According to the blog post here, an any() + generator expression should run quicker than a for loop, and it seems like his reasoning makes sense.
But I've tried to use this method (albeit on some other function), but it seems to take a longer time to run than an explicit for loop.
def with_loop(a, b):
for x in xrange(1, b):
if x * a % b == 1: return True
return False
def with_generator(a, b):
return any(x * a % b == 1 for x in xrange(1, b))
Basically the code loops through all the numbers from 1 to b to find whether the number a has a modular inverse.
The times I got from running the functions are:
>>> from timeit import Timer as T
>>> T(lambda : with_generator(100, 300)).repeat(number = 100000)
[3.4041796334919923, 3.6303230626526215, 3.6714475531563266]
>>> T(lambda : with_loop(100, 300)).repeat(number = 100000)
[2.1977450660490376, 2.2083902291327604, 2.1905292602997406]
>>> T(lambda : with_generator(101, 300)).repeat(number = 100000)
[1.213779524696747, 1.2228346702509043, 1.2216941170634072]
>>> T(lambda : with_loop(101, 300)).repeat(number = 100000)
[0.7431202233722161, 0.7444361146951906, 0.7525384471628058]
with_generator(100,300) returns False and with_generator(101,300) returns True.
It seems that with_generator always takes a longer time to run than with_loop. Is there any reason for this?
EDIT: Is there any other shorter or more elegant way of rewriting with_loop so that we achieve similar or better performance? Thanks!
Context
I think that
any() + generator expression should run quicker than a for loop
means that any does not generate all values but a loop does:
>>> T(lambda : any([x * 101 % 300 == 1 for x in xrange(1, 300)])).repeat(number = 100000)
[5.7612644951345935, 5.742304846931542, 5.746804810873488]
>>> T(lambda : any(x * 101 % 300 == 1 for x in xrange(1, 300))).repeat(number = 100000)
[2.1652204281427814, 2.1640463131248886, 2.164674290446399]
So the quote does not mean that a loop can never achieve the performance of a generator.
The quote means that a loop usually generates all elements and any does not use all of them and a generator only generates the elements that any uses.
Your function with_loop is equivalent to the generator. So you can not expect a different behaviour.
To put it more clearly: any(loop) is slower than any(generator) because the loop generates everything. Your with_loop is equivalent to any(generator) and not to any(loop).
Original Question
>>> profile.run("""T(lambda : with_loop(101, 300)).repeat(number = 100000)""")
600043 function calls (600040 primitive calls) in 6.133 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
3 0.000 0.000 0.000 0.000 :0(append)
6 0.000 0.000 0.000 0.000 :0(clock)
3 0.000 0.000 0.000 0.000 :0(disable)
3 0.000 0.000 0.000 0.000 :0(enable)
3 0.000 0.000 0.000 0.000 :0(globals)
1 0.000 0.000 0.000 0.000 :0(hasattr)
3 0.000 0.000 0.000 0.000 :0(isenabled)
2 0.000 0.000 0.000 0.000 :0(isinstance)
1 0.000 0.000 0.000 0.000 :0(range)
1 0.005 0.005 0.005 0.005 :0(setprofile)
300000 0.579 0.000 5.841 0.000 <string>:1(<lambda>)
4/1 0.000 0.000 6.128 6.128 <string>:1(<module>)
300000 5.262 0.000 5.262 0.000 <string>:1(with_loop)
1 0.000 0.000 6.133 6.133 profile:0(T(lambda : with_loop(101, 300)).repeat(number = 100000))
0 0.000 0.000 profile:0(profiler)
1 0.000 0.000 0.000 0.000 timeit.py:121(__init__)
3 0.000 0.000 0.000 0.000 timeit.py:143(setup)
3 0.000 0.000 6.128 2.043 timeit.py:178(timeit)
1 0.000 0.000 6.128 6.128 timeit.py:201(repeat)
1 0.000 0.000 0.000 0.000 timeit.py:94(_template_func)
3 0.287 0.096 6.128 2.043 timeit.py:96(inner)
>>> profile.run("""T(lambda : with_generator(101, 300)).repeat(number = 100000)""")
31500043 function calls (31500040 primitive calls) in 70.531 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
300000 30.898 0.000 67.590 0.000 :0(any)
3 0.000 0.000 0.000 0.000 :0(append)
6 0.000 0.000 0.000 0.000 :0(clock)
3 0.000 0.000 0.000 0.000 :0(disable)
3 0.000 0.000 0.000 0.000 :0(enable)
3 0.000 0.000 0.000 0.000 :0(globals)
1 0.000 0.000 0.000 0.000 :0(hasattr)
3 0.000 0.000 0.000 0.000 :0(isenabled)
2 0.000 0.000 0.000 0.000 :0(isinstance)
1 0.000 0.000 0.000 0.000 :0(range)
1 0.000 0.000 0.000 0.000 :0(setprofile)
300000 0.667 0.000 70.222 0.000 <string>:1(<lambda>)
4/1 0.000 0.000 70.531 70.531 <string>:1(<module>)
300000 1.629 0.000 69.555 0.000 <string>:6(with_generator)
30600000 37.027 0.000 37.027 0.000 <string>:7(<genexpr>)
1 0.000 0.000 70.531 70.531 profile:0(T(lambda : with_generator(101, 300)).repeat(number = 100000))
0 0.000 0.000 profile:0(profiler)
1 0.000 0.000 0.000 0.000 timeit.py:121(__init__)
3 0.000 0.000 0.000 0.000 timeit.py:143(setup)
3 0.000 0.000 70.531 23.510 timeit.py:178(timeit)
1 0.000 0.000 70.531 70.531 timeit.py:201(repeat)
1 0.000 0.000 0.000 0.000 timeit.py:94(_template_func)
3 0.309 0.103 70.531 23.510 timeit.py:96(inner)
To call the generator every time, 30600000 times, seems to be much slower than just a for loop.
If you know how many elements exist in a list then you can write this:
l[0] * 101 % 300 == 1 or l[1] * 101 % 300 == 1 or l[2] * 101 % 300 == 1 or ....
来源：https://stackoverflow.com/questions/22108103/python-any-generator-expression