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#### Random Access Sequence

###### Description

A Random Access Sequence is a Bidirectional Sequence whose iterators model Random Access Iterator. It guarantees constant time access to arbitrary sequence elements.

###### Refinement of

Bidirectional Sequence

Notation

`s`

  A Random Access Sequence

`S`

  A Random Access Sequence type

`M`

  An MPL integral constant

`N`

  An integral constant

`o`

  An arbitrary object

`e`

  A Sequence element

###### Valid Expressions

In addition to the requirements defined in Bidirectional Sequence, for any Random Access Sequence the following must be met:

Expression | Return type | Type Requirements | Runtime Complexity
--- | --- | --- | ---
``begin`(s)` | Random Access Iterator |  | Constant
``end`(s)` | Random Access Iterator |  | Constant
``at_c`<N>(s)` | Any type |  | Constant
``at_c`<N>(s) = o` | Any type | `s` is mutable and `e = o`, where `e` is the N-th element from the beginning of the sequence, is a valid expression. | Constant
``at`<M>(s)` | Any type |  | Constant
``at`<M>(s) = o` | Any type | `s` is mutable and `e = o`, where `e` is the M-th element from the beginning of the sequence, is a valid expression. | Constant

###### Result Type Expressions

Expression | Compile Time Complexity
--- | ---
``result_of::begin`<S>::type` | Amortized constant time
``result_of::end`<S>::type` | Amortized constant time
``result_of::at`<S, M>::type` | Amortized constant time
``result_of::at_c`<S, N>::type` | Amortized constant time
``result_of::value_at`<S, M>::type` | Amortized constant time
``result_of::value_at_c`<S, N>::type` | Amortized constant time

| Note

``result_of::at`<S, M>` returns the actual type returned by ``at`<M>(s)`. In most cases, this is a reference. Hence, there is no way to know the exact element type using ``result_of::at`<S, M>`.The element at `M` may actually be a reference to begin with. For this purpose, you can use ``result_of::value_at`<S, M>` (Note that, ``result_of::value_at_c`<S, N>` is a counterpart of ``result_of::at_c`<S, N>` as well).

###### Expression Semantics

The semantics of an expression are defined only where they differ from, or are not defined in Bidirectional Sequence.

Expression | Semantics
--- | ---
``at`<M>(s)` | The Mth element from the beginning of the sequence; see `at`.
``at_c`<N>(s)` | The Nth element from the beginning of the sequence; see `at_c`.

###### Models

- `std::pair`
- `boost::array`
- `vector`
- `map`
- `reverse_view`
- `single_view`
- `iterator_range` (where adapted sequence is a Random Access Sequence)
- `transform_view` (where adapted sequence is a Random Access Sequence)
- `zip_view` (where adapted sequences are models of Random Access Sequence)