[Numpy-discussion] Question about numpy.max(<complex matrix>)
Stuart Brorson
sdb@cloud9....
Fri Sep 21 19:45:28 CDT 2007
On Fri, 21 Sep 2007, Robert Kern wrote:
> Stuart Brorson wrote:
>>>> Is it NumPy's goal to be as compatible with Matlab as possible?
>>> No.
>>
>> OK, so that's fair enough. But how about self-consistency?
>> I was thinking about this issue as I was biking home this evening.
>>
>> To review my question:
>>
>> >>> a
>> array([[ 1. +1.j , 1. +2.j ],
>> [ 2. +1.j , 1.9+1.9j]])
>> >>> numpy.max(a)
>> (2+1j)
>>
>> Why does NumPy return 2+1j, which is the element with the largest real
>> part? Why not return the element with the largest *magnitude*?
>> Here's an answer from the list:
>>
>>> There isn't a single, well-defined (partial) ordering of complex numbers. Both
>>> the lexicographical ordering (numpy) and the magnitude (Matlab) are useful, but
>>> the lexicographical ordering has the feature that
>>>
>>> (not (a < b)) and (not (b < a)) implies (a == b)
>> [snip]
>>
>> Sounds good, but actually NumPy is a little schizophrenic when it
>> comes to defining an order for complex numbers. Here's another NumPy
>> session log:
>>
>> >>> a = 2+1j
>> >>> b = 2+2j
>> >>> a>b
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in <module>
>> TypeError: no ordering relation is defined for complex numbers
>> >>> a<b
>> Traceback (most recent call last):
>> File "<stdin>", line 1, in <module>
>> TypeError: no ordering relation is defined for complex numbers
>
> No, that's a Python session log and the objects you are comparing are Python
> complex objects. No numpy in sight. Here is what numpy does for its complex
> scalar objects:
>
>>>> from numpy import *
>>>> a = complex64(2+1j)
>>>> b = complex64(2+2j)
>>>> a < b
> True
OK, fair enough. I was wrong. But, ummmmm, in my example above, when
you find the max of a complex array, you compare based upon the *real*
part of each element. Here, you compare based upon complex
*magnitude*.
Again, I wonder about self-consistency.
I guess the thing which bothers me is that finding the max of a
complex array by finding the element with the largest *real* part
seems..... well..... ummmm, like a bug. Or at least rather
non-intuitive. Yes, you can use any ordering relationship for complex
numbers you want, but, gee, it seems to me that once you choose one
then you should stick to it.
>> Or are NumPy behaviors --
>> once defined -- never changed?
>
> We do try to keep backwards compatibility.
Great! Thank you!
Stuart
More information about the Numpy-discussion
mailing list