Velocity Reviews > Sampling a population

# Sampling a population

Brian Quinlan
Guest
Posts: n/a

 06-02-2006
This is less a Python question and more a optimization/probability
question. Imaging that you have a list of objects and there frequency in
a population e.g.

lst = [(a, 0.01), (b, 0.05), (c, 0.50), (d, 0.30), (e, 0.04), (f, 0.10)]

and you want to drawn n items from that list (duplicates allowed), with
that probability distribution.

The fastest algorithm that I have been able to devise for doing so is:
O(n * log(len(lst))). Can anyone think or a solution with a better time
complexity? If not, is there an obviously better way to do this
(assuming n is big and the list size is small).

Here is the code:

from random import random
from bisect import bisect

def draw(n, lst):
ps = []
last = 0
for p in lst:
ps.append(last + p)
last += p

# ps = [0.01, 0.06, 0.56, 0.86, 0.90, 1.00]

chosen = [0] * len(lst) # track frequency
for i in range(n):
r = random()

chosen[bisect(ps, r)] += 1 # binary search and increment

result = [] # rescale probability based on frequency
for c in chosen:
result.append(float(c) / n)
return result

lst = [0.01, 0.05, 0.50, 0.30, 0.04, 0.10]
print draw(10000, lst)

Duncan Smith
Guest
Posts: n/a

 06-02-2006
Brian Quinlan wrote:
> This is less a Python question and more a optimization/probability
> question. Imaging that you have a list of objects and there frequency in
> a population e.g.
>
> lst = [(a, 0.01), (b, 0.05), (c, 0.50), (d, 0.30), (e, 0.04), (f, 0.10)]
>
> and you want to drawn n items from that list (duplicates allowed), with
> that probability distribution.
>
> The fastest algorithm that I have been able to devise for doing so is:
> O(n * log(len(lst))). Can anyone think or a solution with a better time
> complexity? If not, is there an obviously better way to do this
> (assuming n is big and the list size is small).
>
> Here is the code:
>
> from random import random
> from bisect import bisect
>
> def draw(n, lst):
> ps = []
> last = 0
> for p in lst:
> ps.append(last + p)
> last += p
>
> # ps = [0.01, 0.06, 0.56, 0.86, 0.90, 1.00]
>
> chosen = [0] * len(lst) # track frequency
> for i in range(n):
> r = random()
>
> chosen[bisect(ps, r)] += 1 # binary search and increment
>
> result = [] # rescale probability based on frequency
> for c in chosen:
> result.append(float(c) / n)
> return result
>
> lst = [0.01, 0.05, 0.50, 0.30, 0.04, 0.10]
> print draw(10000, lst)
>

I would do something like the following (maybe with an additional check
that the probabilities do not sum to less than 1),

>>> from random import random
>>> import operator
>>> def draw(n, lst):

lst.sort(key=operator.itemgetter(1), reverse=True)
cumprobs = []
this_cp = 0
for p in lst:
this_cp += p[1]
cumprobs.append(this_cp)
for _ in xrange(n):
rnd = random()
for i, cumprob in enumerate(cumprobs):
if rnd < cumprob:
yield lst[i][0]
break

>>> lst = [('a', 0.01), ('b', 0.05), ('c', 0.50), ('d', 0.30), ('e',

0.04), ('f', 0.10)]
>>> list(draw(8, lst))

['d', 'd', 'c', 'e', 'c', 'd', 'c', 'f']
>>>

The sorting of the list means that iterating over the cumulative
probabilities is minimised (for your density function the inner loop
will be broken out of after the first iteration 50% of the time). (I've
assumed that it doesn't matter to you that the list is sorted.) I'm not
sure that this is in any way optimal.

Duncan

Mitja Trampus
Guest
Posts: n/a

 06-02-2006
Brian Quinlan wrote:
> The fastest algorithm that I have been able to devise for doing so is:
> O(n * log(len(lst))). Can anyone think or a solution with a better time
> complexity? If not, is there an obviously better way to do this
> (assuming n is big and the list size is small).

If list is indeed short, I'd say common sense speaks against complicating and optimizing
further - you can only get log(len(lst))-fold speedup, which is in your case more or less
a small constant. _IF_ this part of code later turns out to be a bottleneck, you might
profit more by porting it to C than searching for an O(n) solution, if it even exists.

Guest
Posts: n/a

 06-02-2006
Brian Quinlan wrote:
> This is less a Python question and more a optimization/probability
> question. Imaging that you have a list of objects and there frequency in
> a population e.g.
>
> lst = [(a, 0.01), (b, 0.05), (c, 0.50), (d, 0.30), (e, 0.04), (f, 0.10)]
>
> and you want to drawn n items from that list (duplicates allowed), with
> that probability distribution.
>
> The fastest algorithm that I have been able to devise for doing so is:
> O(n * log(len(lst))). Can anyone think or a solution with a better time
> complexity? If not, is there an obviously better way to do this
> (assuming n is big and the list size is small).
>

Any way I tried to slice and dice it, I could not get any faster.
draw2 and draw 3 generate code on the fly. draw4 sneakily tries to
trade memory and accuracy for speed but is even slower!

First the times, then the code:

\$ ./timeit.py 'from probDistribution import draw as draw; draw(10000,
[0.01, 0.05, 0.50, 0.30, 0.04, 0.10])'
100 loops, best of 3: 13.4 msec per loop

\$ ./timeit.py 'from probDistribution import draw2 as draw; draw(10000,
[0.01, 0.05, 0.50, 0.30, 0.04, 0.10])'
100 loops, best of 3: 15.2 msec per loop

\$ ./timeit.py 'from probDistribution import draw3 as draw; draw(10000,
[0.01, 0.05, 0.50, 0.30, 0.04, 0.10])'
100 loops, best of 3: 16.2 msec per loop

\$ ./timeit.py 'from probDistribution import draw4 as draw; draw(10000,
[0.01, 0.05, 0.50, 0.30, 0.04, 0.10])'
10 loops, best of 3: 30.5 msec per loop

=== CODE probDistribution.py ===
from random import random, randrange
from bisect import bisect

def draw(n, lst):
ps = []
last = 0
for p in lst:
ps.append(last + p)
last += p

# ps = [0.01, 0.06, 0.56, 0.86, 0.90, 1.00]

chosen = [0] * len(lst) # track frequency
for i in range(n):
r = random()

chosen[bisect(ps, r)] += 1 # binary search and increment

result = [] # rescale probability based on frequency
for c in chosen:
result.append(float(c) / n)
return result

def draw2(n, lst):
""" uses dynamicc code generation of this form:
chosen = [0] * 6
for i in xrange(10000):
r = random()
if r < 0.01: chosen[0]+=1
elif r < 0.06: chosen[1] +=1
...
elif r < 0.90: chosen[4] +=1
else chosen[5]+=1
"""
assert len(lst)>1, "Corner case NOT covered"

codestr = 'chosen = [0] * %i\n' % (len(lst),)
codestr += 'for i in xrange(%i):\n r = random()\n' % (n,)

last = 0.0
lstmax = len(lst)-1
for i,p in enumerate(lst):
last += p
if i==0: codestr += ' if r < %g: chosen[%i] +=1\n' %
(last, i)
elif i==lstmax: codestr += ' else: chosen[%i] +=1\n' % (i,)
else: codestr += ' elif r < %g: chosen[%i] +=1\n' %
(last, i)

exec codestr

result = [] # rescale probability based on frequency
for c in chosen:
result.append(float(c) / n)
return result

def draw3(n, lst):
""" uses dynamicc code generation of this form:
chosen = [0] * 6
for i in xrange(10000):
r = random()
chosen[-1+ (
((r<0.01) and 1)
or ((r<0.06) and 2)
...
or ((r<0.90) and 5)
or 6
)] +=1

"""
assert len(lst)>1, "Corner case NOT covered"

codestr = 'chosen = [0] * %i\n' % (len(lst),)
codestr += 'for i in xrange(%i):\n r = random()\n' % (n,)
codestr += ' chosen[-1+ (\n'

last = 0.0
lstmax = len(lst)-1
for i,p in enumerate(lst):
last += p
if i==0: codestr += ' ((r<%g) and 1)\n' % (last)
elif i==lstmax: codestr += ' or %i\n )] +=1\n' % (i+1,)
else: codestr += ' or ((r<%g) and %i)\n' % (last,
i+1)
#return codestr
exec codestr

result = [] # rescale probability based on frequency
for c in chosen:
result.append(float(c) / n)
return result

def draw4(n, lst, precision = 0.01, maxbins=10000):
""" Memory/speed tradeoff by coarse quantizing of frequency values.

"""
assert len(lst)>1, "Corner case NOT covered"
assert 0.0 < precision < 1.0 and (1.0/precision) < maxbins

binmax = int(1.0/precision)
chosenbin = [0] * binmax
for i in xrange(n):
chosenbin[randrange(binmax)] +=1

left, right = 0, 0 # extract bin range for summation
chosen = [0] * len(lst)
last = 0.0
for i,p in enumerate(lst):
last += p
right = int(last/precision)
chosen[i] = sum( chosenbin[left:right] )
left = right

result = [] # rescale probability based on frequency
for c in chosen:
result.append(float(c) / n)
return result

=== END CODE ===

Terry Reedy
Guest
Posts: n/a

 06-02-2006

"Paddy" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) ups.com...
> Brian Quinlan wrote:
>> This is less a Python question and more a optimization/probability
>> question. Imaging that you have a list of objects and there frequency in
>> a population e.g.
>>
>> lst = [(a, 0.01), (b, 0.05), (c, 0.50), (d, 0.30), (e, 0.04), (f, 0.10)]
>>
>> and you want to drawn n items from that list (duplicates allowed), with
>> that probability distribution.

For a set of probabilities that uneven, you *might* do better to sort high
to low and do a linear search. Or check the two common cases and then do a
binary search on the rest.

tjr

Roger Miller
Guest
Posts: n/a

 06-02-2006
For your example, since the probabilities are all multiples of 0.01 you
could make a list of 100 elements. Set one of them to a, 5 of them to
b, 50 of them to c, etc. Then just randomly sample from the table
(which is O(1)). Of course if the probabilities can be arbitrary
floating point values then this won't work. But if you can live with
rounding to 3 or 4 digits this is probably the fastest and easiest
approach.