Python tips

1、xrange

Docstring:
xrange(stop) -> xrange object
xrange(start, stop[, step]) -> xrange object

Like range(), but instead of returning a list, returns an object that
generates the numbers in the range on demand.  For looping, this is 
slightly faster than range() and more memory efficient.

2、int自动转换为long

In [1]:
import sys

In [2]:
sys.version
Out[2]:
'2.7.10 (default, May 23 2015, 09:44:00) [MSC v.1500 64 bit (AMD64)]'

In [3]:
a = sys.maxint
b = a + 1    #超出int限制，自动转换为long类型  
type(a),type(b)
Out[3]:
(int, long)

注：Python里的长整型所能表达的范围远远超过C语言中的长整型，事实上，Python长整型仅受限于用户计算机的虚拟内存，类似于java中的BigInter类型，从Python2.3开始，不存在报整型溢出错误，结果会自动转换为长整型。

3、删除列表中的重复元素

>>> A = [1,2,3,4,2,3,4,1,2]
>>> B = ['a','b','c','a','b','c']
>>> list(set(A)),list(set(B))
([1, 2, 3, 4], ['a', 'c', 'b'])
>>> _[0]      #下划线 _ 在解释器中有特别的含义，表示最后一个表达式的值
[1, 2, 3, 4]

4、删除字符串中的空格

In [1]:
s = '   Hello World   '

In [2]:
s.lstrip()    #删除左边空格
Out[2]:
'Hello World   '

In [3]:
s.rstrip()    #删除右边空格
Out[3]:
'   Hello World'

In [4]:
s.strip()    #删除字符串首尾空格
Out[4]:
'Hello World'

In [5]:
s.split()
Out[5]:
['Hello', 'World']

In [6]:
''.join(s.split())   #删除字符串中所有空格
Out[6]:
'HelloWorld'　

5、列表转换为字典，value为列表元素，key为列表元素对应的序号

In [1]:
a = ['a','b','c']
c = list(enumerate(a))
d = [(n,a[n]) for n in xrange(len(a))]

In [2]:
dict(c)
Out[2]:
{0: 'a', 1: 'b', 2: 'c'}

In [3]:
dict(d)
Out[3]:
{0: 'a', 1: 'b', 2: 'c'}　

6、re.search() vs re.match()

Python offers two different primitive operations based on regular expressions: re.match() checks for a match only at the beginning of the string, while re.search() checks for a match anywhere in the string (this is what Perl does by default).

>>> re.match("c", "abcdef") # No match
>>> re.search("c", "abcdef") # Match
<_sre.SRE_Match object at ...>

Regular expressions beginning with '^' can be used with search() to restrict the match at the beginning of the string:

>>> re.match("c", "abcdef") # No match
>>> re.search("^c", "abcdef") # No match
>>> re.search("^a", "abcdef") # Match
<_sre.SRE_Match object at ...>

Note however that in MULTILINE mode match() only matches at the beginning of the string, whereas using search() with a regular expression beginning with '^' will match at the beginning of each line.

>>> re.match('X', 'A\nB\nX', re.MULTILINE) # No match
>>> re.search('^X', 'A\nB\nX', re.MULTILINE) # Match
<_sre.SRE_Match object at ...>

7、计算字符串之间的汉明距离

In [1]:
def hamming_distance(s1, s2):
    if len(s1) != len(s2):
        raise ValueError("Undefined for sequence of unequal length")
    return sum([ch1 != ch2 for ch1, ch2 in zip(s1, s2)])

In [2]:
s1 = 'Hello Ronaldinho'
s2 = 'Hallo Ronalhuang'
hamming_distance(s1,s2)
Out[2]:
6

8、由rand7构造rand10

rand7()返回1~7随机自然数，利用rand7构造rand10()随机返回1~10

import random

def rand7():
    return random.randrange(1,7)

def rand10():
    temp = (rand7()-1)*7 + rand7()  #构造出1~49上的均匀分布
    while temp > 40:  #如果temp>10,则有39/49的概率需要循环，容易死循环，效率降低
        temp = (rand7()-1)*7 + rand7()
    return temp%10+1

9、二维数组转置

[[r[col] for r in arr] for col in range(len(arr[0]))]

10、pass