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Re: floating point conversions && how to read standards

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Stanley Rice Wrote:

> Hi all, I got confused while referencing to the 4.8/1 in draft n3242.
> 
> 4.8 Floating point conversions
> A prvalue of floating point type can be converted to a prvalue of
> another floating point type. If the source value can be exactly
> represented in the destination type, the result of the conversion is
> that exactly representation. If the source value is between two
> adjacent destination values, the result of the conversion is an
> implementation-defined choice of either of those values. Otherwise,
> the behavior is undefined.
> 
> I do not exactly know what the 'destination type' and 'two adjacent
> destination value' mean. Could you explain to me and show me some
> example if possible.

Lets assume, for the purpose of this explanation, that you have a system
where double can represent 3 decimal digits of precision and float can
represent 2 decimal digits of precision.
On such a system, if you have the initialisation

  float pi = 3.14;

the constrant 3.14 (of type double) has to be converted to type float.
This makes float the 'destination type' of clause 4.8.
As float can not represent the value 3.14 exactly (given that float can only
represent 2 digits), the compiler has to round the value to one that can be
represented in float. This would be either 3.1f or 3.2f, which are the two
nearest representable values.

> 
> What's more, I found reading the standard is quite difficult. There
> are so many new terms that were never heard before. I have to
> keep cross referencing, which in return, I forget where I got started.
> Is there experienced ones who can give me some suggestions
> about how to read standards.

Reading standards is not for the faint of heart. They are not meant as a
teaching aid, but are more like a law-text.
The only advice I can give is to keep practicing. With practice, the common
terms and the phraseology become more familiar.

Bart v Ingen Schenau

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Thread

floating point conversions && how to read standards Stanley Rice <heconghui@gmail.com> - 2011-10-07 00:55 -0700
  Re: floating point conversions && how to read standards Juha Nieminen <nospam@thanks.invalid> - 2011-10-07 08:05 +0000
    Re: floating point conversions && how to read standards Stanley Rice <heconghui@gmail.com> - 2011-10-08 00:18 -0700
      Re: floating point conversions && how to read standards Robert Wessel <robertwessel2@yahoo.com> - 2011-10-08 02:36 -0500
  Re: floating point conversions && how to read standards Bart van Ingen Schenau <bart@ingen.ddns.info> - 2011-10-07 11:23 +0200
    Re: floating point conversions && how to read standards Stanley Rice <heconghui@gmail.com> - 2011-10-08 00:30 -0700

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