Difference between revisions of "cpp/numeric/random/random device"
From cppreference.com
m (→Example: emphasized the spread of "uniform distribution") |
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#include <iostream> | #include <iostream> | ||
#include <string> | #include <string> | ||
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std::uniform_int_distribution<int> dist(0, 9); | std::uniform_int_distribution<int> dist(0, 9); | ||
for (int n = 0; n < 20000; ++n) { | for (int n = 0; n < 20000; ++n) { | ||
| − | ++hist[dist(rd)]; // note: demo only: the performance of many | + | ++hist[dist(rd)]; // note: demo only: the performance of many |
// implementations of random_device degrades sharply | // implementations of random_device degrades sharply | ||
// once the entropy pool is exhausted. For practical use | // once the entropy pool is exhausted. For practical use | ||
| − | // random_device is generally only used to seed | + | // random_device is generally only used to seed |
// a PRNG such as mt19937 | // a PRNG such as mt19937 | ||
} | } | ||
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for (auto p : hist) { | for (auto p : hist) { | ||
| − | std::cout << p.first << " : " << std::string | + | std::cout << p.first << " : " << std::string(p.second/100, '*') << '\n'; |
} | } | ||
} | } | ||
| output= | | output= | ||
| − | + | 0 : ******************** | |
| − | 0 : ****** | + | 1 : ******************* |
| − | + | 2 : ******************** | |
| − | + | 3 : ******************** | |
| − | + | 4 : ******************** | |
| − | + | 5 : ******************* | |
| − | + | 6 : ******************** | |
| − | 6 : ******* | + | 7 : ******************** |
| − | + | 8 : ******************* | |
| − | 8 : | + | 9 : ******************** |
| − | 9 : | + | |
}} | }} | ||
{{langlinks|de|es|fr|it|ja|pt|ru|zh}} | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | ||
Revision as of 20:49, 7 December 2020
| 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 |
Member types
| Member type | Definition |
result_type
|
unsigned int |
Member functions
Construction | |
| constructs the engine (public member function) | |
| operator= (deleted) |
the assignment operator is deleted (public member function) |
Generation | |
| advances the engine's state and returns the generated value (public member function) | |
Characteristics | |
| (C++11) |
obtains the entropy estimate for the non-deterministic random number generator (public member function) |
| [static] |
gets the smallest possible value in the output range (public static member function) |
| [static] |
gets the largest possible value in the output range (public static member function) |
Notes
A notable implementation where std::random_device is deterministic is old versions of MinGW (bug 338, fixed since GCC 9.2). The latest MinGW Versions can be downloaded from GCC with the MCF thread model.
Example
Run this code
#include <iostream> #include <string> #include <map> #include <random> 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 p : hist) { std::cout << p.first << " : " << std::string(p.second/100, '*') << '\n'; } }
Possible output:
0 : ******************** 1 : ******************* 2 : ******************** 3 : ******************** 4 : ******************** 5 : ******************* 6 : ******************** 7 : ******************** 8 : ******************* 9 : ********************
