python列表解析
列表解析来自函数式编程语言(haskell),语法如下:
[expr for iter_var in iterable] [expr for iter_var in iterable if cond_expr]
第一种语法:首先迭代iterable里所有内容,每一次迭代,都把iterable里相应内容放到iter_var中,再在表达式中应用该iter_var的内容,最后用表达式的计算值生成一个列表。
第二种语法:加入了判断语句,只有满足条件的内容才把iterable里相应内容放到iter_var中,再在表达式中应用该iter_var的内容,最后用表达式的计算值生成一个列表。
例子如下:
>>> seq = [11,10,9,9,5,35,8,20,31,72,54,53] >>> filter(lambda x:x%2,seq) [11, 9, 9, 5, 35, 31, 53] >>> [x for x in seq if x%2] [11, 9, 9, 5, 35, 31, 53]
更复杂的如求矩阵:
#3行5列的矩阵 >>> [(x+1,y+1) for x in range(3) for y in range(5)] [(1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5)] #简单的3行3列 >>> [(x,y) for x in range(3) for y in range(3)] [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)] #模拟 >>> for x in range(3): for y in range(3): print x,y 0 0 0 1 0 2 1 0 1 1 1 2 2 0 2 1
计算一个文件的字数和大小:
>>> f = open('demo.txt','r') >>> #计算单词的个数 >>> len([word for line in f for word in line.split()]) 1942 >>> #计算文件大小 >>> os.stat('demo.txt').st_size 14051L >>> f.seek(0) #seek()函数回到文件头部,因为迭代器已经访问完了文件的所有行 >>> sum([len(word) for line in f for word in line.split()]) 10806 >>> f.close() >>>
经典例子:
book=[
{"name":u"C#从入门到精通","price":23.7,"store":u"卓越"}, {"name":u"ASP.NET高级编程","price":44.5,"store":u"卓越"}, {"name":u"Python核心编程","price":24.7,"store":u"当当"}, {"name":u"JavaScript大全","price":45.7,"store":u"当当"}, {"name":u"Django简明教程","price":26.7,"store":u"新华书店"}, {"name":u"深入Python","price":55.7,"store":u"新华书店"}, ]
1、书籍信息中价格最低的
#No1:原始方式: >>> price=[] >>> for item in books: for p in item: if p == 'price': price.append(item[p]) >>> min(price) 23.699999999999999 #No2:列表解析: >>> min([item[p] for item in books for p in item if p=='price']) 23.699999999999999
#Python相关书籍检索 for item in books: for p in item: if item['name'].find('Python')>=0: print item[p], #24.7 Python核心编程 当当 55.7 深入Python 新华书店 total = [item[p] for item in books for p in item if item['name'].find('Python')>=0] print total #[24.699999999999999, u'Python\u6838\u5fc3\u7f16\u7a0b', u'\u5f53\u5f53', 55.700000000000003, u'\u6df1\u5165Python', u'\u65b0\u534e\u4e66\u5e97']