vector addition

[Chris Rebert]

On Sat, Jun 5, 2010 at 6:20 PM, GZ <zyzhu2000 at gmail.com> wrote:
> Hi,
>
> I am looking for a fast internal vector representation so that
> (a1,b2,c1)+(a2,b2,c2)=(a1+a2,b1+b2,c1+c2).
>
> So I have a list
>
> l = ['a'a,'bb','ca','de'...]
>
> I want to count all items that start with an 'a', 'b', and 'c'.
>
> What I can do is:
>
> count_a = sum(int(x[1]=='a') for x in l)
> count_b = sum(int(x[1]=='b') for x in l)
> count_c = sum(int(x[1]=='c') for x in l)
>
> But this loops through the list three times, which can be slow.

I don't really get how that relates to vectors or why you'd use that
representation, and it looks like you're forgotten that Python uses
0-based indexing, but anyway, here's my crack at something more
efficient:

from collections import defaultdict

cared_about = set('abc')
letter2count = defaultdict(int)
for item in l:
    initial = item[0]
    if initial in cared_about:
        letter2count[initial] += 1

count_a = letter2count['a']
count_b = letter2count['b']
count_c = letter2count['c']

Cheers,
Chris
--
http://blog.rebertia.com