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 -- http://mail.python.org/mailman/listinfo/python-list