Boost Random Generators returning identical values

Question

WITHOUT USING C++ 11 RANDOM

Looking for a boost random expert... I need to generate random numbers in between many, many different ranges. I've written the below functions:

#include <iostream> #include <boost/random.hpp> #include <boost/generator_iterator.hpp> boost::mt19937 g_oRng; int generateIntVariate(int p_iMin, int p_iMax){ boost::uniform_int<> min_to_max(p_iMin, p_iMax); boost::variate_generator< boost::mt19937, boost::uniform_int<> > oGen(g_oRng, min_to_max); return oGen(); } float generateFloatVariate(int p_fMin, p_fMax){ boost::uniform_real<> min_to_max(p_fMin, p_fMax); boost::variate_generator< boost::mt19937, boost::uniform_real<> > oGen(m_oRng, min_to_max); return oGen(); } int main(){ struct timeval tp; gettimeofday(&tp, NULL); g_oRng = boost::mt19937((int)tp.tv_sec); for(int i = 0 ; i < 10 ; ++i){ std::cout << generateIntVariate(0, 10) << ", " << generateFloatVariate(0.0f, 10.0f) << std::endl; } }

The problem is that both functions return the same exact number for a given range, every time it's executed.

(gdb) p generateIntVariate(0, 10) $40 = 8 (gdb) p generateIntVariate(0, 10) $41 = 8 (gdb) p generateIntVariate(0, 10) $42 = 8 (gdb) p generateIntVariate(0, 10) $43 = 8

The same thing as above happens with the float function. Is there any way I can accomplish what I'm trying to do using the boost random distros?

Can you give us a minimal reproducible example? Right from the start I see your not seeding the PRNG so you will always get the same sequence. — NathanOliver
– NathanOliver, Commented Jun 27, 2017 at 19:03
@NathanOliver I am actually, it's just not shown here. I seed it with gettimeofday's tv_sec — Riptyde4
– Riptyde4, Commented Jun 27, 2017 at 19:04
note that there's no need to use boost here, C++ bascially has exactly the same random functions — The Techel
– The Techel, Commented Jun 27, 2017 at 19:04
@Riptyde4 Still waiting for you to fix the missing parameter type and missing : in boost:mt19937. Why are you doing this? — LogicStuff
– LogicStuff, Commented Jun 27, 2017 at 19:27

LogicStuff · Accepted Answer · 2017-06-27 19:18:14Z

boost::variate_generator's constructor looks like this:

variate_generator(Engine e, Distribution d);

- meaning it won't change the state of passed-in m_oRng and makes a copy of it instead.

I suggest omitting variate_generator and just using boost::uniform_int<>::operator().

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