On Fri, May 22, 2009 at 10:42 PM, Kevin Horton <khorto...@rogers.com> wrote:
>
> On 22 May 2009, at 16:07, Robert Bradshaw wrote:
>
>> On May 22, 2009, at 3:36 AM, Kevin Horton wrote:
>>
>>> I am using sage-4.0.rc0 on 32 bit ubuntu 8.10.
>>>
>>> I have discovered a very strange interaction between the
>>> "Integer=int"
>>> declaration, division, and the number of times a worksheet is
>>> evaluated.
>>
>> This is an error due to literal constant extraction, I thought there
>> was a ticket for this on trac but was unable to find it for the
>> moment.
>>
>>> Consider the following worksheet. The very first time it is
>>> evaluated, division is handled as float division, and the results are
>>> what I hoped for. But, if I evaluate the cell again, I get a
>>> different answer. This time the division seems to be python integer
>>> division, and I get a different result. I keep getting this second
>>> result until I restart the worksheet, then I get the first result for
>>> a single evaluation.
>>>
>>> Regardless of what the correct behaviour is, I would hope to get the
>>> same behaviour every time I evaluate the worksheet. This looks
>>> like a
>>> bug to me.
>>>
>>> ================================
>>> First evaluation after restarting worksheet:
>>>
>>> Division Error?
>>> system:sage
>>>
>>> {{{id=2|
>>> Integer=int
>>>
>>> print float(500/1000), float(1000/1000)
>>> print float(500/1000.), float(1000/1000.)
>>> ///
>>>
>>> 0.5 1.0
>>> 0.5 1.0
>>> }}}
>>
>> To see what is really going on, this gets turned into
>>
>> {{{
>> print sage.misc.preparser.preparse_file("""
>> Integer=int
>>
>> print float(500/1000), float(1000/1000)
>> print float(500/1000.), float(1000/1000.)
>> """)
>> ///
>> _sage_const_500 = Integer(500); _sage_const_1000 = Integer(1000);
>> _sage_const_1000p = RealNumber('1000.')
>> Integer=int
>>
>> print float(_sage_const_500 /_sage_const_1000 ), float
>> (_sage_const_1000/_sage_const_1000 )
>> print float(_sage_const_500 /_sage_const_1000p ), float
>> (_sage_const_1000/_sage_const_1000p )
>> }}}
>>
>> We do this to avoid the (large) overhead of re-creating constants in
>> the bodies of loops, functions, etc. Perhaps we need to detect the
>> Integer=xxx line explicitly?
>
> Thanks for the info. I still don't understand why the results are
> different the second time the cell is evaluated. But, I'm not a
> developer, so it really isn't important that I understand this.
In the above preparsed code, you find the line
_sage_const_500 = Integer(500);
and it appears before the "Integer = int" definition. This means that
the first time the cell is executed, _sage_const_500 is defined using
the original definition of Integer, whereas the second (and
subsequent) times the cell is executed, _sage_const_500 is defined to
be int(500) because Integer has been redefined.
So if you execute Integer=int in a different cell, then you won't see
this problem.
Franco
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