# Array and array index expressions

## Array expressions

> Syntax
> ArrayExpression :
> `[` `]`
> | `[` Expression ( `,` Expression )* `,`? `]`
> | `[` Expression `;` Expression `]`

An array expression can be written by enclosing zero or more comma-separated expressions of uniform type in square brackets. This produces and array containing each of these values in the order they are written.

Alternatively there can be exactly two expressions inside the brackets, separated by a semi-colon. The expression after the `;` must be a have type `usize` and be a constant expression, such as a literal or a constant item. `[a; b]` creates an array containing `b` copies of the value of `a`. If the expression after the semi-colon has a value greater than 1 then this requires that the type of `a` is `Copy`.

    # #![allow(unused_variables)]
    #fn main() {
    [1, 2, 3, 4];
    ["a", "b", "c", "d"];
    [0; 128];              // array with 128 zeros
    [0u8, 0u8, 0u8, 0u8,];
    [[1, 0, 0], [0, 1, 0], [0, 0, 1]]; // 2D array
    #}

## Array and slice indexing expressions

> Syntax
> IndexExpression :
> Expression `[` Expression `]`

Array and slice-typed expressions can be indexed by writing a square-bracket-enclosed expression of type `usize` (the index) after them. When the array is mutable, the resulting memory location can be assigned to.

For other types an index expression `a[b]` is equivalent to `*std::ops::Index::index(&a, b)`, or `*std::ops::IndexMut::index_mut(&mut a, b)` in a mutable place expression context. Just as with methods, Rust will also insert dereference operations on `a` repeatedly to find an implementation.

Indices are zero-based for arrays and slices. Array access is a constant expression, so bounds can be checked at compile-time with a constant index value. Otherwise a check will be performed at run-time that will put the thread in a panicked state if it fails.

    # #![allow(unused_variables)]
    #fn main() {
    // lint is deny by default.
    #![warn(const_err)]

    ([1, 2, 3, 4])[2];        // Evaluates to 3

    let b = [[1, 0, 0], [0, 1, 0], [0, 0, 1]];
    b[1][2];                  // multidimensional array indexing

    let x = (["a", "b"])[10]; // warning: index out of bounds

    let n = 10;
    let y = (["a", "b"])[n];  // panics

    let arr = ["a", "b"];
    arr[10];                  // warning: index out of bounds
    #}

The array index expression can be implemented for types other than arrays and slices by implementing the Index and IndexMut traits.