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-   -   Sorting: too different times. Why? (http://www.velocityreviews.com/forums/t706161-sorting-too-different-times-why.html)

n00m 11-22-2009 09:21 AM

Sorting: too different times. Why?
 
Any comment:

class Vector:
def __init__(self, x, y):
self.x = x
self.y = y
def __cmp__(self, v):
if self.x < v.x and self.y > v.y:
return -1
return 0

def v_cmp(v1, v2):
if v1.x < v2.x and v1.y > v2.y:
return -1
return 0

from random import randint
from time import time

a = []
for i in range(200000):
a += [Vector(randint(0, 500000), randint(0, 500000))]
b = a[:]
c = a[:]

print 'Sorting...'

t = time()
b.sort(cmp=v_cmp)
print time() - t

t = time()
c.sort()
print time() - t

print b == c



>>> ===================================== RESTART ======
>>>

Sorting...
0.906000137329
6.57799983025
True


n00m 11-22-2009 10:56 AM

Re: Sorting: too different times. Why?
 

I was expecting the 1st method would be *slower* than the 2nd one :-)
Or at least equal... Just random ("intuitive") expectations

Steven D'Aprano 11-22-2009 11:04 AM

Re: Sorting: too different times. Why?
 
In the subject line, you write "too different times". You actually want
"two", the number, not "too" as in "too many", "too much". Lots of native
English speakers get this wrong too :)

On Sun, 22 Nov 2009 01:21:42 -0800, n00m wrote:

> Any comment:
>
> class Vector:
> def __init__(self, x, y):
> self.x = x
> self.y = y
> def __cmp__(self, v):
> if self.x < v.x and self.y > v.y:
> return -1
> return 0



Modern versions of Python (since 2.2 I think?) use __lt__ or __gt__ for
sorting. If the class doesn't have a __lt__ method, Python falls back on
__cmp__.

> b.sort(cmp=v_cmp)


This is relatively fast, because you pass a comparison function directly,
so Python doesn't need to look for a __lt__ method then fall back to
__cmp__. It just uses v_cmp, every time.


> c.sort()


This is slower, because every comparison looks up the __lt__ and fails,
then tries the __cmp__.

If you change the definition of Vector to include rich comparison methods
as detailed here:

http://docs.python.org/reference/dat...#object.__lt__

sorting will probably be significantly faster still.



--
Steven

Diez B. Roggisch 11-22-2009 11:06 AM

Re: Sorting: too different times. Why?
 
n00m schrieb:
> Any comment:
>
> class Vector:
> def __init__(self, x, y):
> self.x = x
> self.y = y
> def __cmp__(self, v):
> if self.x < v.x and self.y > v.y:
> return -1
> return 0
>
> def v_cmp(v1, v2):
> if v1.x < v2.x and v1.y > v2.y:
> return -1
> return 0
>
> from random import randint
> from time import time
>
> a = []
> for i in range(200000):


Use xrange instead (unless you are under python3), because for loops you
don't need the full list range creates - xrange is just a generator.

> a += [Vector(randint(0, 500000), randint(0, 500000))]


Better use .append here, looks nicer and should also be a bit faster.

> b = a[:]
> c = a[:]
>
> print 'Sorting...'
>
> t = time()
> b.sort(cmp=v_cmp)
> print time() - t
>
> t = time()
> c.sort()
> print time() - t
>
> print b == c
>
>
>
>>>> ===================================== RESTART ======
>>>>

> Sorting...
> 0.906000137329
> 6.57799983025


I think the main reason is that method-dispatch is more expensive than
function-dispatch. The former must create a bound method before calling,
the latter just works out of the box.

Things get better if you do this:

t = time()
c.sort(cmp=Vector.__cmp__)
print time() - t


Although not the exact same performance - I get

Sorting...
0.677843093872
1.4283311367
True

Diez


Chris Rebert 11-22-2009 11:07 AM

Re: Sorting: too different times. Why?
 
On Sun, Nov 22, 2009 at 2:56 AM, n00m <n00m@narod.ru> wrote:
> I was expecting the 1st method would be *slower* than the 2nd one :-)
> Or at least equal... Just random ("intuitive") expectations


The second method repeatedly looks up left_item.__class__.__cmp__
(i.e. Vector.__cmp__) when doing the necessary comparisons between the
list items; while these lookups are optimized and are fast, they are
not free and cannot be skipped because Python doesn't know the list
contains only Vectors.
The first method uses the single provided comparison function and thus
does no such lookups; hence, it's faster.

That's my guess anyway.

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

Mark Dickinson 11-22-2009 11:44 AM

Re: Sorting: too different times. Why?
 
On Nov 22, 9:21*am, n00m <n...@narod.ru> wrote:
> Any comment:
>
> class Vector:
> * * def __init__(self, x, y):
> * * * * self.x = x
> * * * * self.y = y
> * * def __cmp__(self, v):
> * * * * if self.x < v.x and self.y > v.y:
> * * * * * * return -1
> * * * * return 0
>
> def v_cmp(v1, v2):
> * * if v1.x < v2.x and v1.y > v2.y:
> * * * * return -1
> * * return 0
>
> from random import randint
> from time import time
>
> a = []
> for i in range(200000):
> * * a += [Vector(randint(0, 500000), randint(0, 500000))]
> b = a[:]
> c = a[:]
>
> print 'Sorting...'
>
> t = time()
> b.sort(cmp=v_cmp)
> print time() - t
>
> t = time()
> c.sort()
> print time() - t
>
> print b == c
>
> >>> ===================================== RESTART ======

>
> Sorting...
> 0.906000137329
> 6.57799983025
> True


Do you get the same magnitude difference if you make Vector a new-
style
class? (I.e., use "class Vector(object)" instead of "class Vector
()".)

Mark

Dave Angel 11-22-2009 12:28 PM

Re: Sorting: too different times. Why?
 
n00m wrote:
> Any comment:
>
> <snip>
> def v_cmp(v1, v2):
> if v1.x < v2.x and v1.y > v2.y:
> return -1
> return 0
>
>
>

The second part of the compound if is backwards. So if this is headed
for production code, it better get fixed.

DaveA


n00m 11-22-2009 12:45 PM

Re: Sorting: too different times. Why?
 
> Do you get the same magnitude difference
> if you make Vector a new-style class?


Yes (I mean "No"): new-style's much faster

And now it's elephants instead of vectors.
Def: an elephant is smarter than another one IIF
its size is strictly less but its IQ is strictly
greater

I.e. you can't compare (2, 8) to (20, 50)
or let count them as equally smart elephants.
================================================

class Elephant(object):
def __init__(self, size, iq):
self.size = size
self.iq = iq
def __cmp__(self, e):
if self.size < e.size and self.iq > e.iq:
return -1
if self.size > e.size and self.iq < e.iq:
return 1
return 0

def e_cmp(e1, e2):
if e1.size < e2.size and e1.iq > e2.iq:
return -1
if e1.size > e2.size and e1.iq < e2.iq:
return 1
return 0

from random import randint
from time import time

a = []
for i in xrange(200000):
a.append(Elephant(randint(1, 50000), randint(1, 50000)))
b = a[:]
c = a[:]

print 'Sorting...'

t = time()
b.sort(cmp=e_cmp)
print time() - t

t = time()
c.sort()
print time() - t

print b == c



>>> ===================================== RESTART =====
>>>

Sorting...
1.56299996376
1.95300006866
True

n00m 11-22-2009 12:49 PM

Re: Sorting: too different times. Why?
 

> The second part of the compound if is backwards. *So if this is headed
> for production code, it better get fixed.
>
> DaveA


Not sure I'm understanding your remark.

MRAB 11-22-2009 04:17 PM

Re: Sorting: too different times. Why?
 
Steven D'Aprano wrote:
> In the subject line, you write "too different times". You actually want
> "two", the number, not "too" as in "too many", "too much". Lots of native
> English speakers get this wrong too :)
>

[snip]
It could mean that the times are not just different, they're _too_
different, ie a lot more than they are expected to be.


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