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Groups > comp.os.linux.development.apps > #452

Re: cast double to short problem

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Grant Edwards wrote, On 3/7/2012 2:13 PM:
> On 2012-03-07, Ersek, Laszlo<lacos@caesar.elte.hu>  wrote:
>>
>>> Ah, there's your problem: expecting to get 3.  You should expect to
>>> get either 2 or 3.  And you do. :)
>>
>> He should get the same result both times.
>
> Yes.  He should.  My point is that he shouldn't expect to.
>
> His code should not be written to depend on either (int)(0.3 * 10.0)
> == 2 or on (int)(0.3 * 10.0) == 3.
>
>> - The floating point environment is the same.
>> - dval starts out the same (as in "object representation").
>> - 10 (int) is converted to double the same way (with exact value).
>> - Both double multiplications happen with the same operands (as in
>> "object representation").
>> - Initialization of (~ simple assignment to) a double does not change the
>> double product (as in "object representation").
>> - Conversion to short is the same (truncated toward zero).
>
> I agree: I think it should produce the same result in both cases.
>
Thanks for all the great replies!! For some silly reason I was thinking 
that the cast took care of the rounding. Even more silly, that my value 
was getting truncated had not occurred to me.

In this situation, I don't need to be concerned with rounding up or 
down. I always want to round to the nearest decimal.

The scenario is this:

recipe_parms.dispense_volume1 = (int)round(atof(value) * 10);

where dispense_volume1 has a precision of 0.1

or

recipe_parms.suckback_volume = (int)round(atof(value) * 100);

where suckback_volume has a precision of 0.01

So now the question of why are they different remains, but it does not 
affect my application.


Thanks!!

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cast double to short problem Bill M <wpmccormick@just_about_everywhere.com> - 2012-03-06 16:52 -0600
  Re: cast double to short problem Grant Edwards <invalid@invalid.invalid> - 2012-03-06 23:21 +0000
    Re: cast double to short problem Bill M <wpmccormick@just_about_everywhere.com> - 2012-03-06 17:36 -0600
      Re: cast double to short problem Grant Edwards <invalid@invalid.invalid> - 2012-03-07 00:30 +0000
        Re: cast double to short problem Noob <root@127.0.0.1> - 2012-03-07 11:17 +0100
          Re: cast double to short problem Grant Edwards <invalid@invalid.invalid> - 2012-03-07 15:23 +0000
    Re: cast double to short problem "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2012-03-07 19:33 +0100
      Re: cast double to short problem Grant Edwards <invalid@invalid.invalid> - 2012-03-07 20:13 +0000
        Re: cast double to short problem Bill M <wpmccormick@just_about_everywhere.com> - 2012-03-08 11:45 -0600

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