__peter__ at web
Nov 17, 2009, 2:24 AM
Post #8 of 8
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Gabriel Genellina wrote:
> En Mon, 16 Nov 2009 19:30:27 -0300, Hyunchul Kim
> <hyunchul.mailing [at] gmail> escribió:
>
>> I want to improve speed of following simple function.
>> Any suggestion?
>>
>> **********
>> def triple(inputlist):
>> results = []
>> for x in inputlist:
>> results.extend([x,x,x])
>> return results
>> **********
>
> These are my best attempts:
>
> def triple3(inputlist):
> results = []
> append = results.append
> for x in inputlist:
> append(x); append(x); append(x)
> return results
>
> def triple4(inputlist, _three=xrange(3)):
> return [x for x in inputlist for _ in _three]
>
> For a 400-items list, triple3 is 40% faster and triple4 25% faster than
> yours.
[I didn't see the original post]
If inputlist is actually a list or tuple the following should beat them all:
def triple_repeat(items):
result = [None] * (3*len(items))
result[::3] = result[1::3] = result[2::3] = items
return result
For small n the generalization should still be quite competitive:
def n_repeat(items, n):
items = tuple(items)
result = [None]*(len(items) * n)
for offset in range(n):
result[offset::n] = items
return result
Some measurements:
$ cat extend.py
from itertools import *
def triple_repeat(items):
result = [None] * (3*len(items))
result[::3] = result[1::3] = result[2::3] = items
return result
def n_repeat(items, n):
items = tuple(items)
result = [None]*(len(items) * n)
for offset in range(n):
result[offset::n] = items
return result
def t1(i):
for x in i:
yield x
yield x
yield x
def triple3(inputlist, list=list, chain_from_iterable=chain.from_iterable, izip=izip):
return list(chain_from_iterable(izip(inputlist, inputlist, inputlist)))
data = range(1000)
$ python -m timeit -s 'from extend import triple_repeat, data' 'triple_repeat(data)'
10000 loops, best of 3: 52.4 usec per loop
$ python -m timeit -s 'from extend import n_repeat, data' 'n_repeat(data, 3)'
10000 loops, best of 3: 60.7 usec per loop
$ python -m timeit -s 'from extend import t1, data' 'list(t1(data))'
1000 loops, best of 3: 337 usec per loop
$ python -m timeit -s 'from extend import triple3, data' 'triple3(data)'
1000 loops, best of 3: 231 usec per loop
Peter
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