第7章

Posted 2020-06-17

7-1：

dict.update()

7-2:

当元组中只包括像数字和字符串这样的不可变参数时，可以作为字典中的键。对于不能用作字典键的对象类型，原因在于它们不是可哈希的，解释器调用哈希函数，根据字典中键的值来计算所存储数据的位置。如果键是可变对象，它的值可改变。如果键发生变化，哈希函数会映射到不同的地址来存储数据，如果这样的情况发生，哈希函数就不可能可靠地存储或获取相关的数据。

7-3：

（a）

adict = {‘a‘:1, ‘c‘:3, ‘q‘:123, ‘m‘:7}
for key in sorted(adict):
    print key,

（b）

adict = {‘a‘:1, ‘c‘:3, ‘q‘:123, ‘m‘:7}
for key in sorted(adict):
    print key, adict[key]

（c）

adict = {‘a‘:1, ‘c‘:3, ‘q‘:123, ‘m‘:7}
values = adict.values()
values.sort()
print values
for value in values:
    for key in adict:
        if adict[key] == value:
            print key, value
            break

7-4：

def list2dict(l1, l2):
    adict = {}
    for item in range(len(l1)):
        adict.update({l1[item]:l2[item]})
    return adict

7-5：

（a）

 1 #!/usr/bin/env python
 2 
 3 import time
 4 
 5 db = {}
 6 
 7 def newuser():
 8     prompt = ‘login desired: ‘
 9     while True:
10         name = raw_input(prompt)
11         if db.has_key(name):
12             prompt = ‘name taken, try another: ‘
13             continue
14         else:
15             break
16     pwd = raw_input(‘passed: ‘)
17     db[name] = [pwd, time.asctime()]
18 
19 def olduser():
20     print ‘Please enter your username and password‘
21     name = raw_input(‘login: ‘)
22     pwd = raw_input(‘passed: ‘)
23     passwd = db.get(name)[0]
24     if passwd == pwd:
25         print ‘welcome back‘, name
26         if (int(time.asctime()[-13:-11]) - int(db[name][1][-13:-11])) <= 4:
27             print "You already logged in at:", db[name][1]
28         db[name][1] = time.asctime()
29     else:
30         print ‘login incorrect‘
31 
32 def showmenu():
33     prompt = """
34 (N)ew User login
35 (E)xisting User login
36 (Q)uit
37 
38 Enter choice: """
39 
40     done = False
41     while not done:
42 
43         chosen = False
44         while not chosen:
45             try:
46                 choice = raw_input(prompt).strip()[0].lower()
47             except (EOFError, KeyboardInterrupt):
48                 choice = ‘q‘
49             print ‘\nYou picked: [%s]‘ % choice
50             if choice not in ‘neq‘:
51                 print ‘invalid option, try again‘
52             else:
53                 chosen = True
54 
55         if choice == ‘q‘: done = True
56         if choice == ‘n‘: newuser()
57         if choice == ‘e‘: olduser()
58 
59 if __name__ == ‘__main__‘:
60     showmenu()

（b）