {
    w.do_something(); // Only reachable when present
}
}

5. References Cannot Be Reseated

// C++: a reference is an alias — it can't be rebound
int a = 1, b = 2;
int& r = a;
r = b;  // This ASSIGNS b's value to a — it does NOT rebind r!
// a is now 2, r still refers to a

#![allow(unused)]
fn main() {
// Rust: let bindings can shadow, but references follow different rules
let a = 1;
let b = 2;
let r = &a;
// r = &b;   // ❌ Cannot assign to immutable variable
let r = &b;  // ✅ But you can SHADOW r with a new binding
             // The old binding is gone, not reseated

// With mut:
let mut r = &a;
r = &b;      // ✅ r now points to b — this IS rebinding (not assignment through)
}

Mental model: In C++, a reference is a permanent alias for one object. In Rust, a reference is a value (a pointer with lifetime guarantees) that follows normal variable binding rules — immutable by default, rebindable only if declared mut.