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# 3-4 python逻辑之程序循环for
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- for循环是python语言中重要语句，做重复事情就是它最重要的功能
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- 举个例子，要求依次输出从1到5的5个数
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```python
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# 1.不用for循环的写法,要求也能完成，但这太低效了，完全没有利用计算机的优势
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print(1) #重复5次，把5个数输出
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print(2)
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print(3)
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print(4)
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print(5)
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# 2.用for循环的写法,for语句重复从列表中取数，依次打印出来
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for x in [1,2,3,4,5]:
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print(x) #通过for语句，循环5次，重复输出即可
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# 3.用 range函数，直接生成1-5的列表，这样写更简单了
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for x in range(1,6):
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print(x)
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```
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- **同样的事情重复N遍处理，这才是for循环的目的，下图简单清楚的说明的for循环的整个流程**
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![674](https://user-images.githubusercontent.com/103555341/163699595-f34bd435-de45-4614-8fab-896227ea9bda.jpg)
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# 最后总结for循环的基本结构
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```python
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#for循环的基本结构 ( for 和 in 组成 )
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for 变量 in 序列: # 结尾一定有冒号':', 依次把序列元素赋值给变量
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语句1 # 主要前面有空格，语句缩进，代表这些语句1是for循环中的
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语句2 # 语句缩进，代表这些语句2是for循环中的
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....
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语句3 # 没有空格缩进，同for层级，不是for循环内的
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语句4 # 没有空格缩进，同for层级，不是for循环内的
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```
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### 代码区 1
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```python
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1 # for循环演示,你可以通过下面代码感受一下循环
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2 num=[1,2,3,4,5] #创建5个数组成的列表,给for用
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3
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4 for x in num: #for开始循环，顺序取数赋值给X
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5 print(x) #第1次取数是1,打印输出
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6 #第2次取数是2,循环5次,到头退出
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7 print('end') #循环结束，程序结束,输出 end
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8
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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```python
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1
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2
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3
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4
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5
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end
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```
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### 代码区 2
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```python
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1 #for循环：用range函数，直接生成1-5的列表，这样写更简单了
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2 for x in range(1,6): #生成1-6直接的数
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3 print(x)
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4
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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```python
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1
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2
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3
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4
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5
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```
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### 代码区 3
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```python
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1 # for循环之遍历一个字符串
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2 v = 'python' #字符串和列表一样都是一种可迭代的序列
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3
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4 for c in v: #for依次把每个字母取出
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5 print(c)
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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```python
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p
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y
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t
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h
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o
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n
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```
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# for循环的具体应用
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- **只要是重复循环的，就可以用for语句，比如写一个九九乘法表，您就不用写81行了**
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### 代码区 4
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```python
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1 # for循环：九九乘法表你可以试试输出正三角
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2 for i in range(1, 10):
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3 for j in range(i, 10):
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4 print('%d * %d = %d' % (i, j, i * j), end='\t')
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5 print('') # 控制换行
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6
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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```python
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1 * 1 = 1 1 * 2 = 2 1 * 3 = 3 1 * 4 = 4 1 * 5 = 5 1 * 6 = 6 1 * 7 = 7 1 * 8 = 8 1 * 9 = 9
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2 * 2 = 4 2 * 3 = 6 2 * 4 = 8 2 * 5 = 10 2 * 6 = 12 2 * 7 = 14 2 * 8 = 16 2 * 9 = 18
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3 * 3 = 9 3 * 4 = 12 3 * 5 = 15 3 * 6 = 18 3 * 7 = 21 3 * 8 = 24 3 * 9 = 27
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4 * 4 = 16 4 * 5 = 20 4 * 6 = 24 4 * 7 = 28 4 * 8 = 32 4 * 9 = 36
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5 * 5 = 25 5 * 6 = 30 5 * 7 = 35 5 * 8 = 40 5 * 9 = 45
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6 * 6 = 36 6 * 7 = 42 6 * 8 = 48 6 * 9 = 54
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7 * 7 = 49 7 * 8 = 56 7 * 9 = 63
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8 * 8 = 64 8 * 9 = 72
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9 * 9 = 81
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```
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### 代码区 5
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```python
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1 # for 循环访问列表
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2 name = ['python', 'java', 'go','php','c++','delphi','Basic']
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3 for x in name: #依次把列表所有元素循环一遍
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4 print(x)
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5
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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```python
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python
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java
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go
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php
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c++
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delphi
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Basic
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```
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### 代码区 6
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```python
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1 # for循环打印等腰三角形
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2
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3 x=10
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4 for i in range(0,x):
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5 print(" "*(x-i)+"* "*(i+1))
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6
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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```python
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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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* * * * * * * * * *
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```
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### 代码区 7
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```python
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1 # for循环打印ASICII值
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2
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3 text = 'ABabc123#'
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4 for x in text:
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5 print('字符:',x,' ASICII值:',ord(x))
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6
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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```python
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字符: A ASICII值: 65
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字符: B ASICII值: 66
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字符: a ASICII值: 97
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字符: b ASICII值: 98
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字符: c ASICII值: 99
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字符: 1 ASICII值: 49
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字符: 2 ASICII值: 50
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字符: 3 ASICII值: 51
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字符: # ASICII值: 35
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```
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### 代码区 8
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```python
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1 # 练习题 定义一个列表(list) 并遍历打印出list中的每一个元素
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2
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```
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![4](https://user-images.githubusercontent.com/103555341/163546933-bee710b5-943e-454e-b00d-922d2b897614.jpg)
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