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HomeBackend DevelopmentPython TutorialPython中利用sorted()函数排序的简单教程

排序算法

排序也是在程序中经常用到的算法。无论使用冒泡排序还是快速排序,排序的核心是比较两个元素的大小。如果是数字,我们可以直接比较,但如果是字符串或者两个dict呢?直接比较数学上的大小是没有意义的,因此,比较的过程必须通过函数抽象出来。通常规定,对于两个元素x和y,如果认为x y,则返回1,这样,排序算法就不用关心具体的比较过程,而是根据比较结果直接排序。

Python内置的sorted()函数就可以对list进行排序:

>>> sorted([36, 5, 12, 9, 21])
[5, 9, 12, 21, 36]

此外,sorted()函数也是一个高阶函数,它还可以接收一个比较函数来实现自定义的排序。比如,如果要倒序排序,我们就可以自定义一个reversed_cmp函数:

def reversed_cmp(x, y):
  if x > y:
    return -1
  if x < y:
    return 1
  return 0

传入自定义的比较函数reversed_cmp,就可以实现倒序排序:

>>> sorted([36, 5, 12, 9, 21], reversed_cmp)
[36, 21, 12, 9, 5]

我们再看一个字符串排序的例子:

>>> sorted(['bob', 'about', 'Zoo', 'Credit'])
['Credit', 'Zoo', 'about', 'bob']

默认情况下,对字符串排序,是按照ASCII的大小比较的,由于'Z'

现在,我们提出排序应该忽略大小写,按照字母序排序。要实现这个算法,不必对现有代码大加改动,只要我们能定义出忽略大小写的比较算法就可以:

def cmp_ignore_case(s1, s2):
  u1 = s1.upper()
  u2 = s2.upper()
  if u1 < u2:
    return -1
  if u1 > u2:
    return 1
  return 0

忽略大小写来比较两个字符串,实际上就是先把字符串都变成大写(或者都变成小写),再比较。

这样,我们给sorted传入上述比较函数,即可实现忽略大小写的排序:

>>> sorted(['bob', 'about', 'Zoo', 'Credit'], cmp_ignore_case)
['about', 'bob', 'Credit', 'Zoo']

从上述例子可以看出,高阶函数的抽象能力是非常强大的,而且,核心代码可以保持得非常简洁。

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