Re: Copy construction of TR1 random number engines
On 2009-04-08 08:48:04 -0400, "Chaille" <nahoj1@karlfeldt.com> said:
When I tried out the TR1 random number facility, I stumbled across
something strange. Trying to create two low-correlation sequences gives
different results depending on how I do it! It seems that the cause for
the
problem is that the copy constructors for the random number engines do
actually not create copies holding the identical state, but initializes
the
"copies" with the same initial seed.
These are pseudo-random number generators, so the sequence is
pre-determined. A copy of a generator generates the same sequence as
the generator. It's not a matter of state versus seed; the seed
determines the initial state, so copying either one gives you the same
result (and in the case of minstd_rand, the two are the same thing).
And that's a good thing, because if you can't reproduce a given
sequence it's much harder to debug an application that's not doing what
you want.
Does anybody know if this way of doing copy construction is according to
standard, unspecified in the standard, or a bug in my TR1-implementation
(I'm running Visual C++ 2008 Express, which I guess is using a version of
the Dinkumware standard library)?
Here's the code that's trigging the strange behavior:
----8<----
// Feel free to change engine; the result seems to remain
typedef ::std::tr1::minstd_rand Eng;
typedef vector<Eng::result_type> Seq;
Eng eng;
Seq seq1, seq2;
// Doing it the old-fashioned way.
for (int i=0; i < 1000; i++)
seq1.push_back(eng());
for (int i=0; i < 1000; i++)
seq2.push_back(eng());
// This assert is no problem
assert(!equal(seq1.begin(), seq1.end(), seq2.begin()));
seq1.clear();
seq2.clear();
// Using the generate_n algorithm, which causes copy construction
// of the generator.
::std::generate_n(::std::back_insert_iterator<Seq>(seq1), 1000, eng);
::std::generate_n(::std::back_insert_iterator<Seq>(seq2), 1000, eng);
// This assert fires
assert(!equal(seq1.begin(), seq1.end(), seq2.begin()));
----8<----
Yup. And if you run eng() you'll get the same sequence, too.
--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)
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