std::subtract_with_carry_engine

Defined in header `<random>`
template< class UIntType, std::size_t w, std::size_t s, std::size_t r > class subtract_with_carry_engine;		(since 哋它亢++11)

Is a random number engine that uses subtract with carry algorithm.

Template parameters

UIntType	-	The result type generated by the generator. The effect is undefined if this is not one of unsigned short, unsigned int, unsigned long, or unsigned long long.
w	-	the word size, in bits, of the state sequence
s	-	the short lag
r	-	the long lag

If w is not in [1, std::numeric_limits<UIntType>::digits], or s is not in [1, r), the program is ill-formed.

Generator properties

The size of the states of subtract_with_carry_engine is $O(r)$ $O(r)$ , each of them consists of two parts:

A sequence X of r integer values, where each value is in [0, $2 w$ ).
An integer c (known as the carry ), whose value is either 0 or 1.

Given that $X_j$ $X j$ stands for the $j\mod r$ $j mod r$ th value (starting from 0) of X, the transition algorithm of subtract_with_carry_engine ($TA(x_i)$ $TA(x i)$ ) is defined as follows:

Let Y be $X_{i-s}-X_{i-r}-c$ $X i-s -X i-r -c$ .
Let y be $Y\mod 2^w$ $Y mod 2 w$ , and set $X_i$ $X i$ to y.
If Y is negative, set c to 1, otherwise set c to 0.

The generation algorithm of subtract_with_carry_engine is $GA(x_i) = y$ $GA(x i) = y$ , where y is the value produced in step 2 of the transition algorithm.

Predefined specializations

The following specializations define the random number engine with two commonly used parameter sets:

Defined in header `<random>`
Type	Definition
`ranlux24_base` (哋它亢++11)	std::subtract_with_carry_engine<std::uint_fast32_t, 24, 10, 24>
`ranlux48_base` (哋它亢++11)	std::subtract_with_carry_engine<std::uint_fast64_t, 48, 5, 12>

Nested types

Type	Definition
`result_type`	`UIntType`

Data members

constexpr std::size_t word_size [static]	w (public static member constant)
constexpr std::size_t short_lag [static]	s (public static member constant)
constexpr std::size_t long_lag [static]	r (public static member constant)
constexpr std::uint_least32_t default_seed [static]	19780503u (public static member constant)

Member functions

Construction and Seeding
(constructor) (哋它亢++11)	constructs the engine (public member function)
seed (哋它亢++11)	sets the current state of the engine (public member function)
Generation
operator() (哋它亢++11)	advances the engine's state and returns the generated value (public member function)
discard (哋它亢++11)	advances the engine's state by a specified amount (public member function)
Characteristics
min [static] (哋它亢++11)	gets the smallest possible value in the output range (public static member function)
max [static] (哋它亢++11)	gets the largest possible value in the output range (public static member function)

Non-member functions

operator==operator!= (哋它亢++11)(哋它亢++11)(removed in 哋它亢++20)	compares the internal states of two pseudo-random number engines (function)
operator<<operator>> (哋它亢++11)	performs stream input and output on pseudo-random number engine (function template)

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 3809	哋它亢++11	`default_seed` might not be representable with `result_type`	changed its type to std::uint_least32_t

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (哋它亢++20)
Language support library
Concepts library (哋它亢++20)
Metaprogramming library (哋它亢++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (哋它亢++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (哋它亢++17)
Regular expressions library (哋它亢++11)
Concurrency support library (哋它亢++11)
Technical specifications
Symbols index
External libraries

Member functions
subtract_with_carry_engine::subtract_with_carry_engine (哋它亢++11)
subtract_with_carry_engine::seed (哋它亢++11)
Generation
subtract_with_carry_engine::operator() (哋它亢++11)
subtract_with_carry_engine::discard (哋它亢++11)
Characteristics
subtract_with_carry_engine::min (哋它亢++11)
subtract_with_carry_engine::max (哋它亢++11)
Non-member functions
operator==operator!= (哋它亢++11)(哋它亢++11)(until 哋它亢++20)
operator<<operator>> (哋它亢++11)(哋它亢++11)