1.2 使用字典对象

准备工作

• 学习使用脚本计算词频，也就是每个词在文本中出现的次数。

操作方法

sentence = "Peter Piper picked a peck of pickled peppers A peck of pickled \
                peppers Peter Piper picked If Peter Piper a peck of picked\
                peppers Wheres the peck of pickled peppers Peter Piper Picked"

word_dict = {}

for word in sentence.split():
    if word not in word_dict:
        word_dict[word] = 1
    else:
        word_dict[word] += 1

for key, value in word_dict.items():
    print("{}:  {}".format(key, value))
#print(word_dict)

输出结果如下：

Peter:  114
Piper:  114
picked:  113
a:  112
peck:  114
of:  114
pickled:  113
peppers:  114
A:  111
If:  111
Wheres:  111
the:  111
Picked:  111

工作原理：

• 字典的内容是一个键值对，这里的键是词，对应的值是出现的词频。
• item() 函数可以遍历字典中的所有键值对。

更多内容：defaultdict

• python中的collection模块，有一个defaultdict类，它会传递一个函数作为参数，使用函数的返回值来初始化字典的未遇到的键.
• 可以看下面一个实例，它接受了num()函数的返回值作为默认初始值，这里的输出结果是上面的输出结果加上110。
• 另外，一般也可以使用python自带的int函数或者float函数。

from collections import defaultdict
def num():
    return 110

sentence = "Peter Piper picked a peck of pickled peppers A peck of pickled \
                peppers Peter Piper picked If Peter Piper a peck of picked\
                peppers Wheres the peck of pickled peppers Peter Piper Picked"

word_dict = defaultdict(num)

for word in sentence.split():
    word_dict[word] += 1

for key, value in word_dict.items():
    print("{}:  {}".format(key, value))

更多内容：Counter子类

• 还可以用COunter来计算词频，它是一个字典子类，得到的结果也是一个字典类型。

from collections import Counter

sentence = "Peter Piper picked a peck of pickled peppers A peck of pickled \
                peppers Peter Piper picked If Peter Piper a peck of picked\
                peppers Wheres the peck of pickled peppers Peter Piper Picked"

words = sentence.split()
word_count = Counter(words)
for key, value in word_dict.items():
    print("{}:  {}".format(key, value))
#print(word_count)