[英]Why is triadic iteration 2 times slower than the recursion?
为了解决我的问题,我需要将所有可能的长度为N的数组(包括-1、0和1)放在一起,并通过一些函数运行它们,以查看它们是否满足特定条件。
我已经实现了2种方法,三元数方法和递归。 它们都返回正确的结果,都检查相同数量的数组,没有危险信号,除了在我的机器上,三元方法慢了近两倍。 有什么理由要这样吗? printIfTargetHit
是一个简单的数字函数,没有任何缓存或其他可能解释此问题的复杂情况。
def triadicIteraction(signsQty):
signs = [None for _ in range(signsQty)]
comb = int(3**signsQty-1)
while (comb >= 0):
combCopy = comb
for n in range(signsQty):
signs[n] = 1-combCopy%3 # [0,1,2] -> [1,0,-1], correct range for signs
combCopy = combCopy//3
printIfTargetHit(signs)
comb = comb - 1
def recursiveIteration(signsQty):
def recursiveIterationInner(signs, newSign, currentOrder):
if (currentOrder >= 0):
signs[currentOrder] = newSign
if currentOrder == (len(signs) - 1):
printIfTargetHit(signs)
else:
recursiveIterationInner(signs,-1, currentOrder+1)
recursiveIterationInner(signs, 0, currentOrder+1)
recursiveIterationInner(signs, 1, currentOrder+1)
recursiveIterationInner(signs = [None for _ in range(signsQty)], newSign = None, currentOrder = -1)
完整代码在github上, https://github.com/Yulia5/workspace/blob/master/P2/P3/PlusesMinuses1ToN.py ,我没有全部发布,因为我相信上面的示例是自给自足的。
性能输出,按datetime.datetime.now()
的差值计算的时间,第一种方法是简单的嵌入式循环。
Method name: <function embeddedLoopsFixed at 0x01D63D30>
Combination quantity : 16560
Combinations evaluated : 14348907
Time elapsed : 0:01:56.440000
Method name: <function recursiveIteration at 0x01D63DB0>
Combination quantity : 16560
Combinations evaluated : 14348907
Time elapsed : 0:02:20.526000
Method name: <function triadicIteraction at 0x01D63D70>
Combination quantity : 16560
Combinations evaluated : 14348907
Time elapsed : 0:04:12.297000
问题是您每次在triadicIteraction
都在执行O(n)
操作:
for n in range(signsQty):
signs[n] = 1-combCopy%3 # [0,1,2] -> [1,0,-1], correct range for signs
combCopy = combCopy//3
要看到这一点,可以使用标准库中的profile
模块并实现该函数,以使每个计算都在单独的函数中进行(此处称为calSigns
):
def triadicIteractionGranular(signsQty):
signs = [None for _ in range(signsQty)]
comb = int(3**signsQty-1)
def calSigns(combCopy):
for n in xrange(signsQty):
signs[n] = 1-combCopy%3 # [0,1,2] -> [1,0,-1], correct range for signs
combCopy = combCopy//3
while (comb >= 0):
calSigns(comb)
printIfTargetHit(signs)
comb = comb - 1
然后:
>> profile.run('runCombinationCheckingMethod(triadicIteractionGranular, [], {"signsQty" : 15})')
Method name: <function triadicIteractionGranular at 0x10f466848>
Combination quantity : 16560
Combinations evaluated : 14348907
Time elapsed : 0:02:34.560533
43394489 function calls in 154.561 seconds
Ordered by: standard name
ncalls tottime percall cumtime percall filename:lineno(function)
1 0.000 0.000 154.561 154.561 <ipython-input-1-bf48bf6dbc36>:119(runCombinationCheckingMethod)
264960 0.068 0.000 0.068 0.000 <ipython-input-1-bf48bf6dbc36>:18(onesZerosToChars)
16560 0.355 0.000 0.488 0.000 <ipython-input-1-bf48bf6dbc36>:27(printEqualityAsString)
14348907 66.392 0.000 66.392 0.000 <ipython-input-1-bf48bf6dbc36>:33(evaluate)
14348907 8.279 0.000 75.159 0.000 <ipython-input-1-bf48bf6dbc36>:54(printIfTargetHit)
16561 0.024 0.000 0.024 0.000 <ipython-input-1-bf48bf6dbc36>:7(combinationNumber)
1 10.580 10.580 154.560 154.560 <ipython-input-26-037769fa8fac>:1(triadicIteractionGranular)
**** Note this line ****
14348907 68.822 0.000 68.822 0.000 <ipython-input-26-037769fa8fac>:5(calSigns)
1 0.000 0.000 154.561 154.561 <string>:1(<module>)
2 0.000 0.000 0.000 0.000 {built-in method now}
16560 0.005 0.000 0.005 0.000 {len}
1 0.000 0.000 0.000 0.000 {method 'disable' of '_lsprof.Profiler' objects}
16560 0.017 0.000 0.017 0.000 {method 'join' of 'str' objects}
16561 0.020 0.000 0.020 0.000 {range}
其他功能的配置文件看起来类似,而无需花费calSign
。
有一种方法可以实现triadicIteraction
而不会导致每次O(n)
花费:
def triadicIteractionFirstPrinciple(signsQty):
signs = [-1 for _ in range(signsQty)]
comb = int(3**signsQty-1)
def addOne(idx=0):
if signs[idx] < 1:
signs[idx] += 1
else:
signs[idx] = -1
addOne(idx+1)
while (comb >= 0):
printIfTargetHit(signs)
if comb > 0: addOne()
comb = comb - 1
这应避免O(n)
成本,并为您提供可与其他实现相比的结果。
在我的系统上:
>> profile.run('runCombinationCheckingMethod(triadicIteractionFirstPrinciple, [], {"signsQty" : 15})')
Method name: <function triadicIteractionFirstPrinciple at 0x10f62e7d0>
Combination quantity : 16560
Combinations evaluated : 14348907
Time elapsed : 0:01:37.383544
50568926 function calls (43394488 primitive calls) in 97.384 seconds
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