substituting list comprehensions for map()

Steven D'Aprano steven at
Wed Nov 4 02:53:15 CET 2009

On Tue, 03 Nov 2009 10:22:28 -0500, J Kenneth King wrote:

> However in this case the procedure by which we derive the value is not
> important or even interesting.  It is much more succinct to think of the
> operation as a value and express it accordingly.  There's no need to
> clutter the mind with extra name bindings and iteration keywords.  They
> won't make our idea any more clear.
> dot_product = map(mul, vec1, vec2)
> vs
> dot_product = [a * b for a, b in zip(vec1, vec2)]
> It's very clear, at least to me, what a dot-product is in this case.

Except it's not.

The dot product of two vectors returns a scalar, not another vector:

So what you want is:

dot_product = sum(map(mul, vec1, vec2))

> Adding in the loop construct and name bindings doesn't enhance my
> understanding of what a dot-product is.  I don't need to see the loop
> construct at all in this case.  A dot product is simply the
> multiplication of each element in a vector sequence.

What you need is to define a function dot-product, and not hijack the 
name for a local value. Then the function's implementation is irrelevant 
to you: it could use a list comp, or could use map, it could use a for-
loop, a while loop, recursion, or black magic:

scalar = dot_product(vec1, vec2)


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