Difference between revisions of "cpp/numeric/random/srand"
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{{cpp/title|srand}} | {{cpp/title|srand}} | ||
{{cpp/numeric/random/navbar}} | {{cpp/numeric/random/navbar}} | ||
| − | {{ddcl | header=cstdlib | | + | {{ddcl|header=cstdlib| |
void srand( unsigned seed ); | void srand( unsigned seed ); | ||
}} | }} | ||
| − | Seeds the pseudo-random number generator used by {{lc|rand()}} with the value {{tt|seed}}. | + | Seeds the pseudo-random number generator used by {{lc|std::rand()}} with the value {{tt|seed}}. |
| − | If {{ | + | If {{lc|std::rand()}} is used before any calls to {{tt|srand()}}, {{lc|std::rand()}} behaves as if it was seeded with {{c|srand(1)}}. |
| − | Each time {{ | + | |
| + | Each time {{lc|std::rand()}} is seeded with the same {{tt|seed}}, it must produce the same sequence of values. | ||
{{tt|srand()}} is not guaranteed to be thread-safe. | {{tt|srand()}} is not guaranteed to be thread-safe. | ||
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===Parameters=== | ===Parameters=== | ||
{{par begin}} | {{par begin}} | ||
| − | {{par | seed | the seed value}} | + | {{par|seed|the seed value}} |
{{par end}} | {{par end}} | ||
| Line 21: | Line 22: | ||
===Notes=== | ===Notes=== | ||
| − | Generally speaking, the pseudo-random number generator should only be seeded once, before any calls to {{tt|rand()}}, | + | Generally speaking, the pseudo-random number generator should only be seeded once, before any calls to {{tt|rand()}}, at the start of the program. |
It should not be repeatedly seeded, or reseeded every time you wish to generate a new batch of pseudo-random numbers. | It should not be repeatedly seeded, or reseeded every time you wish to generate a new batch of pseudo-random numbers. | ||
| − | Standard practice is to use the result of a call to {{c|time(0)}} as the seed. | + | Standard practice is to use the result of a call to {{c|std::time(0)}} as the seed. |
| − | However, {{ | + | However, {{lc|std::time}} returns a {{c|std::time_t}} value, and {{c|std::time_t}} is not guaranteed to be an integral type. |
| − | In practice, though, every major implementation defines {{ | + | In practice, though, every major implementation defines {{c|std::time_t}} to be an integral type, and this is also what POSIX requires. |
===Example=== | ===Example=== | ||
{{example | {{example | ||
| − | + | |code= | |
| − | + | ||
#include <cstdlib> | #include <cstdlib> | ||
| − | |||
#include <ctime> | #include <ctime> | ||
| + | #include <iostream> | ||
int main() | int main() | ||
{ | { | ||
| − | std::srand(std::time(0)); //use current time as seed for random generator | + | std::srand(std::time(0)); // use current time as seed for random generator |
| − | + | std::cout << "Random value on [0, " << RAND_MAX << "]: " << std::rand() << '\n'; | |
| − | std::cout << "Random value on [0 " << RAND_MAX << "]: " | + | |
| − | + | ||
} | } | ||
| − | + | |p=true | |
| − | Random value on [0 2147483647]: 1373858591 | + | |output= |
| + | Random value on [0, 2147483647]: 1373858591 | ||
}} | }} | ||
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{{dsc begin}} | {{dsc begin}} | ||
| − | {{dsc inc | cpp/numeric/random/dsc rand}} | + | {{dsc inc|cpp/numeric/random/dsc rand}} |
| − | {{dsc see c | c/numeric/random/srand}} | + | {{dsc inc|cpp/numeric/random/dsc RAND_MAX}} |
| + | {{dsc inc|cpp/experimental/dsc reseed }} | ||
| + | {{dsc see c|c/numeric/random/srand}} | ||
{{dsc end}} | {{dsc end}} | ||
| − | + | {{langlinks|de|es|fr|it|ja|pt|ru|zh}} | |
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Latest revision as of 04:22, 6 May 2023
| Defined in header <cstdlib>
|
||
| void srand( unsigned seed ); |
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Seeds the pseudo-random number generator used by std::rand() with the value seed.
If std::rand() is used before any calls to srand(), std::rand() behaves as if it was seeded with srand(1).
Each time std::rand() is seeded with the same seed, it must produce the same sequence of values.
srand() is not guaranteed to be thread-safe.
Contents |
[edit] Parameters
| seed | - | the seed value |
[edit] Return value
(none)
[edit] Notes
Generally speaking, the pseudo-random number generator should only be seeded once, before any calls to rand(), at the start of the program.
It should not be repeatedly seeded, or reseeded every time you wish to generate a new batch of pseudo-random numbers.
Standard practice is to use the result of a call to std::time(0) as the seed. However, std::time returns a std::time_t value, and std::time_t is not guaranteed to be an integral type. In practice, though, every major implementation defines std::time_t to be an integral type, and this is also what POSIX requires.
[edit] Example
Run this code
Possible output:
Random value on [0, 2147483647]: 1373858591
[edit] See also
| generates a pseudo-random number (function) | |
| maximum possible value generated by std::rand (macro constant) | |
| reseeds the per-thread random engine (function) | |
| C documentation for srand
| |
