# std::weibull_distribution

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< cpp‎ | numeric‎ | random

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

The `weibull_distribution` meets the requirements of a RandomNumberDistribution and produces random numbers according to the Weibull distribution:

  \(\small{f(x;a,b)=\frac{a}{b}{(\frac{x}{b})}^{a-1}\exp{(-{(\frac{x}{b})}^{a})} }\)f(x;a,b) =

  a
  b

  ⎛
  ⎜
  ⎝

  x
  b

  ⎞
  ⎟
  ⎠a-1
  exp⎛
  ⎜
  ⎝-⎛
  ⎜
  ⎝

  x
  b

  ⎞
  ⎟
  ⎠a
  ⎞
  ⎟
  ⎠

a is the shape parameter and b the scale parameter.

`std::weibull_distribution` satisfies 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
a b (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 <cmath>
    #include <iomanip>
    #include <iostream>
    #include <map>
    #include <random>
    #include <string>

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

        std::weibull_distribution<> d;

        std::map<int, int> hist;
        for (int n = 0; n != 10000; ++n)
            ++hist[std::round(d(gen))];

        std::cout << std::fixed << std::setprecision(1) << std::hex;
        for (auto [x, y]: hist)
            std::cout << x << ' ' << std::string(y / 200, '*') << '\n';
    }

Possible output:

    0 *******************
    1 *******************
    2 ******
    3 **
    4
    5
    6
    7
    8

### External links

1. | Weisstein, Eric W. "Weibull Distribution." From MathWorld — A Wolfram Web Resource.
2. | Weibull distribution — From Wikipedia.

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