# Tuple and tuple indexing expressions

## Tuple expressions

> **Syntax**\
> _TupleExpression_ :\
>    `(` [_InnerAttribute_]\* _TupleElements_? `)`
>
> _TupleElements_ :\
>    ( [_Expression_] `,` )+ [_Expression_]?

A *tuple expression* constructs [tuple values][tuple type].

The syntax for tuple expressions is a parenthesized, comma separated list of expressions, called the *tuple initializer operands*.
1-ary tuple expressions require a comma after their tuple initializer operand to be disambiguated with a [parenthetical expression].

Tuple expressions are a [value expression] that evaluate into a newly constructed value of a tuple type.
The number of tuple initializer operands is the arity of the constructed tuple.
Tuple expressions without any tuple initializer operands produce the unit tuple.
For other tuple expressions, the first written tuple initializer operand initializes the field `0` and subsequent operands initializes the next highest field.
For example, in the tuple expression `('a', 'b', 'c')`, `'a'` initializes the value of the field `0`, `'b'` field `1`, and `'c'` field `2`.

Examples of tuple expressions and their types:

| Expression           | Type         |
| -------------------- | ------------ |
| `()`                 | `()` (unit)  |
| `(0.0, 4.5)`         | `(f64, f64)` |
| `("x".to_string(), )` | `(String, )`  |
| `("a", 4usize, true)`| `(&'static str, usize, bool)` |

### Tuple expression attributes

[Inner attributes] are allowed directly after the opening parenthesis of a tuple expression in the same expression contexts as [attributes on block expressions].

## Tuple indexing expressions

> **Syntax**\
> _TupleIndexingExpression_ :\
>    [_Expression_] `.` [TUPLE_INDEX]

A *tuple indexing expression* accesses fields of [tuples][tuple type] and [tuple structs][tuple struct].

The syntax for a tuple index expression is an expression, called the *tuple operand*, then a `.`, then finally a tuple index.
The syntax for the *tuple index* is a [decimal literal] with no leading zeros, underscores, or suffix.
For example `0` and `2` are valid tuple indices but not `01`, `0_`, nor `0i32`.

The type of the tuple operand must be a [tuple type] or a [tuple struct].
The tuple index must be a name of a field of the type of the tuple operand.

Evaluation of tuple index expressions has no side effects beyond evaluation of its tuple operand.
As a [place expression], it evaluates to the location of the field of the tuple operand with the same name as the tuple index.

Examples of tuple indexing expressions:

```rust
// Indexing a tuple
let pair = ("a string", 2);
assert_eq!(pair.1, 2);

// Indexing a tuple struct
# struct Point(f32, f32);
let point = Point(1.0, 0.0);
assert_eq!(point.0, 1.0);
assert_eq!(point.1, 0.0);
```

> **Note**: Unlike field access expressions, tuple index expressions can be the function operand of a [call expression] as it cannot be confused with a method call since method names cannot be numbers.

> **Note**: Although arrays and slices also have elements, you must use an [array or slice indexing expression] or a [slice pattern] to access their elements.

[_Expression_]: ../expressions.md
[_InnerAttribute_]: ../attributes.md
[array or slice indexing expression]: array-expr.md#array-and-slice-indexing-expressions
[attributes on block expressions]: block-expr.md#attributes-on-block-expressions
[call expression]: ./call-expr.md
[decimal literal]: ../tokens.md#integer-literals
[field access expressions]: ./field-expr.html#field-access-expressions
[inner attributes]: ../attributes.md
[operands]: ../expressions.md
[parenthetical expression]: grouped-expr.md
[place expression]: ../expressions.md#place-expressions-and-value-expressions
[slice pattern]: ../patterns.md#slice-patterns
[tuple type]: ../types/tuple.md
[tuple struct]: ../types/struct.md
[TUPLE_INDEX]: ../tokens.md#tuple-index
[value expression]: ../expressions.md#place-expressions-and-value-expressions
