Why is the sequence produced by this random number generator always the same? [duplicate]

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Possible Duplicate:
Why do I always get the same sequence of random numbers with rand()?

I tried to implement the following generator class for random integers from a given range:

class RandomInteger { protected: std::default_random_engine randomEngine; std::uniform_int_distribution<> distribution; public: RandomInteger(int64_t lower, int64_t upper); virtual ~RandomInteger(); virtual int64_t generate(); }; RandomInteger::RandomInteger(int64_t lower, int64_t upper) : distribution(lower, upper) { } RandomInteger::~RandomInteger() { // TODO Auto-generated destructor stub } int64_t RandomInteger::generate() { int64_t i = this->distribution(this->randomEngine); return i; }

It produces integers in the range, BUT the sequence of values it produces is the same each time - not very random. Why?

@honk I'm guessing there isn't any, which would be the problem. — Mark Ransom
– Mark Ransom, Commented Jan 10, 2013 at 18:12
// TODO Auto-generated destructor stub Whatever is auto-generating that for you is doing you quite a disservice... — ildjarn
– ildjarn, Commented Jan 16, 2013 at 19:14
@ildjarn This is Eclipse CDT. Why exactly is it a disservice? — clstaudt
– clstaudt, Commented Jan 16, 2013 at 22:10
Because having a user-declared destructor changes the triviality of your type. Never declare special members when the compiler-generated implementation will do. — ildjarn
– ildjarn, Commented Jan 16, 2013 at 22:15

Community · Accepted Answer · 2017-05-23 12:12:34Z

2

in general a pseudo-random generator takes a seed.

if you never change the seed, you get always the same pseudo-random output/sequence.

edit: I insist on the word pseudo-random (somtimes pseudo-chaos) instead of the abusivily used random

edit2: there should be the c++11 solution to you problem in this other question (look at @R. Martinho Fernandes 's answer)

edited May 23, 2017 at 12:12

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answered Jan 10, 2013 at 18:14

Stephane Rolland

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13 Comments

clstaudt Over a year ago

Thanks for the only constructive comment. The "exact duplicate" does not give a C++11 solution.

Stephane Rolland Over a year ago

There are interesting links on the quoted question however: stackoverflow.com/questions/1108780/…

Benjamin Bannier Over a year ago

@StephaneRolland I also liked stackoverflow.com/questions/7217791/random-numbers-in-c0x?rq=1, but I agree with Griwes that OP just didn't think of the seed.

Mark Ransom Over a year ago

"Pseudo-chaotic" returns 4,890 results from Google while "pseudo-random" returns 1,980,000. I honestly had never heard of it before, thanks for introducing me to something new.

Stephane Rolland Over a year ago

"pseudochaotique" returns 17000 from Google. I most probably have this bias because what I read was in french. It was with Iannis Xenakis about his Stochastic Synthesis, and with french books about Chaos Theory... As a consequence, in english it's probably more correct to say pseudo-random. ;-). I re-edit :-)

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