pairs from a list

[Alan Isaac]

I want to generate sequential pairs from a list.
Here is a way::

from itertools import izip, islice
for x12 in izip(islice(x,0 ,None,2),islice (x,1,None,2)):
print x12

(Of course the print statement is just illustrative.)
What is the fastest way? (Ignore the import time.)

Thanks,
Alan Isaac

[Paul Rubin]

Alan Isaac <ai****@america n.eduwrites:

(Of course the print statement is just illustrative.)
What is the fastest way? (Ignore the import time.)

You have to try a bunch of different ways and time them. One
idea (untested):

def pairs(seq):
while True:
yield (seq.next(), seq.next())

[George Sakkis]

On Jan 21, 10:20 pm, Alan Isaac <ais...@america n.eduwrote:

I want to generate sequential pairs from a list.
Here is a way::

from itertools import izip, islice
for x12 in izip(islice(x,0 ,None,2),islice (x,1,None,2)):
print x12

(Of course the print statement is just illustrative.)
What is the fastest way? (Ignore the import time.)

Look up the timeit module and test yourself the various alternatives;
that's the most reliable way to tell for sure.

George

[Paddy]

On Jan 22, 3:20 am, Alan Isaac <ais...@america n.eduwrote:

I want to generate sequential pairs from a list.

<<snip>>

What is the fastest way? (Ignore the import time.)

1) How fast is the method you have?
2) How much faster does it need to be for your application?
3) Are their any other bottlenecks in your application?
4) Is this the routine whose smallest % speed-up would give the
largest overall speed up of your application?

- Paddy.

[George Sakkis]

On Jan 22, 12:15 am, Paddy <paddy3...@goog lemail.comwrote :

On Jan 22, 3:20 am, Alan Isaac <ais...@america n.eduwrote:I want to generate sequential pairs from a list.
<<snip>>

What is the fastest way? (Ignore the import time.)

1) How fast is the method you have?
2) How much faster does it need to be for your application?
3) Are their any other bottlenecks in your application?
4) Is this the routine whose smallest % speed-up would give the
largest overall speed up of your application?

I believe the "what is the fastest way" question for such small well-
defined tasks is worth asking on its own, regardless of whether it
makes a difference in the application (or even if there is no
application to begin with). Just because cpu cycles are cheap these
days is not a good reason to be sloppy. Moreover, often the fastest
pure Python version happens to be among the most elegant and concise,
unlike other languages where optimization usually implies obfuscation.

George

[Arnaud Delobelle]

On Jan 22, 3:20*am, Alan Isaac <ais...@america n.eduwrote:

I want to generate sequential pairs from a list.
Here is a way::

* * from itertools import izip, islice
* * for x12 in izip(islice(x,0 ,None,2),islice (x,1,None,2)):
* * * * print x12

(Of course the print statement is just illustrative.)
What is the fastest way? (Ignore the import time.)

Thanks,
Alan Isaac

Don't know the fastest, but here's a very concise way:

from itertools import izip

def ipairs(seq):
it = iter(seq)
return izip(it, it)

>>list(pairs(xr ange(10)))

[(0, 1), (2, 3), (4, 5), (6, 7), (8, 9)]

>>list(pairs('h ello'))

[('h', 'e'), ('l', 'l')]

--
Arnaud

[Alan Isaac]

I suppose my question should have been,
is there an obviously faster way?
Anyway, of the four ways below, the
first is substantially fastest. Is
there an obvious reason why?

Thanks,
Alan Isaac

PS My understanding is that the behavior
of the last is implementation dependent
and not guaranteed.

def pairs1(x):
for x12 in izip(islice(x,0 ,None,2),islice (x,1,None,2)):
yield x12

def pairs2(x):
xiter = iter(x)
while True:
yield xiter.next(), xiter.next()

def pairs3(x):
for i in range( len(x)//2 ):
yield x[2*i], x[2*i+1],

def pairs4(x):
xiter = iter(x)
for x12 in izip(xiter,xite r):
yield x12

[Arnaud Delobelle]

On Jan 22, 1:19*pm, Alan Isaac <ais...@america n.eduwrote:
[...]

PS My understanding is that the behavior
of the last is implementation dependent
and not guaranteed.

[...]

def pairs4(x):
* * xiter = iter(x)
* * for x12 in izip(xiter,xite r):
* * * * yield x12

According to the docs [1], izip is defined to be equivalent to:

def izip(*iterables ):
iterables = map(iter, iterables)
while iterables:
result = [it.next() for it in iterables]
yield tuple(result)

This guarantees that it.next() will be performed from left to right,
so there is no risk that e.g. pairs4([1, 2, 3, 4]) returns [(2, 1),
(4, 3)].

Is there anything else that I am overlooking?

[1] http://docs.python.org/lib/itertools-functions.html

--
Arnaud

[Arnaud Delobelle]

On Jan 22, 1:19*pm, Alan Isaac <ais...@america n.eduwrote:

I suppose my question should have been,
is there an obviously faster way?
Anyway, of the four ways below, the
first is substantially fastest. *Is
there an obvious reason why?

Can you post your results?

I get different ones (pairs1 and pairs2 rewritten slightly to avoid
unnecessary indirection).

====== pairs.py ===========
from itertools import *

def pairs1(x):
return izip(islice(x,0 ,None,2),islice (x,1,None,2))

def pairs2(x):
xiter = iter(x)
while True:
yield xiter.next(), xiter.next()

def pairs3(x):
for i in range( len(x)//2 ):
yield x[2*i], x[2*i+1],

def pairs4(x):
xiter = iter(x)
return izip(xiter,xite r)

def compare():
import timeit
for i in '1234':
t = timeit.Timer('l ist(pairs.pairs %s(l))' % i,
'import pairs; l=range(1000)')
print 'pairs%s: %s' % (i, t.timeit(10000) )

if __name__ == '__main__':
compare()
=============== ======

marigold:python arno$ python pairs.py
pairs1: 0.789824962616
pairs2: 4.08462786674
pairs3: 2.90438890457
pairs4: 0.536775827408

pairs4 wins.

--
Arnaud

[Alan Isaac]

Arnaud Delobelle wrote:

According to the docs [1], izip is defined to be equivalent to:

def izip(*iterables ):
iterables = map(iter, iterables)
while iterables:
result = [it.next() for it in iterables]
yield tuple(result)

This guarantees that it.next() will be performed from left to right,
so there is no risk that e.g. pairs4([1, 2, 3, 4]) returns [(2, 1),
(4, 3)].

Is there anything else that I am overlooking?

[1] http://docs.python.org/lib/itertools-functions.html

<URL:http://bugs.python.org/issue1121416>

fwiw,
Alan Isaac