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.
I think it is a very bad situation if a worksheet cell yields
different results depending on how many times it has been evaluated.
In my opinion this is a big bug.
The conclusion I have drawn for the moment is that I need to stop
using Integer=int and Float=float. Instead I will have to wrap each
problematic call to matplotlib.pyplot in float() or int(). I will
also look deeper into sage's plotting, to see if there is a better way
to make my plots, rather than using matplotlib.pyplot.
--
Kevin Horton
Ottawa, Canada
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