Dot product?

[Mike Fletcher]

Maybe not what you were asking, but some food for thought :) .  Of course,
we're told that all this mapping and reducing will get replaced by list
comprehensions, so maybe I shouldn't post this... oh well, I'll be a rebel
;) .

>>> import operator
>>> a = (2,3,4)
>>> b = (3,4,5)
>>> def Transpose( a, b ):
... 	return map( None, a, b )
... 
>>> Transpose( a, b )
[(2, 3), (3, 4), (4, 5)]
>>> def Dot( a, b ):
... 	return reduce( operator.add, map( operator.mul, a, b))
... 
>>> Dot( a, b )
38
>>> def Transpose( *args ):
... 	'''General version, takes any number of sequences (kind of silly
anyway :) )'''
... 	return apply( map, (None,)+args )
... 
>>> Transpose( a, b )
[(2, 3), (3, 4), (4, 5)]
>>> Transpose( a, b, () )
[(2, 3, None), (3, 4, None), (4, 5, None)]
>>> 

Enjoy,
Mike


-----Original Message-----
From: David C. Ullrich [mailto:ullrich at math.okstate.edu]
Sent: Saturday, December 11, 1999 3:51 PM
To: python-list at python.org
Subject: Dot product?


    Ok, so I'm dense. How do I take the traditional "dot product"
of two vectors? In a nice Pythonic sort of way, I mean. Say
the vectors are represented as sequences of numerics - then
of course I can say just

def Dot(X, Y):
    res = 0
    for j in range(len(X)):
        res = res + X[j] * Y[j]
    return res

and that gives the right answer, but it doesn't look right -
seems like the Python way would involve a "for x in X"
instead of that "for j in range(len(X))".

    For example if Transpose returns the transpose of a
sequence of sequences (ie Transpose(s)[j][k] = s[k][j])
I can say

def Dot(X, Y):
    res = 0
    for (x,y) in Transpose((X,Y)):
        res = res + x * y
    return res

and that looks "right". For that matter once I have a
Transpose I could just use reduce. But when I write
a Transpose routine I have the same complaints with
the way it looks, only more so.

    Is there a keen way to do this sort of thing, or do I
just have to use the index as above? (Come to think
of it I think what I really want is a nice Transpose -
that's been coming up a lot, Dot is just one place.)

DU

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