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std::subtract_with_carry_engine

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< cpp‎ | numeric‎ | random
 
 
 
 
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.

Contents

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

[edit] Generator properties

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

  • A sequence X of r integer values, where each value is in [02w
    ).
  • An integer c (known as the carry ), whose value is either 0 or 1.

Given that X
j
stands for the j mod rth value (starting from 0) of X, the transition algorithm of subtract_with_carry_engine (TA(x
i
)
) is defined as follows:

  1. Let Y be X
    i-s
    -X
    i-r
    -c
    .
  2. Let y be Y mod 2w
    , and set 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
, where y is the value produced in step 2 of the transition algorithm.

[edit] 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>[edit]
ranlux48_base (C++11) std::subtract_with_carry_engine<std::uint_fast64_t, 48, 5, 12>[edit]

[edit] Nested types

Type Definition
result_type UIntType

[edit] 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)

[edit] Member functions

Construction and Seeding
constructs the engine
(public member function) [edit]
(C++11)
sets the current state of the engine
(public member function) [edit]
Generation
advances the engine's state and returns the generated value
(public member function) [edit]
(C++11)
advances the engine's state by a specified amount
(public member function) [edit]
Characteristics
[static] (C++11)
gets the smallest possible value in the output range
(public static member function) [edit]
[static] (C++11)
gets the largest possible value in the output range
(public static member function) [edit]

[edit] Non-member functions

(C++11)(C++11)(removed in C++20)
compares the internal states of two pseudo-random number engines
(function) [edit]
performs stream input and output on pseudo-random number engine
(function template) [edit]

[edit] Example

[edit] 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