Python高效编程技巧实战(3)

开发环境:以Python2.x为主  IPython


第三天 : 03day

如何统计序列元素的出现频度

解决方案:使用collections.Counter对象
将序列传入Counter的构造器,得到Counter对象是元素频度的字典
Counter.most_common(n)方法得到频度最高的n个元素的列表

1.某随机序列[12,5,6,6,5,5,7...]    中找到出现次数最高的3个元素,它们出现次数是多少？
In [41]: from random import randint

In [42]: data = [randint(0,20) for _ in xrange(30)]

In [43]: data
Out[43]:
[18,
 15,
 2,
 2,
 15,
 7,
 6,
 0,
 1,
 8,
 15,
 9,
 15,
 8,
 19,
 14,
 6,
 17,
 8,
 1,
 8,
 15,
 2,
 3,
 2,
 13,
 0,
 19,
 6,
 4]

In [44]: c = dict.fromkeys(data,0)

In [45]: c
Out[45]:
{0: 0,
 1: 0,
 2: 0,
 3: 0,
 4: 0,
 6: 0,
 7: 0,
 8: 0,
 9: 0,
 13: 0,
 14: 0,
 15: 0,
 17: 0,
 18: 0,
 19: 0}

In [46]: for x in data:
   ....:     c[x] += 1
   ....:

In [47]: c
Out[47]:
{0: 2,
 1: 2,
 2: 4,
 3: 1,
 4: 1,
 6: 3,
 7: 1,
 8: 4,
 9: 1,
 13: 1,
 14: 1,
 15: 5,
 17: 1,
 18: 1,
 19: 2}


In [48]: from collections import Counter

In [49]: c2 = Counter(data)

In [50]: c2
Out[50]: Counter({15: 5, 2: 4, 8: 4, 6: 3, 0: 2, 1: 2, 19: 2, 3: 1, 4: 1, 7: 1, 9: 1, 13: 1, 14: 1, 17: 1, 18: 1})

In [51]: c2[15]
Out[51]: 5

In [52]: c2[2]
Out[52]: 4


In [53]: c2.most_common(3)
Out[53]: [(15, 5), (2, 4), (8, 4)]

2.对某英文文章的单词，进行词频统计,找到出现次数最多的10个单词,它们出现的次数是多少？
import re
txt = open('/etc/passwd').read()
c3 = re.split('\W+',txt)
c4 = Counter(c3)
print c4.most_common(10)