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# std::lognormal_distribution

From cppreference.com
< 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:

  f(x; m,s) =

  1
  sx√2 π

  exp⎛
  ⎜
  ⎝-

  (ln x - m)2
  2s2

  ⎞
  ⎟
  ⎠

The parameter m is the mean and the parameter s the standard deviation.

`std::lognormal_distribution` satisfies all requirements of `RandomNumberDistribution`

### [edit] 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.

### [edit] Member types

Member type | Definition
`result_type` | RealType
`param_type` | the type of the parameter set, see `RandomNumberDistribution`.

### [edit] Member functions

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

### [edit] Non-member functions

operator==operator!= | compares two distribution objects (function) [edit]
operator<<operator>> | performs stream input and output on pseudo-random number distribution (function template) [edit]

### [edit] Example

Run this code

    #include <iostream>
    #include <iomanip>
    #include <string>
    #include <map>
    #include <random>
    #include <cmath>
    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<10000; ++n) {
            ++hist[std::round(d(gen))];
        }
        for(auto p : hist) {
            std::cout << std::fixed << std::setprecision(1) << std::setw(2)
                      << p.first << ' ' << std::string(p.second/200, '*') << '\n';
        }
    }

Output:

    2
     3 ***
     4 *************
     5 ***************
     6 *********
     7 ****
     8 *
     9
    10
    11
    12

### [edit] External links

- Weisstein, Eric W. "Log Normal Distribution." From MathWorld--A Wolfram Web Resource.
- Log-normal distribution. From Wikipedia.

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