Dictionary introduction
Think about it:
if there is a list
nameList = ['xiaoZhang', 'xiaoWang', 'xiaoLi'];It is necessary to spell the name "xiaoWang" wrong, and modify it through the code:
nameList[1] = 'xiaoxiaoWang'If the order of the list has changed, as follows
nameList = ['xiaoWang', 'xiaoZhang', 'xiaoLi'];At this point, you need to modify the subscript to complete the modification of the name.
nameList[0] = 'xiaoxiaoWang'
有没有方法，既能存储多个数据，还能在访问元素的很方便就能够定位到需要的那个元素呢？
answer:
dictionary
Another scene:
Student information list, each student information includes student number, name, age, etc. How to find the information of a certain student?
>>> studens = [[1001, "王宝强", 24], [1002, "马蓉", 23], [1005, "宋喆"，24], ...]
Loop through? No!
<2> Dictionary in software development
The variable info is of dictionary type:
info = { ' name ' : ' Monitor ' , ' id ' : 100, ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
illustrate:
- Dictionaries, like lists, can also store multiple pieces of data
- When finding an element in a list, it is based on the subscript
- When looking for an element in the dictionary, it is based on the 'name' (that is, the value in front of the colon: such as 'name', 'id', 'sex' in the above code)
- Each element of the dictionary consists of 2 parts, key:value. For example 'name': 'squad leader', 'name' is the key, and 'squad leader' is the value
<3> Access the value according to the key
info = { ' name ' : ' Monitor ' , ' id ' : 100, ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
print(info['name'])
print(info['address'])
result:
班长
地球亚洲中国北京
If you access a non-existent key, you will get an error:
>>> info['age']
Traceback (most recent call last):
File "<stdin>", line 1, in <module> KeyError: 'age'
When we are not sure if a key exists in the dictionary and want to get its value, we can use the get method, and we can also set a default value:
>>> age = info.get('age')
>>> age #'age'键不存在，所以age为None
>>> type(age) <type 'NoneType'> >>> age = info.get('age', 18) # 若info中不存在'age'这个键，就返回默认值18 >>> age 18
Common operations on dictionaries
<1> Modify elements
The data in each element of the dictionary can be modified, as long as it is found through the key, it can be modified
demo:
info = { ' name ' : ' Monitor ' , ' id ' : 100, ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
newId = input( ' Please enter a new student ID ' )
info[ ' id ' ] = int(newId)
print ( ' The modified id is %d: ' %info[ ' id ' ])
Results of the:
<2>Add elements
demo:
info = { ' name ' : ' Monitor ' , ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
# print('id is:%d'%info['id'])#Program The terminal will run because the non-existent key
newId = input( ' please enter a new student ID ' ) is accessed
info[ ' id ' ] = newId
print ( ' The id after adding is: %s ' %info[ ' id ' ])
Results of the:
<3> Delete elements
There are several ways to delete a dictionary:
- the
- clear()
demo:del deletes the specified element
info = { ' name ' : ' Monitor ' , ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
print ( ' Before delete, %s ' %info[ ' name ' ])
del info[ ' name ' ]
print ( ' After deleting, %s ' %info[ ' name ' ])
Results of the:
demo:del删除整个字典
info = { ' name ' : ' Monitor ' , ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
print ( ' Before deleting, %s ' % info)
del info
print ( ' After deleting, %s ' %info)
Results of the:
<4>len()
Measure the number of key-value pairs in the dictionary
info = { ' name ' : ' Monitor ' , ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
result = len(info)
print ( ' Number of key-value pairs: ,%d ' %result)
Results of the:
<5>keys
Returns a list containing all the keys of the dictionary
info = { ' name ' : ' Monitor ' , ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
result=info.keys()
print('键,%s'%result)
<6>values
Returns a list containing all the values of the dictionary
info = { ' name ' : ' Monitor ' , ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
result=info.values()
print('值,%s'%result)
<7>items
Returns a list containing all (key, value) tuples
info = { ' name ' : ' Monitor ' , ' sex ' : ' f ' , ' address ' : ' Earth Asia China Beijing ' }
result=info.items()
print('%s'%result)
