On 22 May 2009, at 16:56, Franco Saliola wrote:
> 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.
Ahh. Now I understand. Thanks.
But, this tells me that I have another issue to look for. I had noted
that a couple of cells in a large worksheet did not always produce the
same result. I was slowly paring down the code, trying to find the
specific code and triggers for the problem. I noted the anomaly in
this thread, and thought that it could have been the cause for the
strange behaviour I was seeing. But this worksheet only has the
Integer=int at the very top, so this could not have caused what I
saw. I will keep looking.
--
Kevin Horton
Ottawa, Canada
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