# Iteration over two sequences

Henrik Holm news+0409 at henrikholm.com
Wed Jan 12 10:54:12 EST 2005

```I am just starting to learn Python, mostly by going through the examples
in Dive Into Python and by playing around.

Quite frequently, I find the need to iterate over two sequences at the
same time, and I have a bit of a hard time finding a way to do this in a
"pythonic" fashion.  One example is a dot product.  The straight-ahead
C-like way of doing it would be:

def dotproduct(a, b):
psum = 0
for i in range(len(a)):
psum += a[i]*b[i]
return psum

However, the range(len(a)) term is awfully un-pythonic :)
The built-in function map() gives me a way of "transposing" the a list
and the b list, and  now I can handle it with a list comprehension:

def dotproduct(a, b):
return sum([x*y for x, y in map(None, a, b)])

My concern is one of efficiency: it seems to me that I have two loops
there: first one implied with map(...) and then the for loop -- which
seems like a waste since I feel I should be able to do the
multiplication via an argument to map.  So far I have come up with an
alternative via defining a separate function:

def dotproduct(a, b):
def prod(x,y): return x*y
return sum(map(prod, a, b))

I suppose I could also use a lambda here -- but is there a different,
efficient, and obvious solution that I'm overlooking?

Thanks,
Henrik

--
"On some great and glorious day the plain folks of the land will reach
in their heart's desire at last and the White House will be adorned by
a downright moron."
-H.L. Mencken (1880-1956) American Writer

```