# std::lognormal_distribution

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< cpp‎ | numeric‎ | random

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

The lognormal_distribution random number distribution produces random numbers x > 0 according to a Log-normal distribution:

  \({\small f(x;m,s) = \frac{1}{sx\sqrt{2\pi} } \exp{(-\frac{ {(\ln{x} - m)}^{2} }{2{s}^{2} })} }\)f(x; m,s) =

  1
  sx√2 π

  exp⎛
  ⎜
  ⎝-

  (ln x - m)2
  2s2

  ⎞
  ⎟
  ⎠

The parameters m and s are, respectively, the mean and standard deviation of the natural logarithm of x.

`std::lognormal_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 s (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::lognormal_distribution<> d(1.6, 0.25);

        std::map<int, int> hist;
        for (int n = 0; n < 1e4; ++n)
            ++hist[std::round(d(gen))];

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

Possible output:

    2
    3 ***
    4 *************
    5 ***************
    6 *********
    7 ****
    8 *
    9
    a
    b
    c

### External links

Weisstein, Eric W. "Log Normal Distribution." From MathWorld — A Wolfram Web Resource.

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