90

由于我对这个线程的回答中的评论，我想知道+=操作员和操作员之间的速度差异是多少''.join()

那么两者的速度对比是多少呢？

4

6 回答 6

125

来自：高效的字符串连接

方法一： 

def method1():
  out_str = ''
  for num in xrange(loop_count):
    out_str += 'num'
  return out_str

方法四： 

def method4():
  str_list = []
  for num in xrange(loop_count):
    str_list.append('num')
  return ''.join(str_list)

现在我意识到它们并不具有严格的代表性，第 4 种方法在迭代和加入每个项目之前附加到一个列表中，但这是一个公平的指示。

字符串连接比连接快得多。

为什么？字符串是不可变的，不能原地更改。要改变一个，需要创建一个新的表示（两者的串联）。

替代文字

于 2010-06-16T17:07:24.557 回答
12

我的原始代码是错误的，似乎+连接通常更快（尤其是在较新硬件上使用较新版本的 Python）

时间如下：

Iterations: 1,000,000

Windows 7、Core i7 上的 Python 3.3

String of len:   1 took:     0.5710     0.2880 seconds
String of len:   4 took:     0.9480     0.5830 seconds
String of len:   6 took:     1.2770     0.8130 seconds
String of len:  12 took:     2.0610     1.5930 seconds
String of len:  80 took:    10.5140    37.8590 seconds
String of len: 222 took:    27.3400   134.7440 seconds
String of len: 443 took:    52.9640   170.6440 seconds

Windows 7 上的 Python 2.7，Core i7

String of len:   1 took:     0.7190     0.4960 seconds
String of len:   4 took:     1.0660     0.6920 seconds
String of len:   6 took:     1.3300     0.8560 seconds
String of len:  12 took:     1.9980     1.5330 seconds
String of len:  80 took:     9.0520    25.7190 seconds
String of len: 222 took:    23.1620    71.3620 seconds
String of len: 443 took:    44.3620   117.1510 seconds

在 Linux Mint、Python 2.7 上，一些较慢的处理器

String of len:   1 took:     1.8840     1.2990 seconds
String of len:   4 took:     2.8394     1.9663 seconds
String of len:   6 took:     3.5177     2.4162 seconds
String of len:  12 took:     5.5456     4.1695 seconds
String of len:  80 took:    27.8813    19.2180 seconds
String of len: 222 took:    69.5679    55.7790 seconds
String of len: 443 took:   135.6101   153.8212 seconds

这是代码：

from __future__ import print_function
import time

def strcat(string):
    newstr = ''
    for char in string:
        newstr += char
    return newstr

def listcat(string):
    chars = []
    for char in string:
        chars.append(char)
    return ''.join(chars)

def test(fn, times, *args):
    start = time.time()
    for x in range(times):
        fn(*args)
    return "{:>10.4f}".format(time.time() - start)

def testall():
    strings = ['a', 'long', 'longer', 'a bit longer', 
               '''adjkrsn widn fskejwoskemwkoskdfisdfasdfjiz  oijewf sdkjjka dsf sdk siasjk dfwijs''',
               '''this is a really long string that's so long
               it had to be triple quoted  and contains lots of
               superflous characters for kicks and gigles
               @!#(*_#)(*$(*!#@&)(*E\xc4\x32\xff\x92\x23\xDF\xDFk^%#$!)%#^(*#''',
              '''I needed another long string but this one won't have any new lines or crazy characters in it, I'm just going to type normal characters that I would usually write blah blah blah blah this is some more text hey cool what's crazy is that it looks that the str += is really close to the O(n^2) worst case performance, but it looks more like the other method increases in a perhaps linear scale? I don't know but I think this is enough text I hope.''']

    for string in strings:
        print("String of len:", len(string), "took:", test(listcat, 1000000, string), test(strcat, 1000000, string), "seconds")

testall()
于 2010-06-16T17:10:19.713 回答
12

现有的答案写得非常好和研究，但这是 Python 3.6 时代的另一个答案，因为现在我们有文字字符串插值（AKA，f-strings）：

>>> import timeit
>>> timeit.timeit('f\'{"a"}{"b"}{"c"}\'', number=1000000)
0.14618930302094668
>>> timeit.timeit('"".join(["a", "b", "c"])', number=1000000)
0.23334730707574636
>>> timeit.timeit('a = "a"; a += "b"; a += "c"', number=1000000)
0.14985873899422586

在 2012 Retina MacBook Pro 上使用 CPython 3.6.5 执行测试，该 MacBook Pro 配备 Intel Core i7，频率为 2.3 GHz。

这绝不是任何正式的基准，但看起来使用f-strings 的性能大致与使用+=串联一样；当然，欢迎任何改进的指标或建议。

于 2018-04-26T01:25:26.453 回答
0

如果我期望良好，对于一个有 k 个字符串的列表，总共有 n 个字符，连接的时间复杂度应该是 O(nlogk)，而经典连接的时间复杂度应该是 O(nk)。

这与合并 k 排序列表的相对成本相同（有效的方法是 O(nlkg)，而简单的方法，类似于串联是 O(nk) ）。

于 2021-11-09T07:07:04.780 回答
0

我重写了最后一个答案，你可以分享你对我测试方式的看法吗？

import time

start1 = time.clock()
for x in range (10000000):
    dog1 = ' and '.join(['spam', 'eggs', 'spam', 'spam', 'eggs', 'spam','spam', 'eggs', 'spam', 'spam', 'eggs', 'spam'])

end1 = time.clock()
print("Time to run Joiner = ", end1 - start1, "seconds")


start2 = time.clock()
for x in range (10000000):
    dog2 = 'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'+' and '+'spam'+' and '+'eggs'+' and '+'spam'

end2 = time.clock()
print("Time to run + = ", end2 - start2, "seconds")

注意：这个例子是用 Python 3.5 编写的，其中 range() 的作用类似于以前的 xrange()

我得到的输出：

Time to run Joiner =  27.086106206103153 seconds
Time to run + =  69.79100515996426 seconds

就我个人而言，我更喜欢 ''.join([]) 而不是 'Plusser 方式'，因为它更干净，更具可读性。

于 2016-09-21T19:32:29.170 回答
-1

这就是愚蠢的程序旨在测试的内容:)

使用加号

import time

if __name__ == '__main__':
    start = time.clock()
    for x in range (1, 10000000):
        dog = "a" + "b"

    end = time.clock()
    print "Time to run Plusser = ", end - start, "seconds"

输出：

Time to run Plusser =  1.16350010965 seconds

现在加入....

import time
if __name__ == '__main__':
    start = time.clock()
    for x in range (1, 10000000):
        dog = "a".join("b")

    end = time.clock()
    print "Time to run Joiner = ", end - start, "seconds"

输出：

Time to run Joiner =  21.3877386651 seconds

所以在 Windows 上的 python 2.6 上，我会说 + 比 join 快 18 倍 :)

于 2010-06-16T17:06:18.633 回答