# std::subtract_with_carry_engine

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< cpp‎ | numeric‎ | random

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.

The state of a `subtract_with_carry_engine` consists of a sequence \(\small{\{ X_i\}(0 \le i <r)}\){X
i} (0 ≤ i <r), every \(\small{X_i}\)X
i is in interval \(\small{[0, 2^w)}\)[0, 2w
).

Let all subscripts applied to sequence be taken modulo `r`, `c` be the carry value which is either `0` or `1`. The state transition is performed as follows:

- let \(\small{Y=X_{i-s}-X_{i-r}-c}\)Y=X
  i-s-X
  i-r-c,
- set \(\small{X_i}\)X
  i to \(\small{Y \mod 2^w}\)Y mod 2w
  ,
- set `c` to `1`, if \(\small{Y<0}\)Y<0, to `0` otherwise.

The following typedefs 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>

### 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, 10 < w <= std::numeric_limits<UIntType>::digits.
s | - | The short lag.
r | - | The long lag, where 0 < s < r.

### Member types

Member type | Definition
`result_type` (C++11) | The integral type generated by the engine. Results are undefined if this is not an unsigned integral type.

### 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)

### Member constants

constexpr size_t word_size [static] (C++11) | template parameter `w`, the word size (public static member constant)
constexpr size_t short_lag [static] (C++11) | template parameter `s`, the short lag (public static member constant)
constexpr size_t long_lag [static] (C++11) | template parameter `r`, the long lag (public static member constant)
constexpr UIntType default_seed [static] (C++11) | constant value 19780503u (public static member constant)

### Example

| This section is incomplete Reason: no example

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