NAME¶

std::subtract_with_carry_engine - std::subtract_with_carry_engine

Synopsis¶

Defined in header <random>
template<

class UIntType, (since C++11)
std::size_t w, std::size_t s, std::size_t r

> class subtract_with_carry_engine;

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

Template parameters¶

The result type generated by the generator. The effect is undefined if
UIntType - 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 std::subtract_with_carry_engine<std::uint_fast32_t, 24, 10,
(C++11) 24>
ranlux48_base std::subtract_with_carry_engine<std::uint_fast64_t, 48, 5,
(C++11) 12>

Nested types

Type Definition
result_type UIntType

Data members

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

Member functions¶

Construction and Seeding¶

constructor constructs the engine
(C++11) (public member function)
seed sets the current state of the engine
(C++11) (public member function)

Generation¶

operator() advances the engine's state and returns the generated value
(C++11) (public member function)
discard advances the engine's state by a specified amount
(C++11) (public member function)

Characteristics¶

min gets the smallest possible value in the output range
[static] (C++11) (public static member function)
max gets the largest possible value in the output range
[static] (C++11) (public static member function)

Non-member functions¶

operator== compares the internal states of two pseudo-random number
operator!= engines
(C++11) (function)
(C++11)(removed in C++20)
operator<< performs stream input and output on pseudo-random number
operator>> engine
(C++11) (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 changed its type to
representable with result_type std::uint_least32_t

Category:¶

* Todo no example