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 unsignedshort, unsignedint, unsignedlong, or unsignedlonglong. |
| 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 \(\scriptsize O(r)\)O(r), each of them consists of two parts:
- A sequence
Xofrinteger values, where each value is in[0,2w). - An integer
c(known as the carry ), whose value is either0or1.
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)) is defined as follows:
- Let
Ybe \(\scriptsize X_{i-s}-X_{i-r}-c\)Xi-s-Xi-r-c. - Let
ybe \(\scriptsize Y\mod 2^w\)Y mod 2w, and set \(\scriptsize X_i\)Xi toy. - If
Yis negative, setcto1, otherwise setcto0.
The generation algorithm of subtract_with_carry_engine is \(\scriptsize GA(x_i) = y\)GA(xi) = 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 |
|
| constructs the engine (public member function) |
|
| sets the current state of the engine (public member function) |
|
Generation |
|
| advances the engine's state and returns the generated value (public member function) |
|
| advances the engine's state by a specified amount (public member function) |
|
Characteristics |
|
|
[static] | gets the smallest possible value in the output range (public static member function) |
|
[static] | gets the largest possible value in the output range (public static member function) |
Non-member functions
|
(C++11)(C++11)(removed in C++20) | compares the internal states of two pseudo-random number engines (function) |
|
(C++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 C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3809 | C++11 |
default_seed might not berepresentable with result_type | changed its type tostd::uint_least32_t |
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