Using '__mul__' within a class

[James Stroud]

Additionally, your __mul__() returns a new FibonnacciMatrix. You do not want a 
new FibbonacciMatrix, you want to operate on an existing matrix (otherwise, 
you would want to go with Ivan Voras's solution, where you re-assign outside 
of the class). If you don't want the overhead of creating a instance of your 
class, you may want to be more explicit in Square(), eg:

     def Multiply(self, other):
         self.a = self.a * other.a + self.b * other.b
         self.b = self.a * other.b + self.b * other.c
         self.c = self.b * other.b + self.c * other.c

     def Square(self, other):
         self.Multiply(other)


With __imul__(), as Terry Reedy suggested, you can also save the overhead of 
creating a new instance, but it works a little differently than Square() 
above, because you need to return self (assuming Square() is as above):

     def __imul__(self, other):
         self.Multiply(other)
         return self

With these methods, __mul__() can be factored:

     def __mul__(self, other):
       result = FibonacciMatrix()
       result.Multiply(other)
       return result

Leaving all of the messiest stuff in the Multiply method.

James


On Saturday 24 September 2005 08:36, Gerard Flanagan wrote:
> Hello
>
> I'm pretty new to Python and was wondering why the 'Square' method in
> the following code doesn't work. It doesn't fail, just doesn't do
> anything ( at least, not what I'd like! ). Why doesn't 'A.a' equal 2
> after squaring?
>  TIA.
>
>
> class FibonacciMatrix:
>     def __init__( self ):
>         self.a = 1
>         self.b = 1
>         self.c = 0
>
>     def __mul__( self, other ):
>         result = FibonacciMatrix()
>         result.a = self.a * other.a + self.b * other.b
>         result.b = self.a * other.b + self.b * other.c
>         result.c = self.b * other.b + self.c * other.c
>         return result
>
>     def Square( self ):
>         self *= self
>
>
> A = FibonacciMatrix()
> A.Square()
>
> print A.a   #prints '1'
>
> A = FibonacciMatrix()
> B = A * A
>
> print B.a   #prints '2'
>
> ------------------------------

-- 
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/