Python presentations

[Alexander Blinne]

On 14.09.2012 14:19, Chris Angelico wrote:
> Err, yes, I did mean ** there. The extra multiplications may be
> slower, but which is worse? Lots of list additions, or lots of integer
> powers? In the absence of clear and compelling evidence, I'd be
> inclined to go with the list comp - and what's more, to skip this
> function altogether and use the list comp directly where it's needed.

I did some timing with the following versions of the function:

def powerlist1(x, n):
    result=[1]
    for i in xrange(n-1):
        result.append(result[-1]*x)
    return result

def powerlist2(x,n):
    if n==1:
        return [1]
    p = powerlist3(x,n-1)
    p.append(p[-1]*x)
    return p

def powerlist3(x,n):
  return [x**i for i in xrange(n)]

with Python 2.7 you are quite right, i used x=4. Below n=26 powerlist3
is the fastest version, for n>26 powerlist1 is faster, powerlist2 is
always slower than both.

With Pypy there is a completely different picture, with n<30 powerlist2
is way faster then the other two, but then powerlist1 is again faster
for greater n, somehow because now C long int cannot be used any longer.

for really big n powerlist3 always takes very much time :)

Alex