Re: Boost.Random

From: pbristow_at_[hidden]
Date: 2020-06-22 11:59:26

Using 

std::mt19937::result_type(std::chrono::system_clock::now().time_since_epoch().count());

 as seed looks nice and 'portable', assuming C++11.

No need to build Boost.Chrono library - the disadvantage that I was trying to avoid.

But using random_device looks even simpler, again assuming Standard C++11.

  using std::random_device;
  random_device seeder;
  static std::mt19937 generator(seeder());
  std::mt19937::result_type result = generator();
  std::cout << "random generated is " << result << std::endl; // 4033698283 3116091012 ...  different each run.

And showing a little more info

  using std::random_device;
  random_device seeder;
  std::mt19937::result_type seed = seeder();
  std::cout << "seed from seeder() is " << seed << std::endl;
  static std::mt19937 generator(seed);
  std::mt19937::result_type result = generator();
  std::cout << "random generated is " << result << std::endl; // 4033698283 3116091012 ...  different each run.

  std::mt19937::result_type is j
  std::is_same<std::mt19937::result_type, std::uint32_t>::value is true

  seed from seeder() is 2113960109
  random generated is 3830391306

Random zealots seem to have reservations about random_device - and pretty much anything?

All seem to work at C++11 with latest MSVC, Clang and GCC.

(Those poor unfortunates stuck in purgatory of the dark ages of c++98 are stuck with time(0)).

Thanks

Paul

> -----Original Message-----
> From: Boost <boost-bounces_at_[hidden]> On Behalf Of Christopher Kormanyos via Boost
> Sent: 20 June 2020 11:41
> To: Paul A Bristow via Boost <boost_at_[hidden]>
> Cc: Christopher Kormanyos <e_float_at_[hidden]>
> Subject: Re: [boost] Boost.Random
> 
> 
> > So would following the crowd and using time(0)> be simplest?
> I never liked time(0) for that particular use casebecause in the old days it had multiple
> millisecondresolution and lacked the seed resolution sometimes.
> I do not know it the code below is best practice,but I usually use some kind of variation of below.You
> can switch system_clock for high_resolution_clock.
> All this is straight off-the-rack C++11.
> 
> 
> #include <chrono>
> #include <iostream>
> #include <random>
> 
> // Use time point now seed to get a different set of values each time.
> const std::mt19937::result_type seed =
>   std::mt19937::result_type(std::chrono::system_clock::now().time_since_epoch().count());
> 
> // uint32_t
> static std::mt19937 gen(seed);
> 
> int main()
> {
>   std::cout << gen() << std::endl;
> }
> 
> 
> 
>     On Thursday, June 18, 2020, 10:27:27 AM GMT+2, Paul A Bristow via Boost <boost_at_[hidden]>
> wrote:
> 
> 
> 
> > -----Original Message-----
> > From: Boost <boost-bounces_at_[hidden]> On Behalf Of John Maddock
> > via Boost
> > Sent: 17 June 2020 18:48
> > To: Paul A Bristow via Boost <boost_at_[hidden]>
> > Cc: John Maddock <jz.maddock_at_[hidden]>
> > Subject: Re: [boost] Boost.Random
> >
> >
> > On 17/06/2020 15:53, Paul A Bristow via Boost wrote:
> > > I have wanted to use boost::random::random_device; as a seeder for my generator.
> > >
> > > #include <boost/random/random_device.hpp> // For
> > > boost::random::random_device; seeder
> > >
> > > But using this requires that I link to a library file // LINK :
> > > fatal error LNK1104: cannot open file 'libboost_random-vc142-mt-gd-x64-1_73.lib'
> > >
> > > So I have instead used C++ std random device successfully
> > >
> > >    using std::random_device;
> > >    random_device seeder;
> > >    // Use seeder to get a different set of values each time.
> > >    static boost::random::mt19937 gen(seeder()); // uint32_t
> > >
> > > But is there any way I can stick to the Boost version (I imagine
> > > that it might prove more
> portable?
> > > Or is this a delusion?)
> >
> > What do you mean by portable?  random_device is inherently
> > non-portable because it's.... random ;)
> 
> By portable I mean 'works on as many platforms and C++ standard versions as possible'.
> 
> > In many ways this is something that the std:: version does best as the
> > system implementer knows
> best
> > how to implement on their OS.  Or you could just link to Boost.Random
> > of course which would work nearly everywhere too I'm sure.
> 
> I was just puzzled why Boost.Random needed to *link* when std:random_device doesn't  appear to.  Is
> it quietly linking to a standard library?
> 
> Paul
> 
> PS Thanks for the even-more-random suggestions but I really, really don't care how randomly random it
> is for my application.
> 
> So would following the crowd and using time(0) be simplest?
> 
> 
> 
> 
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