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Difference between revisions of "cpp/numeric/random/random device"

From cppreference.com
< cpp‎ | numeric‎ | random
m (Text replace - "{{tdcl list end" to "{{dcl list end")
m (MinGW -> MinGW-w64)
 
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{{cpp/title|random_device}}
 
{{cpp/title|random_device}}
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{{cpp/numeric/random/random_device/sidebar}}
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{{cpp/numeric/random/random_device/navbar}}
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{{ddcl | header=random | notes={{mark since c++11}} | 1=
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{{ddcl|header=random|since=c++11|1=
 
class random_device;
 
class random_device;
 
}}
 
}}
  
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{{tt|std::random_device}} is a uniformly-distributed integer random number generator, which produces non-deterministic random numbers, if a non-deterministic source (e.g. a hardware device) is available to the implementation.
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{{tt|std::random_device}} is a uniformly-distributed integer random number generator that produces non-deterministic random numbers.
 +
 
 +
{{tt|std::random_device}} may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation. In this case each {{ttt|std::random_device}} object may generate the same number sequence.
  
 
===Member types===
 
===Member types===
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{{dcl list begin}}
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{{dsc begin}}
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{{tdcl list hitem | Member type | Definition}}
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{{dsc hitem|Member type|Definition}}
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{{tdcl list item | {{tt|result_type}} | {{c|unsigned int}}}}
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{{dsc|{{tt|result_type}} {{mark c++11}}|{{co|unsigned int}}}}
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{{dcl list end}}
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{{dsc end}}
  
 
===Member functions===
 
===Member functions===
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{{dcl list begin}}
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{{dsc begin}}
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{{dcl list h2 | Construction}}
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{{dsc h2|Construction}}
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{{dcl list template | cpp/numeric/random/engine/dcl list constructor |random_device}}
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{{dsc inc|cpp/numeric/random/engine/dsc constructor|random_device}}
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{{dcl list mem fun | nolink=true | operator{{=}} | notes={{mark|deleted}} | the assignment operator is deleted}}
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{{dsc mem fun|nolink=true|operator{{=}}|notes={{mark|deleted}} {{mark c++11}}|the assignment operator is deleted}}
  
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{{dcl list h2 | Generation}}
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{{dsc h2|Generation}}
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{{dcl list template | cpp/numeric/random/engine/dcl list operator() | random_device}}
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{{dsc inc|cpp/numeric/random/engine/dsc operator()|random_device}}
  
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{{dcl list h2 | Characteristics}}
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{{dsc h2|Characteristics}}
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{{dcl list template | cpp/numeric/random/random_device/dcl list entropy}}
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{{dsc inc|cpp/numeric/random/random_device/dsc entropy}}
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{{dcl list template | cpp/numeric/random/engine/dcl list min |random_device}}
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{{dsc inc|cpp/numeric/random/engine/dsc min|random_device}}
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{{dcl list template | cpp/numeric/random/engine/dcl list max |random_device}}
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{{dsc inc|cpp/numeric/random/engine/dsc max|random_device}}
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{{dcl list end}}
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{{dsc end}}
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 +
===Notes===
 +
A notable implementation where {{ttt|std::random_device}} is deterministic in old versions of MinGW-w64 ([https://sourceforge.net/p/mingw-w64/bugs/338/ bug 338], fixed since GCC 9.2). The latest MinGW-w64 versions can be downloaded from [https://gcc-mcf.lhmouse.com/ GCC with the MCF thread model].
  
 
===Example===
 
===Example===
 
{{example
 
{{example
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| code=
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|code=
 
#include <iostream>
 
#include <iostream>
−
#include <string>
 
 
#include <map>
 
#include <map>
 
#include <random>
 
#include <random>
 +
#include <string>
 +
 
int main()
 
int main()
 
{
 
{
 
     std::random_device rd;
 
     std::random_device rd;
 
     std::map<int, int> hist;
 
     std::map<int, int> hist;
−
     for(int n=0; n<20000; ++n)
+
    std::uniform_int_distribution<int> dist(0, 9);
−
         ++hist[rd()%10];
+
 
−
     for(auto p : hist)
+
     for (int n = 0; n != 20000; ++n)
−
         std::cout << p.first << " : " << std::string(p.second/100, '*') << '\n';
+
         ++hist[dist(rd)]; // note: demo only: the performance of many
 +
                          // implementations of random_device degrades sharply
 +
                          // once the entropy pool is exhausted. For practical use
 +
                          // random_device is generally only used to seed
 +
                          // a PRNG such as mt19937
 +
 
 +
     for (auto [x, y] : hist)
 +
         std::cout << x << " : " << std::string(y / 100, '*') << '\n';
 
}
 
}
−
| output=
+
|p=true
 +
|output=
 
0 : ********************
 
0 : ********************
 
1 : *******************
 
1 : *******************
Line 56: Line 70:
 
9 : ********************
 
9 : ********************
 
}}
 
}}
 +
 +
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 21:34, 20 November 2023

 
C++
 
Numerics library
Common mathematical functions
Mathematical special functions (C++17)
Mathematical constants (C++20)
Basic linear algebra algorithms (C++26)
Floating-point environment (C++11)
Complex numbers
Numeric array (valarray)
Pseudo-random number generation
Factor operations
(C++17)
(C++17)
Interpolations
(C++20)
(C++20)
Saturation arithmetic
(C++26)
(C++26)
(C++26)
(C++26)
(C++26)

Generic numeric operations
Bit operations
(C++20)    
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++23)
(C++20)
 
Pseudo-random number generation
 
std::random_device
 
Defined in header <random>
class random_device;
 (since C++11)

std::random_device is a uniformly-distributed integer random number generator that produces non-deterministic random numbers.

std::random_device may be implemented in terms of an implementation-defined pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation. In this case each std::random_device object may generate the same number sequence.

Contents

[edit] Member types

Member type Definition
result_type (C++11) unsigned int

[edit] Member functions

Construction
constructs the engine
(public member function) [edit]
operator=
(deleted) (C++11)
 the assignment operator is deleted
(public member function)
Generation
advances the engine's state and returns the generated value
(public member function) [edit]
Characteristics
(C++11)
 obtains the entropy estimate for the non-deterministic random number generator
(public member function) [edit]
[static]
 gets the smallest possible value in the output range
(public static member function) [edit]
[static]
 gets the largest possible value in the output range
(public static member function) [edit]

[edit] Notes

A notable implementation where std::random_device is deterministic in old versions of MinGW-w64 (bug 338, fixed since GCC 9.2). The latest MinGW-w64 versions can be downloaded from GCC with the MCF thread model.

[edit] Example

Run this code
#include <iostream>
#include <map>
#include <random>
#include <string>
 
int main()
{
    std::random_device rd;
    std::map<int, int> hist;
    std::uniform_int_distribution<int> dist(0, 9);
 
    for (int n = 0; n != 20000; ++n)
        ++hist[dist(rd)]; // note: demo only: the performance of many
                          // implementations of random_device degrades sharply
                          // once the entropy pool is exhausted. For practical use
                          // random_device is generally only used to seed
                          // a PRNG such as mt19937
 
    for (auto [x, y] : hist)
        std::cout << x << " : " << std::string(y / 100, '*') << '\n';
}

Possible output: 

0 : ********************
1 : *******************
2 : ********************
3 : ********************
4 : ********************
5 : *******************
6 : ********************
7 : ********************
8 : *******************
9 : ********************
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