[Numpy-discussion] Re: indexing problem

Tim Hochberg tim.hochberg at cox.net
Mon Feb 13 11:31:05 CST 2006


I've been trying to look into the problem described below, but I just 
can't find where complex multiplication is being done (all the other 
multiplication, but not complex). Could someone with a grasp of the 
innards of numpy please point me in the right direction?

Thanks,

-tim

Ryan Krauss wrote:

>This may only be a problem for ridiculously large numbers.  I actually
>meant to be dealing with these values:
>
>In [75]: d
>Out[75]:
>array([   246.74011003,    986.96044011,   2220.66099025,   3947.84176044,
>         6168.50275068,   8882.64396098,  12090.26539133,  15791.36704174,
>        19985.94891221,  24674.01100272])
>
>In [76]: s=d[-1]*1.0j
>
>In [77]: s
>Out[77]: 24674.011002723393j
>
>In [78]: type(s)
>Out[78]: <type 'complex128scalar'>
>
>In [79]: s**2
>Out[79]: (-608806818.96251547+7.4554869875188623e-08j)
>
>So perhaps the previous difference of 26 orders of magnitude really
>did mean that the imaginary part was negligibly small, that just got
>obscured by the fact that the real part was order 1e+135.
>
>On 2/13/06, Ryan Krauss <ryanlists at gmail.com> wrote:
>  
>
>>I am having a problem with indexing an array and not getting the
>>expected scalar behavior for complex128scalar:
>>
>>In [44]: c
>>Out[44]:
>>array([  3.31781200e+06,   2.20157529e+13,   1.46088259e+20,
>>         9.69386754e+26,   6.43248601e+33,   4.26835585e+40,
>>         2.83232045e+47,   1.87942136e+54,   1.24711335e+61,
>>         8.27537526e+67])
>>
>>In [45]: s=c[-1]*1.0j
>>
>>In [46]: type(s)
>>Out[46]: <type 'complex128scalar'>
>>
>>In [47]: s**2
>>Out[47]: (-6.848183561893313e+135+8.3863291020365108e+119j)
>>
>>In [48]: s=8.27537526e+67*1.0j
>>
>>In [49]: type(s)
>>Out[49]: <type 'complex'>
>>
>>In [50]: s**2
>>Out[50]: (-6.8481835693820068e+135+0j)
>>
>>Why does result 47 have a non-zero imaginary part?
>>
>>Ryan
>>
>>    
>>
>
>
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