Arrays: the map

Level: 101 → 301 · the map

One line: [T; N] — a fixed number of values of one type, with the length in the type — is taught across a dozen pages in this library: how to write one, how it differs from a slice and a Vec, how it goes in and out of functions, where it lives, and how to iterate over it. This page lists them in reading order and says which one answers your question.

Arrays are one of Rust's two built-in compound types, with tuples. A Vec is not one of them: it comes from the standard library, and Array or Vec? is the page for choosing.

Every array page, in reading order

#	Page	What it answers
1	Writing an array down	[1, 2, 3], [u8; 3] and [0; 3]; why every element must be the same type; how inference picks it from the neighbours; the first loop over one, and the u8 sum that overflows
2	Arrays and slices	the length is part of the type; &[T] moves it into the value; out-of-bounds panics; the methods live on the slice; the &Vec<T> trap
3	Array or Vec?	the four things an array buys, and why Vec is still the default
4	Arrays in and out of functions	returning [i32; 3] and &'static [i32]; a parameter of exactly N or of any length; const generics
5	Where an array lives	stack, heap or static data; Box<[i32; 3]> against Box<[i32]>; the megabyte array that overflows a thread's stack
6	An array in a const or a static	a table of records in a static; what 'static in its type means; comparing a const array with a local
7	A Vec of arrays	Vec<[T; N]>: fixed-width rows in one growable block
8	Grids and nested Vecs	[[T; N]; M], a flat Vec and Vec<Vec<T>> for a 2D matrix
9	Slices of slices	why &[&[T]] will not take your grid, and the signatures that will
10	iter, iter_mut, into_iter	three ways to iterate over an array, and the edition-2021 change to array.into_iter()
11	DoubleEndedIterator and ExactSizeIterator	.rev() and .len() on an array's iterator
12	Vec::into_iter and the three IntoIterator impls	the same three impls arrays have, on Vec
13	slice::iter and the slice methods	a page per method an array reaches through the slice: sort, windows, chunks, contains …
14	Values	the widths behind [u8; 3] and [i32; 3]

Beside the lessons:

• What array explanations get wrong, run — array questions: twenty-four claims from books, docs, answers and chat, each checked on 1.98.0
• Every array error, and its fix — array errors: twenty-nine refusals, by code and by message
• Lints around arrays — twenty-four rustc and clippy warnings, each with a bad and a good program
• Helpful resources for arrays — The Book, the Reference, std, books by chapter, articles and videos, each checked

Which page answers your question

You want to know	Go to
how to write (initialize) array values	Writing an array down
what the two halves of [0; 3] mean: the value, and how many	[value; how many]
why the elements must be the same type, and how one suffix decides it	Inference reads the neighbours, errors 6 to 8
why ["bb".to_string(); 3] does not compile	error 9, and array::from_fn
how to iterate over an array	Iterating over it by reference, iter, iter_mut, into_iter
how to filter an array	claims 11 and 12, errors 22 to 24
what IntoIterator for arrays changed in edition 2021	array_into_iter
array or Vec	Array or Vec?
how to pass an array to a function, or return one	Arrays in and out of functions
whether an array is on the stack or the heap; a heap-allocated array	Where an array lives
how to build a static array, or an array of structs in a static	An array in a const or a static
comparing arrays: a const against a local, or two lengths	A const array, and a local, error 3
what happens on an index past the end	Arrays and slices, error 10
whether arrays stop at 32 elements	claim 4
a 2D matrix, rows and columns	Grids and nested Vecs, the transpose kata
a list of rows that all have the same width	A Vec of arrays

Katas along the way

After	Kata	On
1	The first ten odd numbers, two ways — a block that fills a zeroed array, and array::from_fn	Writing an array down
2	One function, four callers — which signatures turn which callers away — and the call that printed its own receipt	Arrays and slices
3	Two lengths in one election — one is a fact about the problem, one about this run	Array or Vec?
4	Transpose a matrix — two index loops, then const generics and from_fn	Arrays in and out of functions
5	Nine sizes, then a megabyte on a small stack	Where an array lives
6	A static lookup table, checked while it compiles	An array in a const or a static
7	Read the wire, and make the short row impossible	A Vec of arrays
8	Build the same grid twice	Grids and nested Vecs
9	A signature you cannot change	Slices of slices
11	Five references where you meant five numbers	DoubleEndedIterator and ExactSizeIterator

The full sequence, with every other kata in the library, is KATAS.md.

If you are coming from another language

• Python. Python has no fixed-length array in the language: list is Rust's Vec, and tuple is the nearest fixed-length thing, though it may mix types. The byte types come closest to [u8; N]: bytes(5) is five zero bytes ↗, and bytearray is the mutable one ↗, which grows where a Rust array cannot. Slicing differs at the edges: a Python slice clamps an out-of-range bound ↗, so a[1:100] on five elements is the last four. In Rust &a[1..100] panics with range end index 100 out of range for slice of length 5, and a.get(1..100) is None.
• C. A C array passed to a function becomes a pointer, and its length is lost: sizeof then gives the pointer's size ↗. Rust keeps the length, in the type for [T; N] and beside the pointer for &[T]. The overruns that follow from that in C — functions that write past a fixed buffer ↗, and a length prefix nobody checked ↗ — are a panic or a compile error in Rust.
• C++. std::array<T, N> is [T; N], stored inline, with the length in the type, and std::vector<T> is Vec<T>. The C++ library has no arrays page yet.
• Go. Go draws the same line: [3]int is a value with the length in its type, and []int is a view with the length in the value. Go arrays are copied on assignment, as Rust's Copy arrays are. The Go library, a concurrency library, has no arrays page.
• Java. int[] carries its length at run time but not in its type, and it is always a reference to a heap object. split returns a String[] with sharp edges ↗ in the Java library.

See also

• Collections — the section this map belongs to: tuples, arrays, Vec, maps, sets and Box
• How to learn ToOwned — the same kind of map, for a trait
• Glossary: array, slice, repeat expression — one-paragraph definitions

Po polsku

Wszystkie strony o tablicach [T; N] w jednym miejscu, w kolejności czytania. Kolejne strony uczą: jak zapisać tablicę ([1, 2, 3], [u8; 3], [0; 3]); czym różni się od wycinka &[T] i od wektora Vec; jak przekazać tablicę do funkcji i zwrócić ją; gdzie leży (stos, sterta, dane statyczne); jak zbudować tablicę statyczną z rekordami; jak po niej iterować. Obok są strony z pytaniami i odpowiedziami (twierdzenia sprawdzone na 1.98.0), z błędami kompilacji, z ostrzeżeniami clippy i ze źródłami.

Tablica i krotka to dwa wbudowane typy złożone (compound types). Vec do nich nie należy, bo pochodzi z biblioteki standardowej. Po polsku „tablica” to [T; N], a to, co w Pythonie jest listą, to w Ruscie wektor.

Szukaj po polsku: tablice w Ruscie · array - main · array - errors · array - questions · array iterate · static array · filter an array · compound types · array - must be the same type · constants - comparing · heap-allocated array · IntoIterator for arrays · write (initialize) array values