Difference between revisions of "cpp/numeric/random/random device"
From cppreference.com
(why switch from using random_device as a random number generator (which it is) to using it as a seed?) |
(I think I know what the editor was getting at...) |
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| Line 45: | Line 45: | ||
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)]; | + | ++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) { | for (auto p : hist) { | ||
Revision as of 08:08, 27 February 2014
| 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.
Note that std::random_device may be implemented in terms of a pseudo-random number engine if a non-deterministic source (e.g. a hardware device) is not available to the implementation.
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) |
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 : ********************
