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Re: Possible loss of precision

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On 11/14/2013 6:03 PM, Gene Wirchenko wrote:
> On Thu, 14 Nov 2013 20:39:42 +0100, Michael Jung
> <miju@golem.phantasia.org> wrote:
>
>> ram@zedat.fu-berlin.de (Stefan Ram) writes:
>>> char x = 'C' + 1;
>>> x = x + 1;
>>>    in a block, one gets an error message
>>> »possible loss of precision«
>>>    for »x = x + 1«. Ok, fair enough: »x + 1« has type »int«,
>>>    and precision is lost when assigning this to the variable x
>>>    of type char.
>>>    I was somewhat surprised, though, that »char x = 'C' + 1;«
>>>    does not yield an error, even though the type of »'C' + 1«
>>>    is int, too. Possibly, this time, the compiler can figure
>>>    out that the actual value still fits into a char.
>>>    But then, »int i = 2.0;« gives an error, even though the
>>>    compiler should see that »2.0« can be represented as an int.
>>
>> You are right that x+1 is "promoted" to int and the second assignment
>> fails.  In the other cases the compiler will follow JLS 15.28 => 5.2.
>> This concerns constant expressions first and the second allows a
>> narrowing in rare cases, of which the first line is one but the
>> "int i = 2.0" isn't.
>
>       Why isn't it though?

     Speculation: The fact that a floating-point expression happens
to evaluate to a number with no fractional part doesn't mean the
value "is" an integer.  One reasonable view of FP is that a value
is just a representative of a range of real values, namely, all
those real values that round to the representative.  In this
view, converting 2.0 to 2 *does* lose precision, specifically,
the amount of wiggle room.

     Then again, maybe the Java designers were just FP-wary.

-- 
Eric Sosman
esosman@comcast-dot-net.invalid

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Re: Possible loss of precision Michael Jung <miju@golem.phantasia.org> - 2013-11-14 20:39 +0100
  Re: Possible loss of precision Gene Wirchenko <genew@telus.net> - 2013-11-14 15:03 -0800
    Re: Possible loss of precision Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-11-14 18:31 -0500
      Re: Possible loss of precision Joerg Meier <joergmmeier@arcor.de> - 2013-11-15 00:55 +0100
      Re: Possible loss of precision Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-11-14 20:19 -0500
        Re: Possible loss of precision taqmcg@gmail.com - 2013-11-15 07:01 -0800
        Re: Possible loss of precision Martin Gregorie <martin@address-in-sig.invalid> - 2013-11-15 21:36 +0000
    Re: Possible loss of precision Michael Jung <miju@golem.phantasia.org> - 2013-11-15 19:24 +0100
      Re: Possible loss of precision Patricia Shanahan <pats@acm.org> - 2013-11-15 20:48 -0800
        Re: Possible loss of precision Michael Jung <miju@golem.phantasia.org> - 2013-11-17 13:28 +0100
          Re: Possible loss of precision Patricia Shanahan <pats@acm.org> - 2013-11-17 08:05 -0800

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