# std::piecewise_constant_distribution

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< cpp‎ | numeric‎ | random

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

`std::piecewise_constant_distribution` produces random floating-point numbers, which are uniformly distributed within each of the several subintervals [b
i, b
i+1), each with its own weight w
i. The set of interval boundaries and the set of weights are the parameters of this distribution.

The probability density for any b
i ≤ x <b
i+1 is

w i
S (b i+1 - b i)

, where S is the sum of all weights.

`std::piecewise_constant_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
intervals densities | 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 <iostream>
    #include <map>
    #include <random>
    #include <string>

    int main()
    {
        std::random_device rd;
        std::mt19937 gen(rd());
        // 50% of the time, generate a random number between 0 and 1
        // 50% of the time, generate a random number between 10 and 15
        std::vector<double> i {0, 1, 10, 15};
        std::vector<double> w {1, 0, 1};
        std::piecewise_constant_distribution<> d(i.begin(), i.end(), w.begin());

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

        for (std::cout << std::hex << std::uppercase; auto [x, y] : hist)
            std::cout << x << ' ' << std::string(y / 100, '*') << '\n';
    }

Possible output:

    0 **************************************************
    A **********
    B *********
    C *********
    D **********
    E *********

### References

- C++23 standard (ISO/IEC 14882:2023):

  - 28.5.9.6.2 Class template piecewise_constant_distribution [rand.dist.samp.pconst] (p: 1421-1422)

- C++20 standard (ISO/IEC 14882:2020):

  - 29.6.9.6.2 Class template piecewise_constant_distribution [rand.dist.samp.pconst] (p: 1207-1208)

- C++17 standard (ISO/IEC 14882:2017):

  - 29.6.8.6.2 Class template piecewise_constant_distribution [rand.dist.samp.pconst] (p: 1098-1100)

- C++14 standard (ISO/IEC 14882:2014):

  - 26.5.8.6.2 Class template piecewise_constant_distribution [rand.dist.samp.pconst] (p: 962-964)

- C++11 standard (ISO/IEC 14882:2011):

  - 26.5.8.6.2 Class template piecewise_constant_distribution [rand.dist.samp.pconst] (p: 955-957)

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