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range() is not the best way to check range?

it seems that range() can be really slow:

the following program will run, and the last line shows how long it ran
for:

import time

startTime = time.time()

a = 1.0
for i in range(0, 30000):
if i in range (0, 10000):
a += 1
if not i % 1000: print i

print a, " ", round(time.time() - startTime, 1), "seconds"
---------------------------------
the last line of output is
---------------------------------

10001.0 22.8 seconds

so if i change the line

if i in range (0, 10000):

to

if i >= 0 and i < 10000:

the the last line is

10001.0 0.2 seconds

so approximately, the program ran 100 times faster!

or is there an alternative use of range() or something similar that can
be as fast?

Jul 18 '06 #1
45 8233
Su************@gmail.com wrote:
it seems that range() can be really slow:
....
if i in range (0, 10000):
This creates a 10,000-element list and sequentially searches it. Of
course that's gonna be slow.

Jul 18 '06 #2
Su************@gmail.com wrote:
or is there an alternative use of range() or something similar that can
be as fast?
You could use xrange:

leif@ubuntu:~$ python -m timeit -n10000 "1 in range(10000)"
10000 loops, best of 3: 260 usec per loop
leif@ubuntu:~$ python -m timeit -n10000 "1 in xrange(10000)"
10000 loops, best of 3: 0.664 usec per loop
Jul 18 '06 #3
Leif K-Brooks wrote:
Su************@gmail.com wrote:
or is there an alternative use of range() or something similar that can
be as fast?

You could use xrange:

leif@ubuntu:~$ python -m timeit -n10000 "1 in range(10000)"
10000 loops, best of 3: 260 usec per loop
leif@ubuntu:~$ python -m timeit -n10000 "1 in xrange(10000)"
10000 loops, best of 3: 0.664 usec per loop
That's only because you're choosing a number that's early in the list.

~$ python -m timeit -n10000 "1 in xrange(10000)"
10000 loops, best of 3: 1.22 usec per loop
~$ python -m timeit -n10000 "9999 in xrange(10000)"
10000 loops, best of 3: 1.24 msec per loop

That's *milliseconds*, not microseconds.

Jul 18 '06 #4
On 18/07/2006 12:41 PM, Su************@gmail.com wrote:
it seems that range() can be really slow:

the following program will run, and the last line shows how long it ran
for:

import time

startTime = time.time()

a = 1.0
for i in range(0, 30000):
if i in range (0, 10000):
a += 1
if not i % 1000: print i

print a, " ", round(time.time() - startTime, 1), "seconds"
---------------------------------
the last line of output is
---------------------------------

10001.0 22.8 seconds

so if i change the line

if i in range (0, 10000):

to

if i >= 0 and i < 10000:

the the last line is

10001.0 0.2 seconds

so approximately, the program ran 100 times faster!

or is there an alternative use of range() or something similar that can
be as fast?
Some things to try:
1a. Read what the manual has to say about the range() function ... what
does it produce?

1b. Read what the manual has to say about time.time() and time.clock().
Change over to using time.clock(). Change the round(...., 1) to (say) 4.

Alternatively, use something like this:

print "%.1f ... %.4f seconds" % (a, time.clock() - startTime)

1c. Repeat the two ways that you tried already.

2. First alternative:

Do this:

test_range = range(10000)

*once*, just after "a = 1.0".

and change your if test to

if i in test_range:

3. Now change that to:

test_range = set(range(10000))

4. Now forget about test_range, and change your if test to this:

if 0 <= i < 10000:

HTH,
John
Jul 18 '06 #5
Su************@gmail.com wrote:
so if i change the line
if i in range (0, 10000):
to
if i >= 0 and i < 10000:
[snip;]
is there an alternative use of range() or something similar that can
be as fast?

you've found that alternative yourself! just use the comparison operators...

in fact, you can write a little more compact as:
if 0 <= i < 10000 :
[sreeram;]
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Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org

iD8DBQFEvFGdrgn0plK5qqURAlXhAJ9mrod2ofLGlSCksnXqjz WDd0Y35wCgtx0f
+Fn3h07cOFT16NvCX+DDqnY=
=Hk3J
-----END PGP SIGNATURE-----

Jul 18 '06 #6
On 2006-07-18, Su************@gmail.com <Su************@gmail.comwrote:
it seems that range() can be really slow:

the following program will run, and the last line shows how long it ran
for:

import time

startTime = time.time()

a = 1.0
for i in range(0, 30000):
if i in range (0, 10000):
a += 1
if not i % 1000: print i

print a, " ", round(time.time() - startTime, 1), "seconds"
or is there an alternative use of range() or something similar that can
be as fast?
Creating and then searching a 10,000 element list to see if a
number is between two other numbers is insane.

Using xrange as somebody else suggested is also insane.

If you want to know if a number is between two other numders,
for pete's sake use the comparison operator like god intended.

if 0 <= i <= 10000:

--
Grant Edwards grante Yow! ANN JILLIAN'S HAIR
at makes LONI ANDERSON'S
visi.com HAIR look like RICARDO
MONTALBAN'S HAIR!
Jul 18 '06 #7
Grant Edwards wrote:
for pete's sake use the comparison operator like god intended.

if 0 <= i <= 10000:
I'm assuming you used Python's compound comparison as opposed to the
C-style of and'ing two comparisons together to emphasize the fact it is
god's chosen way of doing this ;-)

Jul 18 '06 #8
In <11**********************@h48g2000cwc.googlegroups .com>, tac-tics
wrote:
Grant Edwards wrote:
>for pete's sake use the comparison operator like god intended.

if 0 <= i <= 10000:

I'm assuming you used Python's compound comparison as opposed to the
C-style of and'ing two comparisons together to emphasize the fact it is
god's chosen way of doing this ;-)
Pete doesn't like to be called god in public. ;-)

Ciao,
Marc 'BlackJack' Rintsch
Jul 18 '06 #9
Grant Edwards <gr****@visi.comwrote:
Creating and then searching a 10,000 element list to see if a
number is between two other numbers is insane.
Using xrange as somebody else suggested is also insane.
Aye to both
If you want to know if a number is between two other numders,
for pete's sake use the comparison operator like god intended.

if 0 <= i <= 10000:
Sets are pretty fast too, and have the advantage of flexibility in
that you can put any numbers in you like

$ python2.4 -m timeit -s 's=range(0,10000); i=5000' 'i in s'
1000 loops, best of 3: 228 usec per loop

$ python2.4 -m timeit -s 's=set(range(0,10000)); i=5000' 'i in s'
1000000 loops, best of 3: 0.312 usec per loop

$ python2.4 -m timeit -s 'i=5000' '0 <= i < 10000'
1000000 loops, best of 3: 0.289 usec per loop

The below prints

range) That took 21.512 seconds: result 10001.0
set) That took 0.023 seconds: result 10001.0
comparison) That took 0.024 seconds: result 10001.0

.................................................. ...........

import time

start = time.time()
a = 1.0
for i in range(0, 30000):
if i in range(0, 10000):
a += 1
dt = time.time() - start
print "range) That took %.3f seconds: result %s" % (dt, a)

start = time.time()
a = 1.0
mine = set(range(0, 10000))
for i in range(0, 30000):
if i in mine:
a += 1
dt = time.time() - start
print "set) That took %.3f seconds: result %s" % (dt, a)

start = time.time()
a = 1.0
mine = set(range(0, 10000))
for i in range(0, 30000):
if 0 <= i < 10000:
a += 1
dt = time.time() - start
print "comparison) That took %.3f seconds: result %s" % (dt, a)

--
Nick Craig-Wood <ni**@craig-wood.com-- http://www.craig-wood.com/nick
Jul 18 '06 #10
John Machin wrote:
On 18/07/2006 12:41 PM, Su************@gmail.com wrote:
it seems that range() can be really slow:
[...]
Some things to try:
1a. Read what the manual has to say about the range() function ... what
does it produce?
Indeed. Still, the addition of a __contains__ method to range (and
xrange) would permit acceptable performance for the code given. Perhaps
this is a convenience worth considering for future Python releases.

Paul

Jul 18 '06 #11

Su************@gmail.com wrote:
it seems that range() can be really slow:
if i in range (0, 10000):
RTFM on range()

You're completely mis-using it here, using it with an if ... in ...
test. The purpose of range() in Python is as loop control, not
comparisons! It's not a SQL BETWEEN statement.

Although you _can_ do this (you've done it!) you've also found that
it's slow. Many people would argue that even using range() for loop
control is unusably slow.

Jul 18 '06 #12
Grant Edwards wrote:
Using xrange as somebody else suggested is also insane.
Sorry about that, I somehow got the misguided notion that xrange defines
its own __contains__, so that it would be about the same speed as using
comparison operators directly. I figured the OP might have a better
reason for wanting to use range() than his post mentioned -- perhaps the
range to check was being passed from a function, and it would be easier
to pass an object than a tuple of lower and upper bound -- but since
xrange does looping for a membership test, my suggestion was indeed insane.
Jul 18 '06 #13
On 18/07/2006 7:22 PM, Paul Boddie wrote:
John Machin wrote:
>On 18/07/2006 12:41 PM, Su************@gmail.com wrote:
>>it seems that range() can be really slow:

[...]
>Some things to try:
1a. Read what the manual has to say about the range() function ... what
does it produce?

Indeed. Still, the addition of a __contains__ method to range (and
xrange) would permit acceptable performance for the code given. Perhaps
this is a convenience worth considering for future Python releases.
range() and xrange() are functions. You are suggesting that 2
*functions* should acquire a __contains__ method each? I trust not.

Perhaps you meant that the acquisitors should be the objects that those
functions return.

range() returns a list. Lists already have a __contains__ method. It's
been getting a fair old exercising in the last few hours, but here we go
again:
>python -mtimeit -s"x=range(30000)" "x.__contains__(40000)"
1000 loops, best of 3: 1.09 msec per loop
>python -mtimeit -s"x=range(30000)" "40000 in x"
1000 loops, best of 3: 1.09 msec per loop
>python -mtimeit -s"x=range(30000)" "x.__contains__(1)"
1000000 loops, best of 3: 0.434 usec per loop
>python -mtimeit -s"x=range(30000)" "1 in x"
1000000 loops, best of 3: 0.137 usec per loop

A __contains__ method for xrange objects? The case of x in xrange(lo,
hi) is met by lo <= x < hi. IMHO, anyone who wants x in xrange(lo, hi,
step) should be encouraged to do the necessary algebra themselves; I
wouldn't suggest that any core dev time be wasted on implementing such
folderol.

In any case, isn't the *whole* xrange gizmoid on GvR's I-wish-I-hadn't list?

Cheers,
John
Jul 18 '06 #14
John Machin wrote:
>
range() and xrange() are functions. You are suggesting that 2
*functions* should acquire a __contains__ method each? I trust not.
Well, range is a function in the current implementation, although its
usage is similar to that one would get if it were a class, particularly
a subclass of list or one providing a list-style interface. With such a
class, you could provide a __contains__ method which could answer the
question of what the range contains based on the semantics guaranteed
by a range (in contrast to a normal list).
Perhaps you meant that the acquisitors should be the objects that those
functions return.
The whole point was to avoid expanding the range into a list.

[...]
A __contains__ method for xrange objects? The case of x in xrange(lo,
hi) is met by lo <= x < hi. IMHO, anyone who wants x in xrange(lo, hi,
step) should be encouraged to do the necessary algebra themselves; I
wouldn't suggest that any core dev time be wasted on implementing such
folderol.
Sure, you could always implement your own class to behave like an
existing range and to implement the efficient semantics proposed above.
In any case, isn't the *whole* xrange gizmoid on GvR's I-wish-I-hadn't list?
Yes, he wants range to return an iterator, just like xrange more or
less does now. Given that xrange objects support __getitem__, unlike a
lot of other iterators (and, of course, generators), adding
__contains__ wouldn't be much of a hardship. Certainly, compared to
other notational conveniences bounced around on the various development
lists, this one would probably provide an order of magnitude
improvement on the usual bang per buck development ratio.

Paul

Jul 18 '06 #15
Andy Dingley wrote:
The purpose of range() in Python is as loop control,
No, the purpose of range() is to create a list, as the docs state.

http://docs.python.org/lib/built-in-funcs.html

"""
range(...) - This is a versatile function to create lists containing
arithmetic progressions.
"""

Stefan
Jul 18 '06 #16
Paul Boddie wrote:
Yes, he wants range to return an iterator, just like xrange more or
less does now. Given that xrange objects support __getitem__, unlike a
lot of other iterators (and, of course, generators), adding
__contains__ wouldn't be much of a hardship. Certainly, compared to
other notational conveniences bounced around on the various development
lists, this one would probably provide an order of magnitude
improvement on the usual bang per buck development ratio.
xrange already has __contains__. The problem is, it's implemented by a
highly-inefficient sequential search. Why not modify it to merely
check the bounds and (value - start) % step == 0?

Jul 18 '06 #17
On 2006-07-18, tac-tics <ta*******@gmail.comwrote:
Grant Edwards wrote:
>for pete's sake use the comparison operator like god intended.

if 0 <= i <= 10000:

I'm assuming you used Python's compound comparison as opposed to the
C-style of and'ing two comparisons together to emphasize the fact it is
god's chosen way of doing this ;-)
Exactly!

--
Grant Edwards grante Yow! HUGH BEAUMONT died
at in 1982!!
visi.com
Jul 18 '06 #18
On 2006-07-18, Marc 'BlackJack' Rintsch <bj****@gmx.netwrote:
In <11**********************@h48g2000cwc.googlegroups .com>, tac-tics
wrote:
>Grant Edwards wrote:
>>for pete's sake use the comparison operator like god intended.

if 0 <= i <= 10000:

I'm assuming you used Python's compound comparison as opposed to the
C-style of and'ing two comparisons together to emphasize the fact it is
god's chosen way of doing this ;-)

Pete doesn't like to be called god in public. ;-)
Interesting point. Does the phrase "for pete's sake as god
intended" equate pete with god?

It's possible that pete is not god and yet god's intentions are
in pete's best interest, so something could be "for pete's
sake" and "as god intended" without pete being god.

That said, "for pete's sake" is probably a just an cleaned up
version of "for god's sake", so I probably did call pete god.

--
Grant Edwards grante Yow! This PORCUPINE knows
at his ZIPCODE... And he has
visi.com "VISA"!!
Jul 18 '06 #19
On 2006-07-18, Paul Boddie <pa**@boddie.org.ukwrote:
John Machin wrote:
>>
range() and xrange() are functions. You are suggesting that 2
*functions* should acquire a __contains__ method each? I trust
not.

Well, range is a function in the current implementation,
although its usage is similar to that one would get if it were
a class, particularly a subclass of list or one providing a
list-style interface. With such a class, you could provide a
__contains__ method which could answer the question of what
the range contains based on the semantics guaranteed by a
range (in contrast to a normal list).
>Perhaps you meant that the acquisitors should be the objects
that those functions return.

The whole point was to avoid expanding the range into a list.
It's unclear what you're referring to as "the range".

Perhaps you're thinking of a slice? Somethign like

if (0:10000).contains(x):

--
Grant Edwards grante Yow! Make me look like
at LINDA RONSTADT again!!
visi.com
Jul 18 '06 #20
Grant Edwards wrote:
On 2006-07-18, Marc 'BlackJack' Rintsch <bj****@gmx.netwrote:
>>In <11**********************@h48g2000cwc.googlegroups .com>, tac-tics
wrote:

>>>Grant Edwards wrote:

for pete's sake use the comparison operator like god intended.

if 0 <= i <= 10000:

I'm assuming you used Python's compound comparison as opposed to the
C-style of and'ing two comparisons together to emphasize the fact it is
god's chosen way of doing this ;-)

Pete doesn't like to be called god in public. ;-)


Interesting point. Does the phrase "for pete's sake as god
intended" equate pete with god?

It's possible that pete is not god and yet god's intentions are
in pete's best interest, so something could be "for pete's
sake" and "as god intended" without pete being god.

That said, "for pete's sake" is probably a just an cleaned up
version of "for god's sake", so I probably did call pete god.
No, actually you called god pete ;-)

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

Jul 18 '06 #21

Dan Bishop wrote:
Su************@gmail.com wrote:
it seems that range() can be really slow:
...
if i in range (0, 10000):

This creates a 10,000-element list and sequentially searches it. Of
course that's gonna be slow.
And you're doing it 30000 times.

Jul 18 '06 #22
Nick Craig-Wood wrote:
>
Sets are pretty fast too, and have the advantage of flexibility in
that you can put any numbers in you like
I know this is self-evident to most of the people reading this, but I
thought it worth pointing out that this is a great way to test
membership in range(lo, hi, step) without doing "the necessary
algebra".

i.e. n in set(xrange(0, 10000, 23)) ...
Peace,
~Simon

Jul 18 '06 #23
Simon Forman wrote:
Nick Craig-Wood wrote:
>>
Sets are pretty fast too, and have the advantage of flexibility in
that you can put any numbers in you like

I know this is self-evident to most of the people reading this, but I
thought it worth pointing out that this is a great way to test
membership in range(lo, hi, step) without doing "the necessary
algebra".

i.e. n in set(xrange(0, 10000, 23)) ...
No, its not. It works, but it works by no means faster than

n in range(0, 10000, 23)

Both need O(n), which is a bit slow compared to

(((n - 15) % 23) == 0 and n >= 15 and n < 10000)

that will run in O(1)

Diez
Jul 18 '06 #24
Diez B. Roggisch wrote:
Simon Forman wrote:
Nick Craig-Wood wrote:
>
Sets are pretty fast too, and have the advantage of flexibility in
that you can put any numbers in you like
I know this is self-evident to most of the people reading this, but I
thought it worth pointing out that this is a great way to test
membership in range(lo, hi, step) without doing "the necessary
algebra".

i.e. n in set(xrange(0, 10000, 23)) ...

No, its not. It works, but it works by no means faster than

n in range(0, 10000, 23)

Both need O(n), which is a bit slow compared to

(((n - 15) % 23) == 0 and n >= 15 and n < 10000)

that will run in O(1)

Diez
You're right, of course. I should have said that if you're testing
such a membership, say, 30000 times AND you (like me) are slightly
rusty on the algebra and prefer to let the computer do it, then you
could use something like:

test_set = set(xrange(0, 10000, 23))

if n in test_set:
...

;-)
~Simon

Jul 18 '06 #25
Grant Edwards wrote:
>
It's unclear what you're referring to as "the range".
The notion of something describing a range of values which can be
expanded to a list or, of relevance here, whose boundaries can be
tested efficiently.
Perhaps you're thinking of a slice? Somethign like

if (0:10000).contains(x):
Did you mean...?

(0:10000) # SyntaxError
slice(0, 10000).contains(x) # AttributeError
3 in slice(0, 10000) # TypeError

Something like this might suffice if slice could have a __contains__
method or if people thought of slices as natural things to test
against. Perhaps we could ask the original questioner why they chose to
use range in such a way - it might indicate a background in languages
which encourage the construction of ranges and their use in comparisons
- although being told to RTFM may have scared them off.

Paul

Jul 18 '06 #26
Simon Forman wrote:
Nick Craig-Wood wrote:
>Sets are pretty fast too, and have the advantage of flexibility in
that you can put any numbers in you like
I know this is self-evident to most of the people reading this, but I
thought it worth pointing out that this is a great way to test
membership in range(lo, hi, step) without doing "the necessary
algebra".

i.e. n in set(xrange(0, 10000, 23)) ...
This is very very misleading... here are some timings :

python -mtimeit "n=5000" "n in set(xrange(0,10000))"
1000 loops, best of 3: 1.32 msec per loop

python -mtimeit "n=5000" "n in xrange(0,10000)"
1000 loops, best of 3: 455 usec per loop

python -mtimeit "n=5000" "0 <= n < 10000"
1000000 loops, best of 3: 0.217 usec per loop

sets are fast only if you create them *once* and use them again and
again. even in that case, the sets use up O(n) memory.

with comparison operators, you don't need extra memory *and* there is no
pre-computation required.

[sreeram;]
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Jul 18 '06 #27
On 2006-07-18, Steve Holden <st***@holdenweb.comwrote:
>That said, "for pete's sake" is probably a just an cleaned up
version of "for god's sake", so I probably did call pete god.

No, actually you called god pete ;-)
Well that's certainly wrong, because we all know god's name is
Howard.

Our father who art in heaven,
Howard be thy name,
....

--
Grant Edwards grante Yow! .. I have a
at VISION! It's a RANCID
visi.com double-FISHWICH on an
ENRICHED BUN!!
Jul 18 '06 #28
On 2006-07-18, Paul Boddie <pa**@boddie.org.ukwrote:
>It's unclear what you're referring to as "the range".

The notion of something describing a range of values which can be
expanded to a list or, of relevance here, whose boundaries can be
tested efficiently.
>Perhaps you're thinking of a slice? Somethign like

if (0:10000).contains(x):
I didn't mean to imply that would actually work, but I thought
maybe that's what you were proposing.
Did you mean...?

(0:10000) # SyntaxError
slice(0, 10000).contains(x) # AttributeError
3 in slice(0, 10000) # TypeError

Something like this might suffice if slice could have a __contains__
method or if people thought of slices as natural things to test
against.
A slice seems to me to be the obvious way to represent a finite
length algebraic sequence of integers. However, obvioiusness
is in the eye of the beholder since as you point out below to
the OP, a range() was the obvious way to it.
Perhaps we could ask the original questioner why they chose to
use range in such a way - it might indicate a background in
languages which encourage the construction of ranges and their
use in comparisons - although being told to RTFM may have
scared them off.
--
Grant Edwards grante Yow! Did I say I was a
at sardine? Or a bus???
visi.com
Jul 18 '06 #29
Dan Bishop:
xrange already has __contains__. The problem is, it's implemented by a
highly-inefficient sequential search. Why not modify it to merely
check the bounds and (value - start) % step == 0?
I think this is a nice idea.

Bye,
bearophile

Jul 18 '06 #30
K.S.Sreeram wrote:
Simon Forman wrote:
Nick Craig-Wood wrote:
Sets are pretty fast too, and have the advantage of flexibility in
that you can put any numbers in you like
I know this is self-evident to most of the people reading this, but I
thought it worth pointing out that this is a great way to test
membership in range(lo, hi, step) without doing "the necessary
algebra".

i.e. n in set(xrange(0, 10000, 23)) ...

This is very very misleading... here are some timings :
Yes it is. I'm sorry about that.
python -mtimeit "n=5000" "n in set(xrange(0,10000))"
1000 loops, best of 3: 1.32 msec per loop

python -mtimeit "n=5000" "n in xrange(0,10000)"
1000 loops, best of 3: 455 usec per loop

python -mtimeit "n=5000" "0 <= n < 10000"
1000000 loops, best of 3: 0.217 usec per loop

sets are fast only if you create them *once* and use them again and
again. even in that case, the sets use up O(n) memory.
That's what I meant. But I didn't state it clearly.

One of the things I like most about python is that it allows you to
specify the problem that you want to solve without a great deal of
difficulty as to *how* to specify it. To me, and perhaps others, "T =
set(xrange(0, 10000, 23))" and "n in T" are somewhat easier to read
and write than "not n % 23 and 0 <= n < 10000", YMMV.

In the given case a set of ~(10000 / 23) ints would not usually be too
burdensome on ram, and the code runs close to the same speed as
compared to the direct calculation:

from timeit import Timer

times = 100000
Max = 10000
n = 5000
T = set(xrange(0, Max, 23))

s1 = 'n in T'
s2 = 'not n %% 23 and 0 <= n < %s' % Max

setup = 'from __main__ import n, T'
S1 = Timer(s1, setup).repeat(number=times)
S2 = Timer(s2, setup).repeat(number=times)
print "%.3f usec/pass" % (1000000 * min(S1) / times)
print "%.3f usec/pass" % (1000000 * min(S2) / times)

On my machine this printed:
0.476 usec/pass
0.552 usec/pass

>
with comparison operators, you don't need extra memory *and* there is no
pre-computation required.
When I set Max = 100000000 in the above test code there was serious
disk thrashing... ;-)
>
[sreeram;]

FWIW, in production code I would certainly use the comparison
operators. A kilobyte saved is a kilobyte earned.

Peace,
~Simon

Jul 18 '06 #31

Su************@gmail.com wrote:
it seems that range() can be really slow:

if i in range (0, 10000):

My original use was like this:

if i in range (iStart, iEnd):
listData.append(a)

in which iStart is 1000 and iEnd is 1008

so in that case, the program ran fine...
but later on, i wanted to include all data, so I relaxed the range by
setting iStart to 0 and iEnd to 9999 and later on i found that the
program was slow due to this.

So looks like the usage of

if sDay in ("Tue", "Wed", "Thu"):

is more like good use of "in a list" but in range(0,10000) will be a
big search in a list.

Jul 18 '06 #32
Simon Forman wrote:
To me, and perhaps others, "T =
set(xrange(0, 10000, 23))" and "n in T" are somewhat easier to read
and write than "not n % 23 and 0 <= n < 10000", YMMV.
Eh? How is the first easier to read than the second?? You have a nested
function call in the first!

Regardless, testing if a member is part of a ranged set is always going
to be slower. It's the nature of what you're doing. Building a set and
then searching it takes much longer than a single modulus and
subtraction (which is all an integer comparison is).

Jul 18 '06 #33
On 19/07/2006 1:05 AM, Dan Bishop wrote:
Paul Boddie wrote:
>Yes, he wants range to return an iterator, just like xrange more or
less does now. Given that xrange objects support __getitem__, unlike a
lot of other iterators (and, of course, generators), adding
__contains__ wouldn't be much of a hardship. Certainly, compared to
other notational conveniences bounced around on the various development
lists, this one would probably provide an order of magnitude
improvement on the usual bang per buck development ratio.

xrange already has __contains__.
As pointed out previously, xrange is a function and one would not expect
it to have a __contains__ method.

The objects returned by xrange do not (according to my reading of the
2.4.3 version of Objects/rangeobject.c) have a __contains__ method.

I find it difficult to believe that an inefficient __contains__ has been
implemented since.

Perhaps you are unaware that the mere fact that an object supports the
"in" operation does not mean that this support is provided by a
__contains__ method. The following section of the manual may help:

"""
The membership test operators (in and not in) are normally implemented
as an iteration through a sequence. However, container objects can
supply the following special method with a more efficient
implementation, which also does not require the object be a sequence.

__contains__( self, item)
Called to implement membership test operators. Should return true
if item is in self, false otherwise. For mapping objects, this should
consider the keys of the mapping rather than the values or the key-item
pairs.
"""

Jul 18 '06 #34
John Machin wrote:
On 19/07/2006 1:05 AM, Dan Bishop wrote:

xrange already has __contains__.

As pointed out previously, xrange is a function and one would not expect
it to have a __contains__ method.
Well, you pointed out that range is a function, but xrange seems to be
a type...
>>xrange
<type 'xrange'>
>>dir(xrange)
['__class__', '__delattr__', '__doc__', '__getattribute__',
'__getitem__', '__hash__', '__init__', '__iter__', '__len__',
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__',
'__setattr__', '__str__']

No __contains__ method, though, at least in 2.4.1.
The objects returned by xrange do not (according to my reading of the
2.4.3 version of Objects/rangeobject.c) have a __contains__ method.
As confirmed by the above evidence.
I find it difficult to believe that an inefficient __contains__ has been
implemented since.
So do I. As you go on to say, the usual sequence traversal mechanisms
are probably used to support the "in" operator. Whether it's a pressing
matter to add support for a more efficient mechanism depends on how
often people want to use ranges in the way described. Perhaps I'll
write a patch - who knows? ;-)

Paul

Jul 18 '06 #35
tac-tics wrote:
Simon Forman wrote:
To me, and perhaps others, "T =
set(xrange(0, 10000, 23))" and "n in T" are somewhat easier to read
and write than "not n % 23 and 0 <= n < 10000", YMMV.

Eh? How is the first easier to read than the second?? You have a nested
function call in the first!
I find the first form more immediately comprehensible than the latter.
I know what xrange() does, and I know what set() does, and "nested
function calls" give me no trouble, whereas the latter form with a
modulus, negation, and comparisons would take me a bit longer both to
compose and/or understand.

If this is not the case for you then by all means please disregard my
posting. YMMV.
>
Regardless, testing if a member is part of a ranged set is always going
to be slower.
Yes. Roughly 0.0000001 seconds slower on my five year old computer.
I'm not worried.
It's the nature of what you're doing. Building a set and
then searching it takes much longer than a single modulus and
subtraction (which is all an integer comparison is).
Building the set, yes, but searching the set is very close to the same
speed, even for rather large sets. If you were performing the search
30000 times (like in the OP) it would only take about three thousandths
of a second longer, and that's on my old slow computer.

If I were doing this a thousand times more often, or on a range of a
million or more, or in production code, or with ranges that changed
often, then I would certainly take the time to write out the latter
form.
Peace,
~Simon

Jul 19 '06 #36
Paul Boddie wrote:
John Machin wrote:
On 19/07/2006 1:05 AM, Dan Bishop wrote:
>
xrange already has __contains__.
As pointed out previously, xrange is a function and one would not expect
it to have a __contains__ method.

Well, you pointed out that range is a function, but xrange seems to be
a type...
>xrange
<type 'xrange'>
>dir(xrange)
['__class__', '__delattr__', '__doc__', '__getattribute__',
'__getitem__', '__hash__', '__init__', '__iter__', '__len__',
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__',
'__setattr__', '__str__']

No __contains__ method, though, at least in 2.4.1.
The objects returned by xrange do not (according to my reading of the
2.4.3 version of Objects/rangeobject.c) have a __contains__ method.

As confirmed by the above evidence.
I find it difficult to believe that an inefficient __contains__ has been
implemented since.

So do I. As you go on to say, the usual sequence traversal mechanisms
are probably used to support the "in" operator. Whether it's a pressing
matter to add support for a more efficient mechanism depends on how
often people want to use ranges in the way described. Perhaps I'll
write a patch - who knows? ;-)
My mistake. I should have looked at dir(xrange) before posting.

But the point remains that xrange's "implicit __contains__" runs in
linear time when a constant-time algorithm exists.

Jul 19 '06 #37
Paul Boddie <pa**@boddie.org.ukwrote:
John Machin wrote:

range() and xrange() are functions. You are suggesting that 2
*functions* should acquire a __contains__ method each? I trust not.

Well, range is a function in the current implementation, although its
usage is similar to that one would get if it were a class, particularly
a subclass of list or one providing a list-style interface. With such a
class, you could provide a __contains__ method which could answer the
question of what the range contains based on the semantics guaranteed
by a range (in contrast to a normal list).
You'd also have to override just about every mutating method to switch
back to a "normal" __contains__ (or change self's type on the fly) -- a
pretty heavy price to pay.

I have often noticed that subclassing list, dict and maybe set has this
kind of issue: the need to track every possible change to the object.

Maybe a good mechanism to have for the purpose would be to add to
mutable types a "hook" method, say __mutator__, which gets called either
right before or right after any mutating method (there are different
tradeoffs for before-calls and after-calls), presumably passing along
the *a and **k for generality (although it might be faster for the base
case to avoid that); the base types would have a no-op implementation,
but subtypes could easily override just the hook to facilitate their
task of maintaining extra state (could be as little as a per-instance
flag recording whether the object is guaranteed to be still "pristine").
At C level, that might be an extra slot tp_mutator, left NULL in base
types to indicate "no mutator-hook method implemented here".

Like any other addition of, or change to, functionality, this would of
course be a proposal for 2.6, since 2.5 is feature-frozen now.
Alex
Jul 20 '06 #38
Alex Martelli wrote:
Paul Boddie <pa**@boddie.org.ukwrote:

Well, range is a function in the current implementation, although its
usage is similar to that one would get if it were a class, particularly
a subclass of list or one providing a list-style interface. With such a
class, you could provide a __contains__ method which could answer the
question of what the range contains based on the semantics guaranteed
by a range (in contrast to a normal list).

You'd also have to override just about every mutating method to switch
back to a "normal" __contains__ (or change self's type on the fly) -- a
pretty heavy price to pay.
A subclass of list is probably a bad idea in hindsight, due to various
probable requirements of it actually needing to be a list with all its
contents, whereas we wanted to avoid having anything like a list around
until the contents of this "lazy list" were required by the program. If
we really wanted to subclass something, we could consider subclassing
the slice class/type, but that isn't subclassable in today's Python for
some reason, and it doesn't really provide anything substantial,
anyway. However, Python being the language it is, an appropriately
behaving class is quite easily written from scratch.

Paul

Jul 20 '06 #39
Paul Boddie <pa**@boddie.org.ukwrote:
Alex Martelli wrote:
Paul Boddie <pa**@boddie.org.ukwrote:
>
Well, range is a function in the current implementation, although its
usage is similar to that one would get if it were a class, particularly
a subclass of list or one providing a list-style interface. With such a
class, you could provide a __contains__ method which could answer the
question of what the range contains based on the semantics guaranteed
by a range (in contrast to a normal list).
You'd also have to override just about every mutating method to switch
back to a "normal" __contains__ (or change self's type on the fly) -- a
pretty heavy price to pay.

A subclass of list is probably a bad idea in hindsight, due to various
probable requirements of it actually needing to be a list with all its
contents, whereas we wanted to avoid having anything like a list around
until the contents of this "lazy list" were required by the program. If
we really wanted to subclass something, we could consider subclassing
the slice class/type, but that isn't subclassable in today's Python for
some reason, and it doesn't really provide anything substantial,
anyway. However, Python being the language it is, an appropriately
behaving class is quite easily written from scratch.
Nevertheless, that class will still need to implement every single
method of the list type; making it a subclass of list has some advantage
in that every such implementation of a method can basically fill the
real list, self.__class__=list, and leave all the rest, forevermore
(explicitly here, implicitly in the future), to class list. Performance
should be much better than by working off semi-deprecated UserList.

A "hook method" __mutator__ (ideally called _before_ in this case), as I
was proposing (for 2.6 or later), would make such approaches way easier
and handier (and would help with most use cases I can think of for
subclassing list, dict or set).
Alex
Jul 20 '06 #40
On 2006-07-20, Paul Boddie <pa**@boddie.org.ukwrote:
Alex Martelli wrote:
>Paul Boddie <pa**@boddie.org.ukwrote:
>
Well, range is a function in the current implementation, although its
usage is similar to that one would get if it were a class, particularly
a subclass of list or one providing a list-style interface. With such a
class, you could provide a __contains__ method which could answer the
question of what the range contains based on the semantics guaranteed
by a range (in contrast to a normal list).

You'd also have to override just about every mutating method to switch
back to a "normal" __contains__ (or change self's type on the fly) -- a
pretty heavy price to pay.

A subclass of list is probably a bad idea in hindsight, due to various
probable requirements of it actually needing to be a list with all its
contents, whereas we wanted to avoid having anything like a list around
until the contents of this "lazy list" were required by the program. If
we really wanted to subclass something, we could consider subclassing
the slice class/type, but that isn't subclassable in today's Python for
some reason, and it doesn't really provide anything substantial,
anyway. However, Python being the language it is, an appropriately
behaving class is quite easily written from scratch.
Except that if you write your own class from scratch, you can't use
it as a slice. For a language that is supposed to be about duck typing
I find it strange that if I make my own class with a start, stop and
step attribute, that python barfs on it when I want to use it as a
slice.
>>class sl(object):
.... def __init__(self, start = None, stop = None, step = None):
.... self.start = start
.... self.stop = stop
.... self.step = step
....
>>lst = range(20)
s1 = slice(3,13)
s2 = sl(3,13)
s1.start
3
>>s2.start
3
>>s1.stop
13
>>s2.stop
13
>>s1.step
s2.step
lst[s1]
[3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
>>lst[s2]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: list indices must be integers
>>>
--
Antoon Pardon
Jul 21 '06 #41
Antoon Pardon wrote:
>
[Subclasses of list or slice for ranges]
Except that if you write your own class from scratch, you can't use
it as a slice. For a language that is supposed to be about duck typing
I find it strange that if I make my own class with a start, stop and
step attribute, that python barfs on it when I want to use it as a
slice.
[s2 has start, stop, step...]
>lst[s2]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: list indices must be integers
In fact, the duck typing seems only to really work outside the
interpreter and the various accompanying built-in classes. Consider a
class similar to the one you defined with the name myslice (for
clarity) and with the apparently necessary indices method; consider it
used as follows:
>>range(0, 10)[myslice(1, 5, 2)]
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: list indices must be integers

Here, we start to believe that only traditional index or slice notation
will work. However, we did come across the built-in slice class before;
consider this:
>>range(0, 10)[slice(1, 5, 2)]
[1, 3]

Regardless of whether myslice inherits from object or not, there's no
persuading the interpreter that it is a genuine slice, and remember
that we can't subclass slice (for some reason unknown). So, it would
appear that the interpreter really wants instances from some specific
set of types (presumably discoverable by looking at list_subscript in
listobject.c) rather than some objects conforming to some interface or
protocol, and perhaps it is implemented this way for performance
reasons.

In any case, in the core of Python some types/classes are more equal
than others, and for whatever reason the duck typing breaks down - a
case of "malbik endar" [1] that you just have to be aware of, I
suppose.

Paul

[1] http://www.sciamanna.com/island/pop/...lbik_endar.htm

Jul 21 '06 #42
Paul Boddie wrote:
[...]
Regardless of whether myslice inherits from object or not, there's no
persuading the interpreter that it is a genuine slice, and remember
that we can't subclass slice (for some reason unknown). So, it would
appear that the interpreter really wants instances from some specific
set of types (presumably discoverable by looking at list_subscript in
listobject.c) rather than some objects conforming to some interface or
protocol, and perhaps it is implemented this way for performance
reasons.

In any case, in the core of Python some types/classes are more equal
than others, and for whatever reason the duck typing breaks down - a
case of "malbik endar" [1] that you just have to be aware of, I
suppose.
>>class mySlice(types.SliceType):
... pass
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: Error when calling the metaclass bases
type 'slice' is not an acceptable base type
>>>
Indeed.

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

Jul 21 '06 #43
On 2006-07-21, Paul Boddie <pa**@boddie.org.ukwrote:
Regardless of whether myslice inherits from object or not, there's no
persuading the interpreter that it is a genuine slice, and remember
that we can't subclass slice (for some reason unknown). So, it would
appear that the interpreter really wants instances from some specific
set of types (presumably discoverable by looking at list_subscript in
listobject.c) rather than some objects conforming to some interface or
protocol, and perhaps it is implemented this way for performance
reasons.

In any case, in the core of Python some types/classes are more equal
than others, and for whatever reason the duck typing breaks down - a
case of "malbik endar" [1] that you just have to be aware of, I
suppose.
Is there any chance this will iever change?

Is there any chance the start:stop:step notation will ever
be considered an atom?

--
Antoon Pardon
Jul 23 '06 #44
Antoon Pardon wrote:
>
Except that if you write your own class from scratch, you can't use
it as a slice.
Correct, but we were actually discussing subclassing built-in classes
for use as a replacement for range/xrange. :-)

It may be "hard work" writing all those methods in a totally new
range/xrange class, but passing objects of that class around should
prove satisfactory for the use of most programs. I personally doubt
that it is that much hard work, especially if you stick to a reasonable
selection of list capabilities, for example, rather than attempting to
emulate support for every dodgy trick available to the programmer in
the modern CPython arsenal.
For a language that is supposed to be about duck typing
I find it strange that if I make my own class with a start, stop and
step attribute, that python barfs on it when I want to use it as a
slice.
Yes, my post showed this and gave a reference to where in the CPython
source code the tests for specific types are performed.

Paul

Jul 23 '06 #45
On 2006-07-23, Paul Boddie <pa**@boddie.org.ukwrote:
Antoon Pardon wrote:
>>
Except that if you write your own class from scratch, you can't use
it as a slice.

Correct, but we were actually discussing subclassing built-in classes
for use as a replacement for range/xrange. :-)
I think a slice could be very usefull for that.

To me it feel very natural that a slice would be iterable and
could be used in a form like:

for i in slice(10,20):

or if the slice notation would be usable

for i in (10:20):

And why not make a slice indexable too?
Whether slices really are the tool to use here or not
I don know for sure. But the problem is, that you
can't easily experiment with this idea because slices
are not subclassable.
It may be "hard work" writing all those methods in a totally new
range/xrange class, but passing objects of that class around should
prove satisfactory for the use of most programs.
But the parameters you will have to give such a class are the same
you have to pass to a slice. IMO that means that you should at least
consider giving slices this functionality. At least it should be
easy to convert from one to the other and back.

--
Antoon Pardon
Jul 25 '06 #46

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