From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-03-15 05:54:09
If you want to test round-tripping on your platform and std lib without
actually using serialization,
may I suggest a loop including something like:
double a = some start value;
double aa; // to hold the read back.
s.precision(2+std::numeric_limits<double>::digits * 3010/10000);
// cout << "output " << a;
s << a; // output to string s
//cout << ", s.str() is " << s.str();
s >> aa; // read back in.
//cout << ", read back " << aa << endl;
if (a != aa)
cout << "error " << a << tab << aa << endl;
a = nextafter(a, std::numeric_limits<double>::max()); // Make one
of course this may take too long for the full range of possible double! -
some years ;-))
Took overnight for al possible floats on my aging system.
//a *= 10.; // And times 10 too to make test run in reasonable
time. Ran OK 8.0 16 sep 04
This should give you a feel for the risk of failure.
-- Paul A Bristow Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB Phone and SMS text +44 1539 561830, Mobile and SMS text +44 7714 330204 mailto: pbristow_at_[hidden] http://www.hetp.u-net.com/index.html http://www.hetp.u-net.com/Paul%20A%20Bristow%20info.html | -----Original Message----- | From: boost-bounces_at_[hidden] | [mailto:boost-bounces_at_[hidden]] On Behalf Of John Maddock | Sent: 14 March 2006 19:23 | To: boost_at_[hidden] | Subject: Re: [boost] [serialization] | Serialisation/deserialisation offloating-point values | | While it's true that some decimal values have no exact binary | representation | and vice-versa, I believe you *should* be able to write as a | decimal string | and read back in, and get the same value, provided: | | * You write enough digits to the file, | numeric_limits<T>::digits + 2 seems | to be enough, but I wouldn't want to guarantee that. | * You're std lib is bug free: there certainly have been cases | of std lib's | that don't round-trip numbers in this way (I know because | I've reported | these as bugs!), getting round-trip binary-decimal-binary | conversion right | is actually pretty hard. | | The classic "What Every Computer Scientist Should Know About | Floating-Point | Arithmetic" at | http://docs.sun.com/source/806-3568/ncg_goldberg.html fills | in the details: 9 decimal digits are required for single | precision reals, | and 17 for double precision, a formula is also given that | allows you to | check that you have enough decimal digits for some p-digit | binary number. | It's also apparent that reading in a decimal number correctly | requires | extended precision arithmetic, so I suspect most problems are | likely to | occur when serialising the widest floating point type on the | system. Even | Knuth says "leave it to the experts" when discussing binary-decimal | conversion BTW :-) | | HTH, John.
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk