Re: Copy construction of TR1 random number engines

Gil <>
Wed, 8 Apr 2009 19:47:15 CST
On Apr 8, 2:48 pm, "Chaille" <> wrote:

When I tried out the TR1 random number facility, I stumbled across
something strange.

strange indeed, assert works the other way around i.e.

assert( 0 ); // should abort
assert( "man assert" ); // fine

so when you write things like:

assert(!equal(seq1.begin(), seq1.end(), seq2.begin()));

expect to abort when those sequences _are_ equal.

// Doing it the old-fashioned way.
for (int i=0; i < 1000; i++)
for (int i=0; i < 1000; i++)

you actually insert total different numbers into seq1 and seq2
because you just generate a sequence of 2000 random numbers
(with the same starting seed), even if you use two for loops and
two containers to store them.

// This assert is no problem
assert(!equal(seq1.begin(), seq1.end(), seq2.begin()));

of course it's not because assert's argument expression evaluates
to true;

// 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);

here you generate 2 sequences of 1000 random numbers using same seed.
they should be equal assuming nobody changes eng's seed between the
generate calls.

// This assert fires
assert(!equal(seq1.begin(), seq1.end(), seq2.begin()));

because they are equal.

if you replace in your code the tr1 PRNG with a dummy generator like:

struct DummyEng {
  typedef int result_type;
  result_type operator ()( ) {
    return 518693;

typedef DummyEng Eng;

and just keep the rest as it is you will see that there is no relation
your assertions and <tr1/random> implementation.


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