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451 lines
10 KiB
Markdown
451 lines
10 KiB
Markdown
<div align="center">
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<h1> 30 天 Python：第十天 - Loops</h1>
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<a class="header-badge" target="_blank" href="https://www.linkedin.com/in/asabeneh/">
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<img src="https://img.shields.io/badge/style--5eba00.svg?label=LinkedIn&logo=linkedin&style=social">
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</a>
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<a class="header-badge" target="_blank" href="https://twitter.com/Asabeneh">
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<img alt="Twitter Follow" src="https://img.shields.io/twitter/follow/asabeneh?style=social">
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</a>
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<sub>Author:
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<a href="https://www.linkedin.com/in/asabeneh/" target="_blank">Asabeneh Yetayeh</a><br>
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<small> 第二版：2021 年 7 月</small>
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</sub>
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</div>
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[<< 第 9 天](./09_conditionals.md) | [第 11 天 >>](./11_functions.md)
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- [📘 第十天](#-第十天)
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- [循环](#循环)
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- [while 循环](#while-循环)
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- [break和continue - part 1](#break和continue---part-1)
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- [for 循环](#for-循环)
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- [break 和 continue - part 2](#break-和-continue---part-2)
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- [range() 函数](#range-函数)
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- [嵌套for循环](#嵌套for循环)
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- [for和else](#for和else)
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- [pass语句](#pass语句)
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- [💻 练习：第十天](#-练习第十天)
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- [练习：一级](#练习一级)
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- [练习：二级](#练习二级)
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- [练习：三级](#练习三级)
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# 📘 第十天
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## 循环
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生活充满了循环。在编程中，我们会做很多重复的任务。编程语言使用循环来处理重复的任务，而Python编程语言提供了以下两种类型的循环:
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1. while 循环
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2. for 循环
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### while 循环
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我们使用关键字`while`来创建while循环。它在条件被满足时重复执行代码块。当条件变为false时，会结束循环代码块，执行循环之后的代码。
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```python
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# syntax
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while condition:
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code goes here
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```
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**示例:**
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```python
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count = 0
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while count < 5:
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print(count)
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count = count + 1
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#prints from 0 to 4
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```
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在上面的循环中，当count等于5时，循环条件变为false，此时循环停止。
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如果我们想要在循环条件变为false时运行特定的代码块，我们可以使用`else`关键字。
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```python
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# syntax
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while condition:
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code goes here
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else:
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code goes here
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```
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**示例:**
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```python
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count = 0
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while count < 5:
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print(count)
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count = count + 1
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else:
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print(count)
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```
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当count等于5时，循环条件变为false，循环终止，然后开始执行else块中的代码。因此，5将会被打印输出。
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### break和continue - part 1
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* Break：当我们想要退出循环时，我们使用`break`关键字。
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```python
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# syntax
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while condition:
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code goes here
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if another_condition:
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break
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```
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**Example:**
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```python
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count = 0
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while count < 5:
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print(count)
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count = count + 1
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if count == 3:
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break
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```
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上面的while循环只会打印输出0，1，2，但当count等于3时，循环会终止。
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- Continue：当我们想要跳过当前循环并继续执行下一个循环时，我们使用`continue`关键字。
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```python
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# syntax
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while condition:
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code goes here
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if another_condition:
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continue
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```
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**示例:**
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```python
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count = 0
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while count < 5:
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if count == 3:
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count = count + 1
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continue
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print(count)
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count = count + 1
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```
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上面的while循环只会打印输出0，1，2，4。（3被跳过了）
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### for 循环
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`for`关键字用于创建for循环。和别的编程语言相似，但语法上有一些不同。它可以用于对序列的遍历（也就是列表、元组、字典、集合、字符串等）。
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- 列表的for循环
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```python
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# syntax
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for iterator in lst:
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code goes here
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```
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**示例:**
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```python
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numbers = [0, 1, 2, 3, 4, 5]
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for number in numbers: # number是引用列表项的临时名称，仅在此循环中有效
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print(number) # number将会被逐行打印，从0到5
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```
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- 字符串的for循环
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```python
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# syntax
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for iterator in string:
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code goes here
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```
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**示例:**
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```python
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language = 'Python'
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for letter in language:
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print(letter)
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for i in range(len(language)):
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print(language[i])
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```
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- 元组的for循环
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```python
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# syntax
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for iterator in tpl:
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code goes here
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```
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**示例:**
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```python
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numbers = (0, 1, 2, 3, 4, 5)
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for number in numbers:
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print(number)
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```
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- 字典的for循环
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循环遍历将会遍历字典的键。
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```python
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# syntax
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for iterator in dct:
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code goes here
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```
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**示例:**
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```python
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person = {
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'first_name':'Asabeneh',
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'last_name':'Yetayeh',
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'age':250,
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'country':'Finland',
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'is_marred':True,
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'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
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'address':{
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'street':'Space street',
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'zipcode':'02210'
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}
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}
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for key in person:
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print(key) #仅输出键
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for key, value in person.items():
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print(key, value) # 这样我们可以在迭代的过程中同时访问键和值
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```
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- 集合的for循环
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```python
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# syntax
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for iterator in st:
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code goes here
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```
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**示例:**
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```python
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it_companies = {'Facebook', 'Google', 'Microsoft', 'Apple', 'IBM', 'Oracle', 'Amazon'}
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for company in it_companies:
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print(company)
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```
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### break 和 continue - part 2
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提示：
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_break_:当我们想要在循环完成前退出循环时，我们使用`break`关键字。
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```python
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# syntax
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for iterator in sequence:
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code goes here
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if condition:
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break
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```
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**示例:**
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```python
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numbers = (0,1,2,3,4,5)
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for number in numbers:
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print(number)#输出 0，1，2，3
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if number == 3:
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break
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```
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在上面的例子中，当number等于3时，循环会终止。
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_continue_:当我们想要跳过当前循环并继续执行下一个循环时，我们使用`continue`关键字。
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```python
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# syntax
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for iterator in sequence:
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code goes here
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if condition:
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continue
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```
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**示例:**
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```python
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numbers = (0,1,2,3,4,5)
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for number in numbers:
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print(number)
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if number == 3:
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continue
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print('Next number should be ', number + 1) if number != 5 else print("loop's end") # 简而言之，对于简短的条件，需要同时使用if和else语句
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print('outside the loop')
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```
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在上面的例子中，当number等于3时，在此条件之后的（但在循环中的）语句会被跳过，如果还有未完成的遍历元素，它会继续执行下一个循环。
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### range() 函数
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`range()`函数用于生成一个序列的数字。_range(start, end, step)_函数接受三个参数：起始值、结束值和步长。默认情况下，起始值是0，步长是1。这个函数需要至少一个参数。（结束值end）
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使用`range()`函数生成序列
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```python
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lst = list(range(11))
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print(lst) # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
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st = set(range(1, 11)) # 两个参数分别代表start和stop，步长step为默认值1
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print(st) # {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
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lst = list(range(0,11,2))
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print(lst) # [0, 2, 4, 6, 8, 10]
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st = set(range(0,11,2))
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print(st) # {0, 2, 4, 6, 8, 10}
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```
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```python
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# syntax
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for iterator in range(start, end, step):
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```
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**示例:**
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```python
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for number in range(11):
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print(number) # 打印输出0到10，不包括11。
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```
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### 嵌套for循环
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我们可以在循环中嵌套另一个循环。这种循环称为嵌套循环。
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```python
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# syntax
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for x in y:
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for t in x:
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print(t)
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```
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**示例:**
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```python
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person = {
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'first_name': 'Asabeneh',
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'last_name': 'Yetayeh',
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'age': 250,
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'country': 'Finland',
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'is_marred': True,
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'skills': ['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
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'address': {
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'street': 'Space street',
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'zipcode': '02210'
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}
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}
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for key in person:
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if key == 'skills':
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for skill in person['skills']:
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print(skill)
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```
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### for和else
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如果我们想要在循环结束时执行特定的代码块，我们可以使用`else`关键字。
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```python
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# syntax
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for iterator in range(start, end, step):
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do something
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else:
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print('The loop ended')
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```
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**示例:**
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```python
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for number in range(11):
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print(number) # prints 0 to 10, not including 11
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else:
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print('The loop stops at', number)
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```
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### pass语句
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在python中，当需要一些语句（比如在`:`后），但我们不想执行任何代码时，我们可以使用`pass`关键字来避免报错。此外，我们也可以用它来作为一个占位符，以便在以后填充代码。
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**示例:**
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```python
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for number in range(6):
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pass
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```
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🌕 你完成了伟大的一步，太猛了哥。冲冲冲！你刚刚完成了第10天的挑战，你在通往伟大的道路上迈出了10步。现在我们做一些练习来练练肌肉和大脑。
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## 💻 练习：第十天
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### 练习：一级
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1. 分别使用while和for实现从0到10的迭代。
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2. 分别使用while和for实现从10到0的迭代。
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3. 写一个循环，调用7次`print()`函数，输出如下的三角形：
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```py
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#
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##
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###
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####
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#####
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######
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#######
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```
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4. 使用嵌套循环来实现下面的输出：
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```sh
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# # # # # # # #
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# # # # # # # #
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# # # # # # # #
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# # # # # # # #
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# # # # # # # #
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# # # # # # # #
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# # # # # # # #
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# # # # # # # #
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```
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5. 使用循环实现下面格式的输出：
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```sh
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0 x 0 = 0
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1 x 1 = 1
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2 x 2 = 4
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3 x 3 = 9
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4 x 4 = 16
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5 x 5 = 25
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6 x 6 = 36
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7 x 7 = 49
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8 x 8 = 64
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9 x 9 = 81
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10 x 10 = 100
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```
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6. 用for循环遍历列表`['Python', 'Numpy','Pandas','Django', 'Flask']`，并打印输出每个元素。
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7. 用for循环从0到100遍历并且打印输出所有偶数。
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8. 用for循环从0到100遍历并且打印输出所有奇数。
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### 练习：二级
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1. 使用for循环从0到100遍历并且输出所有数字的和。
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```sh
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The sum of all numbers is 5050.
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```
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2. 使用for循环从0到100遍历并且分别输出所有奇数和所有偶数的和。
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```sh
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The sum of all odd numbers is 2500. And the sum of all even numbers is 2550.
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```
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### 练习：三级
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1. 跳转到data文件夹并使用[countries.py](https://github.com/Asabeneh/30-Days-Of-Python/blob/master/data/countries.py)文件。循环遍历所有国家，并且提取出所有包含字母`land`的国家。
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2. 有一个列表`fruits = ['banana', 'orange', 'mango', 'lemon']`，使用循环反转列表中的元素。
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3. 跳转到data文件夹并使用[countries_data.py](https://github.com/Asabeneh/30-Days-Of-Python/blob/master/data/countries-data.py)文件。
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1. 数据中一共有多少个语言？
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2. 找到被最多国家使用的语言。
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3. 找到人数排名前十的国家。
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🎉 恭喜！ 🎉
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[<< 第 9 天](./09_conditionals.md) | [第 11 天 >>](./11_functions.md) |