Class: Random::Binomial

Inherits:
Object
  • Object
show all
Defined in:
ext/binomial.c

Instance Method Summary (collapse)

Constructor Details

- (Random::Binomial) initialize(rng, n, theta)

Returns a random sampler from a binomial distribution.

This sampler uses table plus square histgram method with Robin Hoot method. This method constructs a table for each distribution. If you once construct the table, you can draw a random sample fast for large n (n >= 40), but the cost of the table construction is expensive. Therefore, if you need to draw many samples from the same binomial distribution, you had better to use this class. Otherwise, you should use Random#binomial.

Parameters:

  • rng (Random) —

    a random number generator

  • n (Integer) —

    the number of trials (n > 0)

  • theta (Float) —

    success probability (0 < theta < 1) 



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# File 'ext/binomial.c', line 218

static VALUE binomial_initialize(VALUE self, VALUE rng, VALUE num, VALUE prob)
{
  binomial_t *bin;
  int n = NUM2INT(num);
  double theta = NUM2DBL(prob);

  check_binomial_params(n, theta, "Random::Binomial.new");
  
  rb_iv_set(self, "rng", rng);
  Data_Get_Struct(self, binomial_t, bin);
  bin->n = n;
  bin->theta = theta;
  bin->p = ALLOC_N(double, bin->n + 1);

  fill_binomial_table(bin);
  fill_binomial_T_VT_table(bin);
  
  return Qnil;
}

Instance Method Details

- (Integer) n

Returns the parameter n, the number of trials.

Returns:

  • (Integer) —

    the parameter n 



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# File 'ext/binomial.c', line 272

static VALUE binomial_n(VALUE self)
{
  binomial_t *bin;
  Data_Get_Struct(self, binomial_t, bin);

  return INT2NUM(bin->n);
}

- (Integer) rand

Draws a sample from the binomimial distribution whose parameters are specified in Random::Binomial.new.

Returns:

  • (Integer) —

    a random sample 



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# File 'ext/binomial.c', line 244

static VALUE binomial_rand(VALUE self)
{
  VALUE rng = rb_iv_get(self, "rng");
  binomial_t *bin;
  uint32_t I0 = rb_random_int32(rng);
  uint32_t ILk;
  double U;
  int J;

  Data_Get_Struct(self, binomial_t, bin);
  
  ILk = I0 & MASK(bin->k);
  if (bin->T[ILk] >= 0)
    return INT2NUM(bin->T[ILk]);
  
  U = rb_random_real(rng);
  J = floor((bin->n + 1)*U);
  if (U < bin->V[J])
    return INT2NUM(J);
  else
    return INT2NUM(bin->K[J]);
}

- (Float) theta

Returns the parameter theta, the success probability.

Returns:

  • (Float) —

    the parameter theta 



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# File 'ext/binomial.c', line 285

static VALUE binomial_theta(VALUE self)
{
  binomial_t *bin;
  Data_Get_Struct(self, binomial_t, bin);

  return DBL2NUM(bin->theta);
}