26 Numerics library [numerics]

26.6 Random number generation [rand]

26.6.8 Random number distribution class templates [rand.dist]

26.6.8.6 Sampling distributions [rand.dist.samp]

26.6.8.6.1 Class template discrete_­distribution [rand.dist.samp.discrete]

A discrete_­distribution random number distribution produces random integers i, , distributed according to the discrete probability function
Unless specified otherwise, the distribution parameters are calculated as: for , in which the values , commonly known as the weights, shall be non-negative, non-NaN, and non-infinity.
Moreover, the following relation shall hold: .
template<class IntType = int>
  class discrete_distribution {
  public:
    // types
    using result_type = IntType;
    using param_type  = unspecified;

    // constructor and reset functions
    discrete_distribution();
    template<class InputIterator>
      discrete_distribution(InputIterator firstW, InputIterator lastW);
    discrete_distribution(initializer_list<double> wl);
    template<class UnaryOperation>
      discrete_distribution(size_t nw, double xmin, double xmax, UnaryOperation fw);
    explicit discrete_distribution(const param_type& parm);
    void reset();

    // generating functions
    template<class URBG>
      result_type operator()(URBG& g);
    template<class URBG>
      result_type operator()(URBG& g, const param_type& parm);

    // property functions
    vector<double> probabilities() const;
    param_type param() const;
    void param(const param_type& parm);
    result_type min() const;
    result_type max() const;
  };
discrete_distribution();
Effects: Constructs a discrete_­distribution object with and .
[Note:
Such an object will always deliver the value 0.
— end note]
template<class InputIterator> discrete_distribution(InputIterator firstW, InputIterator lastW);
Mandates: is_­convertible_­v<iterator_­traits<InputIterator>​::​value_­type, double> is true.
Preconditions: InputIterator meets the Cpp17InputIterator requirements ([input.iterators]).
If firstW == lastW, let and .
Otherwise, forms a sequence w of length .
Effects: Constructs a discrete_­distribution object with probabilities given by the formula above.
discrete_distribution(initializer_list<double> wl);
Effects: Same as discrete_­distribution(wl.begin(), wl.end()).
template<class UnaryOperation> discrete_distribution(size_t nw, double xmin, double xmax, UnaryOperation fw);
Mandates: is_­invocable_­r_­v<double, UnaryOperation&, double> is true.
Preconditions: If , let , otherwise let .
The relation holds.
Effects: Constructs a discrete_­distribution object with probabilities given by the formula above, using the following values: If , let .
Otherwise, let for .
Complexity: The number of invocations of fw does not exceed n.
vector<double> probabilities() const;
Returns: A vector<double> whose size member returns n and whose operator[] member returns when invoked with argument k for .