std::subtract_with_carry_engine

 

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subtract_with_carry_engine (C++11)
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rand	srand	RAND_MAX

 

std::subtract_with_carry_engine

Member functions

subtract_with_carry_engine::subtract_with_carry_engine (C++11)

subtract_with_carry_engine::seed (C++11)

Generation

subtract_with_carry_engine::operator() (C++11)

subtract_with_carry_engine::discard (C++11)

Characteristics

subtract_with_carry_engine::min (C++11)

subtract_with_carry_engine::max (C++11)

Non-member functions

operator==operator!= (C++11)(C++11)(until C++20)

operator<<operator>> (C++11)(C++11)

 

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 C++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​, 2w
).
• 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 rth value (starting from 0) of X, the transition algorithm of subtract_with_carry_engine (\(TA(x_i)\)TA(x
i)) is defined as follows:

1. Let Y be \(X_{i-s}-X_{i-r}-c\)X
   i-s-X
   i-r-c.
2. Let y be \(Y\mod 2^w\)Y mod 2w
   , and set \(X_i\)X
   i to y.
3. 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 (C++11)	std::subtract_with_carry_engine<std::uint_fast32_t, 24, 10, 24>
ranlux48_base (C++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) (C++11)	constructs the engine (public member function)
seed (C++11)	sets the current state of the engine (public member function)
Generation
operator() (C++11)	advances the engine's state and returns the generated value (public member function)
discard (C++11)	advances the engine's state by a specified amount (public member function)
Characteristics
min [static] (C++11)	gets the smallest possible value in the output range (public static member function)
max [static] (C++11)	gets the largest possible value in the output range (public static member function)

Non-member functions

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

Defect reports

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

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