Scalar & Compound Types

Q: What is the default integer type?

i32 when inference has no other constraint. Use explicit suffixes (42u64) in APIs that need a specific width.

Q: Can array size be generic?

Yes, with const generics: struct Buffer { data: [u8; N] }.

Q: How do I initialize an array with the same value?

let a = [0u8; 1024]; repeats 0u8 1024 times. The element must implement Copy.

Q: Are bools one bit?

No. bool is 1 byte for alignment and simple addressing.

Q: Can tuples implement traits?

Only for specific arities via standard impls (e.g. Clone for small tuples). Prefer structs for public APIs.

Q: What is isize?

Pointer-sized signed integer. Use for indexing and len() results (usize).

Q: How do I convert strings to numbers?

"42".parse::() returns Result. See the strings section for FromStr.

Q: Does Rust have a null?

No. Use Option for absent values.

Q: Can I compare arrays?

Yes, if T: PartialEq. [1,2] == [1,2] works element-wise.

Rust's scalar types hold single values. Compound types - tuples and fixed-size arrays - group values with statically known shapes.

Recipe

fn main() {
    let count: u32 = 42;
    let ratio: f64 = 3.14;
    let ok: bool = true;
    let ch: char = '✓';
 
    let pair: (u32, &str) = (count, "items");
    let nums: [i32; 3] = [1, 2, 3];
 
    println!("{count} {ratio} {ok} {ch} {:?} {:?}", pair, nums);
}

When to reach for this: Choosing the right primitive or small aggregate before reaching for Vec or custom structs.

Working Example

fn stats(values: [f64; 4]) -> (f64, f64) {
    let sum: f64 = values.iter().sum();
    let avg = sum / values.len() as f64;
    (sum, avg)
}
 
fn main() {
    let readings = [98.6, 99.1, 98.4, 99.0];
    let (total, average) = stats(readings);
    println!("sum={total:.2}, avg={average:.2}");
}

What this demonstrates:

Fixed-size array passed by value (copies 4 f64s on the stack)
Tuple return type for multiple values before a struct is warranted
as f64 for explicit numeric cast
Array indexing with compile-time-known length

Deep Dive

Scalar Types at a Glance

Type	Size (typical)	Notes
`i32` / `u32`	4 bytes	Default integer type
`i64` / `u64`	8 bytes	Timestamps, IDs
`f32` / `f64`	4 / 8 bytes	`f64` is default float
`bool`	1 byte	Only `true` or `false`
`char`	4 bytes	Unicode scalar, not ASCII-only

Integer Literals and Overflow

let hex = 0xff_u8;
let bin = 0b1010_0110;
let wrapped = 255u8.wrapping_add(1); // 0

In debug builds, overflow panics. Use wrapping_*, checked_*, or saturating_* when overflow behavior matters.

Arrays vs Slices

let arr: [i32; 3] = [1, 2, 3];
let slice: &[i32] = &arr[1..];

Arrays have length in the type. Slices are fat pointers (pointer + length) and can refer to arrays, Vec, or stack data.

Tuples

let t = (1, "a", true);
let (a, b, c) = t;
assert_eq!(t.0, a);

Empty tuple () is the unit type - return type of functions that return nothing meaningful.

Gotchas

Assuming char is one byte - char is UTF-32. Fix: Use u8 for raw bytes; iterate str with .chars().
Indexing past array bounds - Runtime panic in safe Rust. Fix: Use .get(i) for optional access.
Mixing integer types - let x: i32 = 1i16; needs explicit cast. Fix: Use as or TryInto.
Float equality - 0.1 + 0.2 == 0.3 is false. Fix: Compare with epsilon or use integers (cents).
Large arrays on the stack - [u8; 1_000_000] can overflow the stack. Fix: Use Vec or Box<[T]> on the heap.

Alternatives

Alternative	Use When	Don't Use When
Struct with named fields	Tuple has more than 2-3 fields or unclear meaning	One-off `(x, y)` coordinates
`Vec<T>`	Runtime-sized collection	Length is fixed and small
Newtype wrapper	Distinct units (`Meters`, `Seconds`)	Simple internal math only
`SmallVec` / array	Usually small, occasionally large	Always heap-allocated data

FAQs

What is the default integer type?

i32 when inference has no other constraint. Use explicit suffixes (42u64) in APIs that need a specific width.

Can array size be generic?

Yes, with const generics: struct Buffer<const N: usize> { data: [u8; N] }.

How do I initialize an array with the same value?

let a = [0u8; 1024]; repeats 0u8 1024 times. The element must implement Copy.

Are bools one bit?

No. bool is 1 byte for alignment and simple addressing.

Can tuples implement traits?

Only for specific arities via standard impls (e.g. Clone for small tuples). Prefer structs for public APIs.

What is `isize`?

Pointer-sized signed integer. Use for indexing and len() results (usize).

How do I convert strings to numbers?

"42".parse::<i32>() returns Result. See the strings section for FromStr.

Does Rust have a `null`?

No. Use Option<T> for absent values.

Can I compare arrays?

Yes, if T: PartialEq. [1,2] == [1,2] works element-wise.

When should I use `f32`?

GPU/math kernels where memory bandwidth matters. Default to f64 elsewhere.

Variables, Mutability & Shadowing - bindings and destructuring
References & Slices - slice types in depth
The Type System & Inference - inference rules
Structs Basics - named product types

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.
Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language, 2nd Edition (Amazon) - Steve Klabnik & Carol Nichols.
Programming Rust, 2nd Edition (O'Reilly) - Same Programming Rust on O'Reilly.

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 main() {
    let count: u32 = 42;
    let ratio: f64 = 3.14;
    let ok: bool = true;
    let ch: char = '✓';
 
    let pair: (u32, &str) = (count, "items");
    let nums: [i32; 3] = [1, 2, 3];
 
    println!("{count} {ratio} {ok} {ch} {:?} {:?}", pair, nums);
}

When to reach for this: Choosing the right primitive or small aggregate before reaching for Vec or custom structs.

Working Example

fn stats(values: [f64; 4]) -> (f64, f64) {
    let sum: f64 = values.iter().sum();
    let avg = sum / values.len() as f64;
    (sum, avg)
}
 
fn main() {
    let readings = [98.6, 99.1, 98.4, 99.0];
    let (total, average) = stats(readings);
    println!("sum={total:.2}, avg={average:.2}");
}

What this demonstrates:

Fixed-size array passed by value (copies 4 f64s on the stack)
Tuple return type for multiple values before a struct is warranted
as f64 for explicit numeric cast
Array indexing with compile-time-known length

Deep Dive

Scalar Types at a Glance

Type	Size (typical)	Notes
`i32` / `u32`	4 bytes	Default integer type
`i64` / `u64`	8 bytes	Timestamps, IDs
`f32` / `f64`	4 / 8 bytes	`f64` is default float
`bool`	1 byte	Only `true` or `false`
`char`	4 bytes	Unicode scalar, not ASCII-only

Integer Literals and Overflow

let hex = 0xff_u8;
let bin = 0b1010_0110;
let wrapped = 255u8.wrapping_add(1); // 0

In debug builds, overflow panics. Use wrapping_*, checked_*, or saturating_* when overflow behavior matters.

Arrays vs Slices

let arr: [i32; 3] = [1, 2, 3];
let slice: &[i32] = &arr[1..];

Arrays have length in the type. Slices are fat pointers (pointer + length) and can refer to arrays, Vec, or stack data.

Tuples

let t = (1, "a", true);
let (a, b, c) = t;
assert_eq!(t.0, a);

Empty tuple () is the unit type - return type of functions that return nothing meaningful.

Gotchas

Assuming char is one byte - char is UTF-32. Fix: Use u8 for raw bytes; iterate str with .chars().
Indexing past array bounds - Runtime panic in safe Rust. Fix: Use .get(i) for optional access.
Mixing integer types - let x: i32 = 1i16; needs explicit cast. Fix: Use as or TryInto.
Float equality - 0.1 + 0.2 == 0.3 is false. Fix: Compare with epsilon or use integers (cents).
Large arrays on the stack - [u8; 1_000_000] can overflow the stack. Fix: Use Vec or Box<[T]> on the heap.

Alternatives

Alternative	Use When	Don't Use When
Struct with named fields	Tuple has more than 2-3 fields or unclear meaning	One-off `(x, y)` coordinates
`Vec<T>`	Runtime-sized collection	Length is fixed and small
Newtype wrapper	Distinct units (`Meters`, `Seconds`)	Simple internal math only
`SmallVec` / array	Usually small, occasionally large	Always heap-allocated data

FAQs

What is the default integer type?

i32 when inference has no other constraint. Use explicit suffixes (42u64) in APIs that need a specific width.

Can array size be generic?

Yes, with const generics: struct Buffer<const N: usize> { data: [u8; N] }.

How do I initialize an array with the same value?

let a = [0u8; 1024]; repeats 0u8 1024 times. The element must implement Copy.

Are bools one bit?

No. bool is 1 byte for alignment and simple addressing.

Can tuples implement traits?

Only for specific arities via standard impls (e.g. Clone for small tuples). Prefer structs for public APIs.

What is `isize`?

Pointer-sized signed integer. Use for indexing and len() results (usize).

How do I convert strings to numbers?

"42".parse::<i32>() returns Result. See the strings section for FromStr.

Does Rust have a `null`?

No. Use Option<T> for absent values.

Can I compare arrays?

Yes, if T: PartialEq. [1,2] == [1,2] works element-wise.

When should I use `f32`?

GPU/math kernels where memory bandwidth matters. Default to f64 elsewhere.

Variables, Mutability & Shadowing - bindings and destructuring
References & Slices - slice types in depth
The Type System & Inference - inference rules
Structs Basics - named product types

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.
Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language, 2nd Edition (Amazon) - Steve Klabnik & Carol Nichols.
Programming Rust, 2nd Edition (O'Reilly) - Same Programming Rust on O'Reilly.

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+.

Scalar & Compound Types

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Scalar & Compound Types