The "flooring" i.e. round down approach is normally chosen to make sure that
x = a *(x DIV a) + x MOD a
stays valid for negative numbers. That would not necessarily be the case if
rounding towards 0.
Tobias
> Am 05.01.2017 um 16:55 schrieb Martyn Hill <martyn.joseph.h...@gmail.com>:
>
> Thank you Tobias!
>
> Makes sense with that context.
>
> M.
>
> On 05/01/2017 15:49, Tobias Fröschle wrote:
>> Martyn,
>>
>> Integer division for negative numbers is not very well defined. Language
>> designers get to choose if their language will round towards zero, negative
>> infinity, or positive infinity when doing integer division. Different
>> languages have made different choices. S*Basic designers have chosen to use
>> the "flooring" approach (round the floating point result to the next smaller
>> number).
>>
>> Some other reasoning: -1 / 2 should be?
>> (1) -1/2 = 0 remainder -1
>> (2)$FFFF ASR 1 = $FFFF = -1
>>
>> Another note: The original QL allowed word-size divisors and dividends.
>> while SMSQ/E extended this to long integers. And the original QL manual even
>> had an example illustrating what you see here:
>>
>> -5 DIV 2 { will output -3}
>>
>> Tobias
>>
>>
>>> Am 05.01.2017 um 16:27 schrieb Martyn Hill <martyn.joseph.h...@gmail.com>:
>>>
>>> Hi everyone
>>>
>>> Can anyone tell me the expected behaviour for the integer-divide operator
>>> 'DIV' in SBASIC, when provided with a negative dividend/numerator?
>>>
>>> My number-theory is a bit rusty, but I would have thought that, say, -1 DIV
>>> 10 should result in 0 (with remainder/MOD of -1).
>>>
>>> Instead, on QPC2/SBASIC, I get the result -1 for that example - and
>>> (almost) always 1 less than expected for negative dividends, thus:
>>>
>>> 12 DIV 10 = 1
>>> 11 DIV 10 = 1
>>> 10 DIV 10 = 1
>>> 9 DIV 10 = 0
>>> ...
>>> 2 DIV 10 = 0
>>> 1 DIV 10 = 0
>>> 0 DIV 10 = 0
>>> * -1 DIV 10 = -1 - expected '0'**
>>> ** -2 DIV 10 = -1**
>>> **...**
>>> ** -9 DIV 10 = -1*
>>> *-10 DIV 10 = -1 - as expected*
>>> *-11 DIV 10 = -2 - expected '-1'
>>> -12 DIV 10 = -2
>>>
>>> *etc...
>>>
>>> Thanks in advance!
>>>
>>> Martyn.
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>>> QL-Users Mailing List
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>
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> "There are 10 types of people in this world. Those who understand binary and
> those who don't."
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