Python Advanced 01 Dictionary

Basic tutorial introduces basic concepts, especially objects and classes.

The advanced tutorial further expands the basic tutorial and explains the details of Python. I hope that after the advanced tutorial, you will have a more comprehensive understanding of Python.

As we said before, list is a class in Python. A specific table, say nl = [1,3,8], is an object of this class. We can call some methods of this object, such as nl.append(15).

We are going to introduce a new class, dictionary. Similar to lists, dictionaries can store multiple elements. This object that stores multiple elements is called a container.

Basic concepts

Common ways to create a dictionary:

>>>dic = {'tom':11, 'sam':57,'lily':100}
>>>print type(dic)

Dictionaries are similar to tables in that they contain multiple elements, each element separated by commas. But the elements of the dictionary contain two parts, keys and values. It is common to use strings to represent keys, but numbers or true values ​​can also be used to represent keys (immutable objects can be used as keys). The value can be any object. There is a one-to-one correspondence between keys and values.

For example, in the above example, 'tom' corresponds to 11, 'sam' corresponds to 57, and 'lily' corresponds to 100

Different from the table, the elements of the dictionary are not in order. You cannot reference elements via subscripts. Dictionaries are referenced by keys.

>>>print dic['tom']
>>>dic['tom'] = 30
>>>print dic

Build a new empty dictionary:

>>>dic = {}
>>>print dic

How to add a new element in the dictionary:

>>>dic['lilei'] = 99
>>>print dic

Here, we reference a new key and assign it its corresponding value.

Loop call of dictionary elements

dic = {'lilei': 90, 'lily': 100, 'sam': 57, 'tom': 90}
for key in dic:
    print dic[key]

In the loop, each key of dict is extracted and assigned to the key variable.

Through the print results, we can confirm again that the elements in dic are not in order.

Common methods of dictionary

>>>print dic.keys()           # 返回dic所有的键
>>>print dic.values()         # 返回dic所有的值
>>>print dic.items()          # 返回dic所有的元素（键值对）
>>>dic.clear()                # 清空dic，dict变为{}

There is also a very common usage:

>>>del dic['tom']             # 删除 dic 的‘tom’元素

del is a reserved keyword in Python, used to delete objects.

Similar to tables, you can use len() to query the total number of elements in the dictionary.

>>>print(len(dic))

Summary

Each element of a dictionary is a key-value pair. The elements are not ordered.

dic = {'tom':11, 'sam':57,'lily':100}
dic['tom'] = 99
for key in dic: ...
del, len()