nearest neighbor in 2D
Jim Richardson
warlock at eskimo.com
Wed Nov 5 02:53:27 CET 2003
On Tue, 04 Nov 2003 17:13:58 +0100,
Peter Otten <__peter__ at web.de> wrote:
> Alex Martelli wrote:
>
>> Andrew Dalke wrote:
>>
>>> Ron Adam
>>>> for point in p:
>>>> distance = math.sqrt((new_point[0]-point[0])**2 \
>>>> +(new_point[1]-point[1])**2)
>>>
>>>> I really don't know how you can make this faster. There might be a
>>
>> Hmmm, that's what math.hypot is for, isn't it...?
>>
>> [alex at lancelot Lib]$ timeit.py -c -s'import math; p=1.6,2.5; np=2.4,1.3'
>> 'math.sqrt((np[0]-p[0])**2 + (np[1]-p[1])**2)'
>> 100000 loops, best of 3: 3 usec per loop
>>
>> [alex at lancelot Lib]$ timeit.py -c -s'import math; p=1.6,2.5; np=2.4,1.3'
>> 'math.hypot(np[0]-p[0], np[1]-p[1])'
>> 100000 loops, best of 3: 1.9 usec per loop
>>
>>
>>>> library that has a distance between two points function that could
>>>> speed it up.
>>>
>>> An easy way is to move the math.sqrt call outside the loop, since
>>> sqrt(d1) < sqrt(d2) iff d1 < d2 (when d1,d2>=0)
>>
>> Yes, omitting the math.sqrt gives the same speed as calling math.hypot,
>> and it's the classic solution to speed up minimum-distance problems.
>>
>> I vaguely suspect you could shave some further fraction of a microsecond
>> by saving those differences as dx and dy and then computing dx*dx+dy*dy --
>> since another classic tip is that a**2 is slower than a*2. Let's see...:
>>
>> [alex at lancelot Lib]$ timeit.py -c -s'import math; p=1.6,2.5; np=2.4,1.3'
>> 'dx=np[0]-p[0]; dy=np[1]-p[1]; disq=dx*dx+dy*dy'
>> 1000000 loops, best of 3: 1.39 usec per loop
>>
>> ...yep, another small enhancement. Ain't measuring _FUN_?-)
>
> Finally found an application for complex numbers:
>
> ...> timeit.py -s"p= 1.6+2.5j; np=2.4+1.3j" "d=abs(p-np)"
> 1000000 loops, best of 3: 0.436 usec per loop
>
> ...> timeit.py -s"p= 1.6,2.5; np=2.4,1.3" "dx=np[0]-p[0];
> dy=np[1]-p[1];d=dx*dx+dy*dy"
> 1000000 loops, best of 3: 1.15 usec per loop
>
> This is of course all premature optimization as the most promising approach
> is to try hard to reduce the number of candidate points, as David Eppstein
> seems to have done. But then, he could use complex numbers, too.
>
> Peter
>
I am new to timeit.py, but this is odd.
jim at grendel:~$ /usr/lib/python2.3/timeit.py -c ' p=1.6+2.5j;np=2.4+1.3j; d=abs(p-np)'
100000 loops, best of 3: 3.1 usec per loop
vs
jim at grendel:~$ /usr/lib/python2.3/timeit.py -c -s'import math; p=1.6+2.5j;np=2.4+1.3j; d=abs(p-np)'
10000000 loops, best of 3: 0.141 usec per loop
Is it because the builtin math functions are much slower?
--
Jim Richardson http://www.eskimo.com/~warlock
Televangelists: The Pro Wrestlers of Religion
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