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

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< cpp‎ | numeric‎ | random
m (fmt)
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{{cpp/title|rand}}
 
{{cpp/title|rand}}
 
{{cpp/numeric/random/navbar}}
 
{{cpp/numeric/random/navbar}}
−
{{ddcl | header=cstdlib |  
+
{{ddcl|header=cstdlib|  
 
int rand();
 
int rand();
 
}}
 
}}
  
−
Returns a pseudo-random integral value between {{c|0}} and {{lc|RAND_MAX}} ({{c|0}} and {{tt|RAND_MAX}} included).
+
Returns a pseudo-random integral value from the range {{closed range|0|RAND_MAX|]}}.
  
 
{{lc|std::srand()}} seeds the pseudo-random number generator used by {{tt|rand()}}.
 
{{lc|std::srand()}} seeds the pseudo-random number generator used by {{tt|rand()}}.
Line 24: Line 24:
 
===Notes===
 
===Notes===
 
There are no guarantees as to the quality of the random sequence produced.
 
There are no guarantees as to the quality of the random sequence produced.
−
In the past, some implementations of {{tt|rand()}} have had serious shortcomings in the randomness, distribution and period of the sequence produced (in one well-known example, the low-order bit simply alternated between {{tt|1}} and {{tt|0}} between calls).
+
In the past, some implementations of {{tt|rand()}} have had serious shortcomings in the randomness, distribution and period of the sequence produced (in one well-known example, the low-order bit simply alternated between {{c|1}} and {{c|0}} between calls).
 +
 
 +
{{tt|rand()}} is not recommended for serious random-number generation needs. {{rev inl|since=c++11|It is recommended to use C++11's [[cpp/numeric/random|random number generation]] facilities to replace {{tt|rand()}}.}}
  
−
{{tt|rand()}} is not recommended for serious random-number generation needs.
 
−
{{rev inl|since=c++11|It is recommended to use C++11's [[cpp/numeric/random|random number generation]] facilities to replace rand().}}
 
 
===Example===
 
===Example===
 
{{example
 
{{example
−
| code=
+
|code=
 
#include <cstdlib>
 
#include <cstdlib>
−
#include <iostream>
 
 
#include <ctime>
 
#include <ctime>
 +
#include <iostream>
  
 
int main()  
 
int main()  
 
{
 
{
 
     std::srand(std::time(nullptr)); // use current time as seed for random generator
 
     std::srand(std::time(nullptr)); // use current time as seed for random generator
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     int random_variable = std::rand();
+
     int random_value = std::rand();
−
     std::cout << "Random value on [0 " << RAND_MAX << "]: "  
+
     std::cout << "Random value on [0, " << RAND_MAX << "]: " << random_value << '\n';
−
              << random_variable << '\n';
+
  
−
     // roll 6-sided dice 20 times
+
     for (const int times = 8; const int sides : {2, 4, 6, 8})
−
    for (int n=0; n != 20; ++n) {
+
    {
−
         int x = 7;
+
        std::cout << "Roll " << sides << "-sided dice " << times << " times: ";
−
        while(x > 6)  
+
        for (int n = 0; n != times; ++n)
−
            x = 1 + std::rand()/((RAND_MAX + 1u)/6); // Note: 1+rand()%6 is biased
+
         {
−
        std::cout << x << ' ';
+
            int x = sides + 1;
 +
            while (x > sides)  
 +
                x = 1 + std::rand() / ((RAND_MAX + 1u) / sides);  
 +
                    // Note: 1 + rand() % sides is biased
 +
            std::cout << x << ' ';
 +
        }
 +
        std::cout << '\n';
 
     }
 
     }
 
}
 
}
−
| p=true
+
|p=true
−
| output=
+
|output=
−
Random value on [0 2147483647]: 726295113
+
Random value on [0, 2147483647]: 948298199
−
6 3 6 2 6 5 6 3 1 1 1 6 6 6 4 1 3 6 4 2
+
Roll 2-sided dice 8 times: 2 2 1 2 1 1 2 2
 +
Roll 4-sided dice 8 times: 1 3 4 2 1 3 3 1
 +
Roll 6-sided dice 8 times: 3 2 1 6 6 4 4 2
 +
Roll 8-sided dice 8 times: 4 5 6 6 3 6 1 2
 
}}
 
}}
  
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/numeric/random/dsc uniform_int_distribution}}
+
{{dsc inc|cpp/numeric/random/dsc uniform_int_distribution}}
−
{{dsc inc | cpp/numeric/random/dsc srand}}
+
{{dsc inc|cpp/numeric/random/dsc srand}}
−
{{dsc inc | cpp/numeric/random/dsc RAND_MAX}}
+
{{dsc inc|cpp/numeric/random/dsc RAND_MAX}}
−
{{dsc inc | cpp/experimental/dsc randint }}
+
{{dsc inc|cpp/experimental/dsc randint}}
−
{{dsc see c | c/numeric/random/rand}}
+
{{dsc see c|c/numeric/random/rand}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Latest revision as of 03:43, 24 April 2024

 
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
 
Defined in header <cstdlib>
int rand();

Returns a pseudo-random integral value from the range [​0​, RAND_MAX].

std::srand() seeds the pseudo-random number generator used by rand(). If rand() is used before any calls to std::srand(), rand() behaves as if it was seeded with std::srand(1).

Each time rand() is seeded with std::srand(), it must produce the same sequence of values on successive calls.

Other functions in the standard library may call rand. It is implementation-defined which functions do so.

It is implementation-defined whether rand() is thread-safe.

Contents

[edit] Parameters

(none)

[edit] Return value

Pseudo-random integral value between ​0​ and RAND_MAX.

[edit] Notes

There are no guarantees as to the quality of the random sequence produced. In the past, some implementations of rand() have had serious shortcomings in the randomness, distribution and period of the sequence produced (in one well-known example, the low-order bit simply alternated between 1 and ​0​ between calls).

rand() is not recommended for serious random-number generation needs. It is recommended to use C++11's random number generation facilities to replace rand().(since C++11)

[edit] Example

Run this code
#include <cstdlib>
#include <ctime>
#include <iostream>
 
int main() 
{
    std::srand(std::time(nullptr)); // use current time as seed for random generator
    int random_value = std::rand();
    std::cout << "Random value on [0, " << RAND_MAX << "]: " << random_value << '\n';
 
    for (const int times = 8; const int sides : {2, 4, 6, 8})
    {
        std::cout << "Roll " << sides << "-sided dice " << times << " times: ";
        for (int n = 0; n != times; ++n)
        {
            int x = sides + 1;
            while (x > sides) 
                x = 1 + std::rand() / ((RAND_MAX + 1u) / sides); 
                    // Note: 1 + rand() % sides is biased
            std::cout << x << ' ';
        }
        std::cout << '\n';
    }
}

Possible output: 

Random value on [0, 2147483647]: 948298199
Roll 2-sided dice 8 times: 2 2 1 2 1 1 2 2 
Roll 4-sided dice 8 times: 1 3 4 2 1 3 3 1 
Roll 6-sided dice 8 times: 3 2 1 6 6 4 4 2 
Roll 8-sided dice 8 times: 4 5 6 6 3 6 1 2

[edit] See also

(C++11)
 produces integer values evenly distributed across a range
(class template) [edit]
seeds pseudo-random number generator
(function) [edit]
maximum possible value generated by std::rand
(macro constant) [edit]
generates a random integer in the specified range
(function template) [edit]
C documentation for rand
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