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 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 )

Generator properties

The size of the states of subtract_with_carry_engine is \(\scriptsize O(r)\) O(r)

• 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 \(\scriptsize X_j\) Xj stands for the \(\scriptsize j\mod r\) j mod rth value (starting from 0) of X, the transition algorithm of subtract_with_carry_engine (\(\scriptsize TA(x_i)\) TA(xi)

1. Let Y be \(\scriptsize X_{i-s}-X_{i-r}-c\) Xi-s-Xi-r-c
2. Let y be \(\scriptsize Y\mod 2^w\) Y mod 2w
   , and set \(\scriptsize X_i\) Xi 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 \(\scriptsize GA(x_i) = y\) GA(xi) = y, where y

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

Example

This section is incomplete Reason: no example

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