《Python Data Structures》Week5 Dictionary 课堂笔记

Coursera课程《Python Data Structures》 密歇根大学 Charles Severance

Week5 Dictionary

9.1 Dictionaries

字典就像是一个包，而这个包里的每样东西都整整齐齐地贴好了标签，于是我们可以通过标签来找到我们想要的东西。而且注意，字典这个包是无序的，所以它不能根据顺序索引，只能根据标签。

>>> purse = dict()
>>> purse['money'] = 12
>>> purse['candy'] = 3
>>> purse['tissues'] = 75
>>> print(purse)
{'money':12, 'tissues':75, 'candy':3}
>>> print(purse['candy'])
3
>>> purse['candy'] = purse['candy'] + 2
>>> print(purse)
{'money':12, 'tissues':75, 'candy': 5}

9.2 Counting with Dictionaries

我们可以使用"in"来查看，是否某个key在这个字典里。

>>> ccc = dict()
>>> print(ccc['csev'])
Traceback (most recent call last):
    File "<stdin", line 1, in <module>
KeyError: 'csev'
>>> 'csev' in ccc
False

字典的get()方法是查看是否一个key已经在字典里，如果没有在的话，那就新建一个key，把它附上默认值。

比如下面这两段代码，其实功能是一样的。

if name in counts:
    x = counts[name]
else:
    x = 0

 x = counts.get(name, 0)

所以使用get()来计数的话会更加方便。

counts = dict()
names = ['csev', 'cwen', 'csev', 'zqian', 'cwen']
for name in names:
    counts[name] = counts.get(name, 0) + 1
print(counts)

输出结果就是

{'csev':2,'zqian':1,'cwen':2}

9.3 Dictionaries and Files

计数的一个基本模式是这样的。

counts = dict()
print('Enter a line of text:')
line = input('')

words = line.split()

print('Words:', words)

print('Counting...')
for word in words:
    counts[word] = counts.get(word, 0) + 1
print('Counts', counts)

我们可以使用keys(), values(), items()这些方法来分别获得字典的key，值或者全部。

>>> jjj = {'chuck':1, 'fred':42, 'jan':100}
>>> print(list(jjj))
['jan', 'chuck', 'fred']
>>> print(jjj.keys())
['jan', 'chuck', 'fred']
>>> print(jjj.values())
[100, 1, 42]
>>> print(jjj.items())
[('jan', 100), ('chuck', 1), ('fred', 42)]

我们还可以使用两个循环变量同时对key和value进行循环。

jjj = {'chuck':1, 'fred':42, 'jan': 100}
for aaa,bbb in jjj.item():
    print(aaa,bbb)

那么接下来我们就可以读取文件，来对里面的文本进行计数了。

name = input('Enter file:')
handle = open(name)

counts = dict()
for line in handle:
    words = line.split()
    for word in words:
        counts[word] = counts.get(word, 0) + 1

bigcount = None
bigword = None
for word, count in counts.items():
    if bigcount is None or count > bigcount:
        bigword = word
        bigcount = count

print(bigword, bigcount)

9.4 Assignment

作业的代码如下

name = input("Enter file:")
if len(name) < 1 : name = "mbox-short.txt"
handle = open(name)

counts = dict()
for line in handle:
    words = line.split()
    if len(words) < 1 or words[0] != 'From':
        continue
    counts[words[1]] = counts.get(words[1], 0) + 1

bigcount = None
bigword = None
for word, count in counts.items():
    if bigcount is None or count > bigcount:
        bigword = word
        bigcount = count

print(bigword, bigcount)