srand() frequency

a10e29 (83)
How often should someone call srand()? Is it worth it to call it more than once? For example before each call to rand() or after x time.

Thanks for your time
Peter87 (11256)
You should only call it once.
PanGalactic (1658)
I would urge you to consider using <random> if you are using modern C++, or boost::random if you are not, in place of srand()/rand(). They are a little more complex, but they can be used in a thread-safe manner (one generator per thread).
Duthomhas (13290)
It is an easy replace to use <random> too...

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#include <iostream> #include <cstdlib> #include <ctime> void print_n_random_numbers( int n ) { while (n--) std::cout << rand() << "\n"; } int main() { srand( time( nullptr ) ); print_n_random_numbers( 10 ); }


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#include <iostream> #include <chrono> #include <random> unsigned get_random( unsigned min, unsigned max ) { thread_local std::ranlux48 prng( std::chrono::system_clock::now().time_since_epoch().count() ); std::uniform_int_distribution <unsigned> dist( min, max ); return dist( prng ); } unsigned get_random( unsigned range ) { return range ? get_random( 0, range - 1 ) : 0; } unsigned get_random() { return get_random( 0, ~0U ); } void print_n_random_numbers( int n ) { while (n--) std::cout << get_random() << "\n"; } int main() { print_n_random_numbers( 10 ); }

The C++ version is cleaner and more useful.
a10e29 (83)
Thank you for your answers. I'll give <random> a try.
jonnin (11497)
as a side note, you can use these things as a hash function, reseed off each record's key and get the first value. That will always give you the same value for the same key. This has limited usefulness give <map> but it is occasionally handy if you don't feel like writing a real hash function.

I can't think of any other reason to re-seed for most applications. Only if you want a new stream that you can control (know it will be the same sequence for debugging or other reasons) do you need to re-seed.

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