在python中，为什么string.count（）比循环快？

在leetcode中，有一个问题需要检查一系列无序的字符串“U”、“D”、“L”、“R”是否会形成一个圆

我的意见如下：

def judgeCircle(moves):

    l=r=u=d=0

    for i in moves:
        if i == 'L':
            l+=1
        if i == 'D':
            d+=1
        if i == 'R':
            r+=1
        if i == 'U':
            u+=1

    return ((l-r)==0) and ((u-d)==0)

法官认为这花了239毫秒 而另一种单线解决方案：

def judgeCircle(moves):
    return (moves.count('R')==moves.count('L')) and 
           (moves.count('U')==moves.count('D'))

只需39毫秒

虽然我理解的代码越少越好，但我认为第二个会循环4次，我是不是误解了

---

谢谢

两个代码示例的算法复杂度都是

O（n）

，但您不应该被大O所愚弄，因为它只显示了趋势。

O（n）

算法的执行时间可以表示为

C*n

，其中

C

是常数，这取决于许多因素

对于

.count（）

code，您需要在

string\u count（）

C函数中执行4个循环，但是C函数速度很快。它还使用了一些先进的算法，比如。这里只执行字符串搜索，最大限度地减少了解释器的开销

在纯Python代码中，您只需要一个循环，但每次迭代都需要执行更多的低级代码，因为Python是解释语言*。例如，您正在为循环的每次迭代创建新的unicode或string对象，而创建对象是一项非常昂贵的操作。由于整数对象是不可变的，所以需要为每个计数器重新创建它们

---

*假设您使用的是CPython解释器，这几乎是默认的

这里有一些

timeit

代码显示各种方法的速度，使用所有4个键计数相等的完美数据和每个键数量大致相等的随机数据

#!/usr/bin/env python3

''' Test speeds of various algorithms that check
    if a sequence of U, D, L, R moves make a closed circle.

    See https://stackoverflow.com/q/46568696/4014959

    Written by PM 2Ring 2017.10.05
'''

from timeit import Timer
from random import seed, choice, shuffle
from collections import Counter, defaultdict

def judge_JH0(moves):
    l = r = u = d = 0
    for i in moves:
        if i == 'L':
            l += 1
        if i == 'D':
            d += 1
        if i == 'R':
            r += 1
        if i == 'U':
            u += 1
    return ((l-r) == 0) and ((u-d) == 0)

def judge_JH1(moves):
    l = r = u = d = 0
    for i in moves:
        if i == 'L':
            l += 1
        elif i == 'D':
            d += 1
        elif i == 'R':
            r += 1
        elif i == 'U':
            u += 1
    return (l == r) and (u == d)

def judge_count(moves):
    return ((moves.count('R') == moves.count('L')) and 
        (moves.count('U') == moves.count('D')))

def judge_counter(moves):
    d = Counter(moves)
    return (d['R'] == d['L']) and (d['U'] == d['D'])

def judge_dict(moves):
    d = {}
    for c in moves:
        d[c] = d.get(c, 0) + 1
    return ((d.get('R', 0) == d.get('L', 0)) and 
        (d.get('U', 0) == d.get('D', 0)))

def judge_defdict(moves):
    d = defaultdict(int)
    for c in moves:
        d[c] += 1
    return (d['R'] == d['L']) and (d['U'] == d['D'])


# All the functions
funcs = (
    judge_JH0,
    judge_JH1,
    judge_count,
    judge_counter,
    judge_dict,
    judge_defdict,
)

def verify(data):
    print('Verifying...')
    for func in funcs:
        name = func.__name__
        result = func(data)
        print('{:20} : {}'.format(name, result))
    print()

def time_test(data, loops=100):
    timings = []
    for func in funcs:
        t = Timer(lambda: func(data))
        result = sorted(t.repeat(3, loops))
        timings.append((result, func.__name__))
    timings.sort()
    for result, name in timings:
        print('{:20} : {}'.format(name, result))
    print()

# Make some data
keys = 'DLRU'
seed(42)
size = 100

perfect_data = list(keys * size)
shuffle(perfect_data)
print('Perfect')
verify(perfect_data)

random_data = [choice(keys) for _ in range(4 * size)]
print('Random data stats:')
for k in keys:
    print(k, random_data.count(k))
print()
verify(random_data)

loops = 1000
print('Testing perfect_data')
time_test(perfect_data, loops=loops)

print('Testing random_data')
time_test(random_data, loops=loops)

典型输出

Perfect
Verifying...
judge_JH0            : True
judge_JH1            : True
judge_count          : True
judge_counter        : True
judge_dict           : True
judge_defdict        : True

Random data stats:
D 89
L 100
R 101
U 110

Verifying...
judge_JH0            : False
judge_JH1            : False
judge_count          : False
judge_counter        : False
judge_dict           : False
judge_defdict        : False

Testing perfect_data
judge_counter        : [0.11746118000155548, 0.11771785900054965, 0.12218693499744404]
judge_count          : [0.12314812499971595, 0.12353860199800692, 0.12495016200409736]
judge_defdict        : [0.20643479600403225, 0.2069275510002626, 0.20834802299941657]
judge_JH1            : [0.25801684000180103, 0.2689959089984768, 0.27642749399819877]
judge_JH0            : [0.36819701099739177, 0.37400564400013536, 0.40291943999909563]
judge_dict           : [0.3991459790049703, 0.4004156189985224, 0.4040740730051766]

Testing random_data
judge_count          : [0.061543637995782774, 0.06157537500257604, 0.06704995800100733]
judge_counter        : [0.11995147699781228, 0.12068584300141083, 0.1207217440023669]
judge_defdict        : [0.2096717179956613, 0.21544414199888706, 0.220649760995002]
judge_JH1            : [0.261116588000732, 0.26281095200101845, 0.2706491360004293]
judge_JH0            : [0.38465088899829425, 0.38476935599464923, 0.3921787180006504]
judge_dict           : [0.40892754300148226, 0.4094729179996648, 0.4135226650032564]

这些计时是在Linux上运行Python 3.6.0的旧2GHz 32位机器上获得的

---

这里还有几个函数

def judge_defdictlist(moves):
    d = defaultdict(list)
    for c in moves:
        d[c].append(c)
    return (len(d['R']) == len(d['L'])) and (len(d['U']) == len(d['D']))

# Sort to groups in alphabetical order: DLRU
def judge_sort(moves):
    counts = [sum(1 for _ in g) for k, g in groupby(sorted(moves))]
    return (counts[0] == counts[3]) and (counts[1] == counts[2])

judge\u defdictlist

比

judge\u defdict

慢，但比

judge\u JH1

快，当然它比

judge\u defdict

使用更多的RAM

---

judge\u sort

比

judge\u JH0

慢，但比

judge\u dict

快，

.count

方法可以以C速度循环，这比显式Python循环快。但是您可以通过使用

elif

稍微加快Python循环的速度，因为一旦找到匹配项，就不需要对给定的

i

进行进一步的测试。以C速度循环4次（对于CPython）。此外，第二个版本将跳过

（moves.count（'U'）==moves.count（'D'））

part如果第一部分为false，因为

和

短路。根据输入字符串和要计数的数量，

集合。计数器可以为偶数faster@Chris_Rands是的，
计数器
可以比
.count
快，尽管它不能在数据不平衡时通过短路来节省时间。Se这是我对一些统计数据的回答。事实上，由于那些
str
常量是不可变的，解释器在CPython中重用了相同的对象。有很多这样的优化，伙计们！我想我现在更明白了。这真是一个令人印象深刻的实验！我学到了很多，非常感谢！所以这也意味着LeetCode上的计时不是很准确比率。我将来应该做更多的体验，更好地了解它是如何工作的。Tnx！