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How to use the practical application of dictionaries in daily life to introduce the knowledge of dictionaries in Python, many novices are not very clear about this. In order to help you solve this problem, the following editor will explain it in detail. People with this need can come and learn. I hope you can get something.

I. Preface

If you have a list and you need to change the name "xiaoWang", you need to change the code with the corresponding index value.

NameList = ['xiaoZhang',' xiaoWang', 'xiaoLi'] nameList [1] =' xiaoxiaoWang

If the order of the list changes, it is as follows:

NameList = ['xiaoWang',' xiaoZhang', 'xiaoLi']

At this point, you need to modify the subscript to complete the name modification.

NameList [0] = 'xiaoxiaoWang'

Is there a way to store multiple data and locate the desired element with easy access to the element? This is the dictionary.

Introduction of dictionaries

Dictionaries, like lists, can store multiple data.

When you find an element in the list, it is based on the subscript.

When looking for an element in the dictionary, it is based on the 'name' (that is, the colon: the preceding value, such as' name', 'id',' sex' in the above code).

Each element of the dictionary consists of two parts, key: value. For example, 'name':' monitor', where 'name'' is the key and 'monitor' is the value.

Access values based on key

Example:

Info = {'name':' monitor', 'id':100,' sex':'f', 'address':' Earth Asia, Beijing, China'} print (info ['name']) print (info [' address'])

Running result:

If you access a key that does not exist, an error will be reported:

> > info ['age'] Traceback (most recent call last): File ", line 1, in KeyError:' age'

When we are not sure whether a key exists in the dictionary and want to get its value, we can use the get method and set the default value.

> age = info.get ('age') > age #' age' key does not exist, so age is None > type (age) > age = info.get ('age', 18) # if the key' age' 'does not exist in info, return the default value 18 > print (age) 18 # run result

1. Common operations of dictionaries 1

Add elements info = {'name':' monitor', 'sex':'f',' address':' Asia, Beijing, China'} print ('id is:% d'%info ['id'])

Running result:

If the "key" is in the dictionary and does not exist when using the variable name [key] = data, then this element will be added.

Add a new element.

Info = {'name':' monitor', 'sex':'f',' address':' Asia, Beijing, China'} # print ('id is:% d'%info ['id']) # the program will run at the terminal, because the non-existent key newId = input (' please enter a new student ID') info ['id'] = newId print (' added id is:% d'%info ['id'])

Running result:

Please enter the new student number 188. the added id is: 188 to delete the element.

There are several ways to delete dictionaries:

Del

Clear ()

Del deletes the specified element

Info = {'name':' monitor', 'sex':'f',' address':' Asia Beijing, China'} print ('before deletion,% s'%info [' name']) del info ['name'] print (' after deletion,% s'%info ['name'])

Running result:

Del deletes the entire dictionary.

Info = {'name':'monitor',' sex':'f', 'address':'China'} print (' before deletion,% s'%info) del info print ('after deletion,% s'%info)

Running result:

Clear empties the entire dictionary.

Info = {'name':'monitor',' sex':'f', 'address':'China'} print (' before emptying,% s'%info) info.clear () print ('after emptied,% s'%info)

Running result:

Modify element

The data in each element of the dictionary can be modified as long as it is found through key.

Info = {'name':' monitor', 'id':100,' sex':'f', 'address':' Asia, Beijing, China'} newId = input ('Please enter a new student ID') info ['id'] = int (newId) print (' modified id is% d:'%info ['id'])

Running result:

two。 Common operations of dictionaries 2

Len ()

Measure the number of key-value pairs in the dictionary.

Dict= {"name": 'zahnsan','sex':'m'} print (len (dict))

Running result:

Keys

Returns a list of all the KEY of the dictionary.

Dict= {"name": 'zahnsan','sex':'m'} print (dict.keys ())

Running result:

Values

Returns a list of all the value of the dictionary.

Dict= {"name": 'zahnsan','sex':'m'} print (dict.values ())

Running result:

Items

Returns a list of all (keys, values) meta-ancestors.

Dict= {"name": 'zahnsan','sex':'m'} print (dict.items ())

Running result:

Third, traversing

Syntax: through for... In.: syntax structure, we can traverse strings, lists, tuples, dictionaries and other data structures.

Note: indentation of Python syntax

First take a look at how strings, lists, and tuples are traversed.

String traversal

> a_str = "hello itcast" > for char in a_str:. Print (char,end='')... H e l l o i t c a s t # run result

List traversal

> a_list = [1,2,3,4,5] > for num in a_list:. Print (num,end='')... 1 2 3 4 5 # running result

Tuple traversal

> a_turple = (1,2,3,4,5) > for num in a_turple:. Print (num,end= "") 1 2 3 4 5 # running result

Dictionary traversal

1. Traversing the dictionary's key (key)

2. Value (value) of the traversal dictionary

3. Traversing the entries (elements) of a dictionary

4. Traversal dictionary key-value (key-value pair)

5. Enumerate ()

Chars = ['averse,' baked, 'cached,' d'] for I, chr in enumerate (chars): print (I, chr)

Running result:

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