# std::fisher_f_distribution

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< cpp‎ | numeric‎ | random

Defined in header `<random>` |  |
template< class RealType = double > class fisher_f_distribution; |  | (since C++11)

Produces random numbers according to the F-distribution:

  \(P(x;m,n)=\frac{\Gamma{(\frac{m+n}{2})} }{\Gamma{(\frac{m}{2})}\Gamma{(\frac{n}{2})} }{(\frac{m}{n})}^{\frac{m}{2} }x^{\frac{m}{2}-1}{(1+\frac{m}{n}x)}^{-\frac{m+n}{2} }\)P(x;m,n) =

  Γ((m+n)/2)
  Γ(m/2) Γ(n/2)

  (m/n)m/2
  x(m/2)-1
  (1+

  mx
  n

  )-(m+n)/2

\(\small m\)m and \(\small n\)n are the degrees of freedom.

`std::fisher_f_distribution` satisfies all requirements of RandomNumberDistribution.

### Template parameters

RealType | - | The result type generated by the generator. The effect is undefined if this is not one of float, double, or long double.

### Member types

Member type | Definition
`result_type` (C++11) | RealType
`param_type` (C++11) | the type of the parameter set, see RandomNumberDistribution.

### Member functions

(constructor) (C++11) | constructs new distribution (public member function)
reset (C++11) | resets the internal state of the distribution (public member function)
##### Generation
operator() (C++11) | generates the next random number in the distribution (public member function)
##### Characteristics
m n (C++11) | returns the distribution parameters (public member function)
param (C++11) | gets or sets the distribution parameter object (public member function)
min (C++11) | returns the minimum potentially generated value (public member function)
max (C++11) | returns the maximum potentially generated value (public member function)

### Non-member functions

operator==operator!= (C++11)(C++11)(removed in C++20) | compares two distribution objects (function)
operator<<operator>> (C++11) | performs stream input and output on pseudo-random number distribution (function template)

### Example

Run this code

    #include <algorithm>
    #include <cmath>
    #include <iomanip>
    #include <iostream>
    #include <map>
    #include <random>
    #include <vector>

    template<int Height = 5, int BarWidth = 1, int Padding = 1, int Offset = 0, class Seq>
    void draw_vbars(Seq&& s, const bool DrawMinMax = true)
    {
        static_assert(0 < Height and 0 < BarWidth and 0 <= Padding and 0 <= Offset);

        auto cout_n = [](auto&& v, int n = 1)
        {
            while (n-- > 0)
                std::cout << v;
        };

        const auto [min, max] = std::minmax_element(std::cbegin(s), std::cend(s));

        std::vector<std::div_t> qr;
        for (typedef decltype(*std::cbegin(s)) V; V e : s)
            qr.push_back(std::div(std::lerp(V(0), 8 * Height,
                                            (e - *min) / (*max - *min)), 8));

        for (auto h {Height}; h-- > 0; cout_n('\n'))
        {
            cout_n(' ', Offset);

            for (auto dv : qr)
            {
                const auto q {dv.quot}, r {dv.rem};
                unsigned char d[] {0xe2, 0x96, 0x88, 0}; // Full Block: '█'
                q < h ? d[0] = ' ', d[1] = 0 : q == h ? d[2] -= (7 - r) : 0;
                cout_n(d, BarWidth), cout_n(' ', Padding);
            }

            if (DrawMinMax && Height > 1)
                Height - 1 == h ? std::cout << "┬ " << *max:
                              h ? std::cout << "│ "
                                : std::cout << "┴ " << *min;
        }
    }

    int main()
    {
        std::random_device rd {};
        std::mt19937 gen {rd()};

        auto fisher = [&gen](const float d1, const float d2)
        {
            std::fisher_f_distribution<float> d {d1 /* m */, d2 /* n */};

            const int norm = 1'00'00;
            const float cutoff = 0.002f;

            std::map<int, int> hist {};
            for (int n = 0; n != norm; ++n)
                ++hist[std::round(d(gen))];

            std::vector<float> bars;
            std::vector<int> indices;
            for (auto const& [n, p] : hist)
                if (float x = p * (1.0 / norm); cutoff < x)
                {
                    bars.push_back(x);
                    indices.push_back(n);
                }

            std::cout << "d₁ = " << d1 << ", d₂ = " << d2 << ":\n";
            for (draw_vbars<4, 3>(bars); int n : indices)
                std::cout << std::setw(2) << n << "  ";
            std::cout << "\n\n";
        };

        fisher(/* d₁ = */ 1.0f, /* d₂ = */ 5.0f);
        fisher(/* d₁ = */ 15.0f, /* d₂ = */ 10.f);
        fisher(/* d₁ = */ 100.0f, /* d₂ = */ 3.0f);
    }

Possible output:

    d₁ = 1, d₂ = 5:
    ███                                                     ┬ 0.4956
    ███                                                     │
    ███ ▇▇▇                                                 │
    ███ ███ ▇▇▇ ▄▄▄ ▂▂▂ ▂▂▂ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ┴ 0.0021
     0   1   2   3   4   5   6   7   8   9  10  11  12  14

    d₁ = 15, d₂ = 10:
        ███                     ┬ 0.6252
        ███                     │
        ███ ▂▂▂                 │
    ▆▆▆ ███ ███ ▃▃▃ ▁▁▁ ▁▁▁ ▁▁▁ ┴ 0.0023
     0   1   2   3   4   5   6

    d₁ = 100, d₂ = 3:
        ███                                                             ┬ 0.4589
        ███                                                             │
    ▁▁▁ ███ ▅▅▅                                                         │
    ███ ███ ███ ▆▆▆ ▃▃▃ ▂▂▂ ▂▂▂ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ▁▁▁ ┴ 0.0021
     0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16

### External links

Weisstein, Eric W. "F-Distribution." From MathWorld — A Wolfram Web Resource.

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