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This article introduces the relevant knowledge of "detailed introduction of dict and set in python". In the operation process of actual cases, many people will encounter such difficulties. Next, let Xiaobian lead you to learn how to deal with these situations! I hope you can read carefully and learn something!

directory

I. Collections

1. collection definition

2. create a collection

3. deduplication

4. set addition and deletion

5. relational operation

6. sort

7.frozenset

8. practice

9. characteristics

II. Dictionary

1. Dictionary definitions

2. Dictionary printing

3. Delete dictionary elements

4.setdefault

5.defaultdict

I. Collections

1. collection definition

A set is an unordered sequence of non-repeating elements.

2. create a collection

Create a collection using braces { } or the set() function;

To create an empty collection, you must use set() instead of { }

{ } is used to create an empty dictionary.

s = {1,2,3,4}print(s,type(s))

3. print(s,)= {1,2,3,4,3,2,1}

4. Set add delete s = {1,2,3}s.add(4) #single element print(s)s.update({4,5,6}) #multiple elements print(s)s = {1,2,3,4,5,6}s.remove(4) #remove a print(s)s.pop() #remove print(s) randomly

5. Relational operation s1 = {1,2,3}s2 = {1,2,4}print(s1 - s2)print(s1 & s2)print(s1 ^ s2)print(s1| s2)

6. Sort s = {1,6,3,8,5}print(sorted(s))7.frozenset

Frozenset is an immutable version of set, so all methods in set that change the set itself (such as add, remove, discard, xxx_update, etc.) are not supported by frozenset; methods in set that do not change the set itself are supported by fronzenset.

These methods of frozenset function exactly the same as the methods of the same name on set collections. Frozenset has two main functions:

It is safer to use frozenset instead of set when the elements of the set do not need to change.

When some APIs require immutable objects, you must replace set with frozenset. For example, the key of dict must be immutable, so only frozenset can be used; for example, the collection elements of set itself must be immutable, so set cannot contain set, set can only contain frozenset.

s = frozenset({1,2,3})print(s, type(s))

8. practice

Mingming wanted to ask some students to do a questionnaire survey together in school. For the objectivity of the experiment, he first used the computer to generate N random integers between 1 and 1000 (N≤1000). For repeated numbers, only one was retained and the rest of the same numbers were removed. Different numbers correspond to different student numbers. Then sort these numbers from large to small, and find students to do surveys according to the order arranged. Please help Mingming complete the work of "de-duplication" and "sorting"(there may be multiple sets of data in the same test case, I hope you can handle it correctly).

import randoms = set()n = int(input("the count:"))for i in range(n): s.add(random.randint(1,1000))print(sorted(s,reverse=True))9. Characteristic

Connection +, duplicate *, index, slice are not supported

Support in, not in

II. Dictionary

1. Dictionary definitions

Dictionaries are another variable container model and can store any type of object.

Keys are generally unique, and if the last key-value pair is repeated, it replaces the previous one. Values do not need to be unique.

Determine whether the character is in the dictionary. Note that the character here is the content of the key value, not the value.

d = {'name':'jia' , 'age':'18' , 'city':'xian'}print(d,type(d))print('name'in d)print('jia'in d)

2. Dictionary printing

Key, value, dictionary printing

d = {'name':'jia' , 'age':'18' , 'city':'xian'}print(d.keys())print(d.values())print(d.items())print(d['name'])print(d['age'])print(d['city'])

Print value

print(d['name'])print(d['age'])print(d['city'])

According to dictionary format, customize key value and value, get value

print(d.get ('province ', ' shanghai'))

Add and modify dictionary values

d = {'name':'jia', 'age':'18'}d['name'] = 'zhao'print(d)d['city'] = 'xian'print(d)

Cycle printing dictionary contents

for item in d: print(item)for item in d.items(): print(item)

bivariate circular printing dictionary

for k,v in d.items(): print(f'key={k},value={v}')

3. Delete dictionary elements

Method 1: pop () method pops up the specified element

d = {'name':'jia' , 'age':'18' , 'city':'xian'}d.pop('name')print(d)

Method 2: del() method deletes the specified element:

d = {'name':'jia' , 'age':'18' , 'city':'xian'}# d.pop('name')del d['age']print(d)

4.setdefault

Set the default value to the specified value. When the default value is specified, it will not be changed.

d = {'name':'jia' , 'age':'18' , 'city':'xian'}d.setdefault('city','luoyang')print(d)d.setdefault('city','beijing')print(d)

5.defaultdict

collections.defaultdict class, itself provides the function of default values, default values can be plastic, list, collection, etc.

Defaultdict is a subclass of dict. But the biggest difference between it and dict is that if the program tries to access a value based on a non-existent key, it raises a KeyError exception; defaultdict provides a default_factory attribute that automatically generates a default value for a non-existent key.

Requirements:

We want a word that maps a key to multiple values (a so-called one-key multi-value dictionary).

Solution:

1). A dictionary is an associative container in which each key maps to a separate value. If you want the key to map to multiple values, you need to save these multiple values in a container (list or collection).

2). Automatically initialize the first value using the defaultdict class in the collections module, so you only need to focus on adding elements.

from collections import defaultdictd = defaultdict(int)d['num'] += 1print(d)d = defaultdict(list)d['hosts'].append('host1')print(d)d = defaultdict(set)d['media'].add('demo')print(d)

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