GZ 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'd like to have something like this:
count_a, count_b, count_c =
sum( (int(x[1]=='a',int(x[1]=='b',int(x[1]=='c') for x in l)
I hesitate to use numpy array, because that will literally create and
destroy a ton of the arrays, and is likely to be slow.
If you want to do vector addition then numpy is the way to go. However,
first you could try:
from collections import defaultdict
counts = defaultdict(int)
for x in l:
counts[x[0]] += 1
(Note that in Python indexes are zero-based.)
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