subtract_with_carry_engine Class

Generates a random sequence by the subtract-with-carry (lagged Fibonacci) algorithm.


template <class UIntType, size_t W, size_t S, size_t R>
class subtract_with_carry_engine;


The unsigned integer result type. For possible types, see <random>.

Word size. Size of each word, in bits, of the state sequence. Precondition: 0 < W ≤ numeric_limits<UIntType>::digits

Short lag. Number of integer values. Precondition: 0 < S < R

Long lag. Determines recurrence in the series generated.


subtract_with_carry_engine::subtract_with_carry_engine subtract_with_carry_engine::min subtract_with_carry_engine::discard
subtract_with_carry_engine::operator() subtract_with_carry_engine::max subtract_with_carry_engine::seed
default_seed is a member constant, defined as 19780503u, used as the default parameter value for subtract_with_carry_engine::seed and the single value constructor.

For more information about engine members, see <random>.


The substract_with_carry_engine template class is an improvement over the linear_congruential_engine. Neither for these engines is as fast or with as high quality results as the mersenne_twister_engine.

This engine produces values of a user-specified unsigned integral type using the recurrence relation ( period) x(i) = (x(i - R) - x(i - S) - cy(i - 1)) mod M, where cy(i) has the value 1 if x(i - S) - x(i - R) - cy(i - 1) < 0, otherwise 0, and M has the value 2W. The engine's state is a carry indicator plus R values. These values consist of the last R values returned if operator() has been called at least R times, otherwise the N values that have been returned and the last R - N values of the seed.

The template argument UIntType must be large enough to hold values up to M - 1.

Although you can construct a generator from this engine directly, you can also use one of these predefined typedefs:

ranlux24_base: Used as a base for ranlux24. typedef subtract_with_carry_engine<unsigned int, 24, 10, 24> ranlux24_base;

ranlux48_base: Used as a base for ranlux48. typedef subtract_with_carry_engine<unsigned long long, 48, 5, 12> ranlux48_base;

For detailed information about the subract with carry engine algorithm, see the Wikipedia article Lagged Fibonacci generator.


Header: <random>

Namespace: std

See also