Re: [Boost-users] incorrect random library equality implementation?

Scott McMurray wrote:

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The std::equal() call is comparing an array of doubles using operator == which I thought was impossible for floating point types due to their inexact nature et al.

A (radix 2) floating-point number will never be exactly equal to one seventh, for example, because it's not representable. The other issue is that roundoff error will make things such as 1./3*3 not result in exactly 1.

That doesn't mean that you can't compare between them for equality. If one float is a copy of another, then they should be bitwise equal. Also, floating-point operations are deterministic, so the same sequence of operations -- in exactly the same environment -- should also yeild bitwise equal results.

What the random streaming operators are doing is saving and restoring the state of the lagged_fibonacci607 object with text. This involves converting the doubles into strings and back again. I'm not certain if ISO C++ specifies whether we are allowed to assume the conversion from double to string and back always yields the identical floating point number or not. Here is a small test program that demonstrates what the random test suite is essentially doing: #include <iostream> #include <sstream> int main() { double pi = 3.14159; double pi2 = pi; if ( pi != pi2 ) { std::cout << "test failed, pi != pi2\n"; return -1; } std::ostringstream file; file << pi; std::string ostr(file.str()); std::istringstream input(file.str()); input >> pi; std::string istr(input.str()); if ( pi != pi2 ) { std::cout << "test failed, restored pi != pi2\n"; } else { std::cout << "passed\n"; } } This test always fails here. --Steven