[Numpy-discussion] Different behaviour logical_and/and

Huaiyu Zhu huaiyu_zhu at yahoo.com
Sat Mar 23 00:32:02 CST 2002


On Thu, 21 Mar 2002, Pearu Peterson wrote:
> On 20 Mar 2002, Jochen Küpper wrote:
> > >>>> a
> > Aureli> array([0, 1, 0, 0, 1, 0, 1, 0, 0, 1])
> > >>>> b
> > Aureli> array([1, 1, 0, 1, 0, 0, 1, 1, 1, 0])
> > 
> > >>>> Numeric.logical_and(a,b)
> > Aureli> array([0, 1, 0, 0, 0, 0, 1, 0, 0, 0])
> > 
> > This look's correct...
> > 
> > >>>> a and b
> > Aureli> array([1, 1, 0, 1, 0, 0, 1, 1, 1, 0])
> > 
> > ... and this suspiciously like b.
> 
> . and also correct.  It is default behaviour of Python `and'
> operation. From Python Language Reference:
> 
>   The expression `x and y' first evaluates `x'; if `x' is false, its value  
>   is returned; otherwise, `y' is evaluated and the resulting value is
>   returned. 
> 
> So, `and' operation is "object"-operation (unless redefined to something
> else) while logical_and is "elementwise"-operation.

There is a section in PEP-225 for elementwise/objectwise operators to 
extend the meaning of ~ to an "elementizer", so that 

[1, 0, 1, 0] and [0, 1, 1, 0]      =>  [0, 1, 1, 0]
[1, 0, 1, 0] ~and [0, 1, 1, 0]     =>  [0, 0, 1, 0]

There are several other places, entirely unrelated to numerical 
computation, that elementization of an operator makes sense.

Huaiyu





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