Re: [boost] [serialization] Serialisation/deserialisation offloating-point values

From: Paul A Bristow (pbristow_at_[hidden])
Date: 2006-03-15 07:13:13

I have also re-tested this on VS 2005 release :-(((

My test and/or memory was obviously faulty.

Using 100 tests with nextafter, starting with the value you found faulty, I
find 38 failures (all one bit wrong on input)  - about the one third I found
in previous tests.

So I confirm my view that this is a Microsoft 'Lack Of Quality Feature'.

To be fair, this is a rather hard problem, though there are proven solutions
which have been proposed, but not widely implemented, as I asked about and
got some info on a recent post.

lists.boost.org/Archives/boost/2006/02/date.php

But I fear that this does NOT help you just now.  Sorry.

I am having a crack at using the Burger and Dvbvig method but have yet to
fully understand it.

Paul

-- 
Paul A Bristow
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| -----Original Message-----
| From: boost-bounces_at_[hidden] 
| [mailto:boost-bounces_at_[hidden]] On Behalf Of Paul Giaccone
| Sent: 15 March 2006 11:06
| To: boost_at_[hidden]
| Subject: Re: [boost] [serialization] 
| Serialisation/deserialisation offloating-point values
| 
| Paul A Bristow wrote:
| 
| >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.
| >
| >	std::stringstream s;
| >	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
| >bit bigger?
| >
| >  
| >
| [...]
| 
| >This should give you a feel for the risk of failure.
| >
| >Paul
| >  
| >
| Funnily enough, I've just written a program to test the value I 
| originally posted about (below, followed by its output):
| 
| #include <string>
| #include <sstream>
| #include <iostream>
| #include <iomanip>
| #include <limits>
| 
| int main(void)
| {
|     const double orig_value = 0.0019075645054089487;
|     std::stringstream stream;
| 
|     double num;
|     stream << std::setprecision(2 + 
| std::numeric_limits<double>::digits 
| * 3030/10000);
|     stream << orig_value;
|     stream >> num;
| 
|     if (num == orig_value)
|     {
|         std::cout << "Match" << std::endl;
|     }
|     else
|     {
|         std::cout << "Deserialisation error" << std::endl;
|         std::cout << std::setprecision(2 + 
| std::numeric_limits<double>::digits * 3030/10000);
|         std::cout << "Original numerical value: " << orig_value << 
| std::endl;
|         std::cout << "Contents of stream: " << stream.str() 
| << std::endl;
|         std::cout << "Deserialised value: " << num << std::endl;
|     }
| 
|     return 0;
| }
| 
| 
| Output:
| 
| Deserialisation error
| Original numerical value: 0.0019075645054089487
| Contents of stream: 0.0019075645054089487
| Deserialised value: 0.0019075645054089489
| 
| 
| This is the same result as in my original thread, so it does 
| indeed look 
| like a Microsoft issue with redirection (>>), not with 
| serialisation itself.
| 
| Paul
| 
| 
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