---
title: 迴圈 - Python 教學
description: 中興大學資訊研究社1101學期程式分享會主題社課
tags: Python
---
###### [Python 教學/](https://hackmd.io/7-LP9CyOThOdkEbq44FLvw)
# 迴圈
> [name=Hui][time= 109,10,27]
---
## 前言
程式寫作時，我們常常需要重複執行某段程式，這時迴圈的使用就顯得很重要
當我們已知or可計算執行的次數時我們常用for迴圈
而當執行次數難以計算時，就比較適合用while迴圈
# for
用法:
for (一物件) in (所有物件) :
簡單來說 就是 for "自己取的變數名稱" in "list,taple....":
"你要執行的程式碼"
PS:注意縮排
我們每次執行 都會從 後面的 list **依序** 拿其中一個元素對他做處理
Q:那我只是單純的想做100次呢?
那就用"range()"
## for 範例1
for 後面接一個 range() 如
```python=
for i in range(5):
print(i)
# 0,1,2,3,4
```
### range
> class range(stop)
> class range(start, stop[, step])
用法
1. 單純設定一個停止的數
如range(30)
```python=
# 0,1,2,3,4,....29
```
2. 設定開頭和結尾
如range(5,30)
```python=
# 5,6,7,8,9,10...29
```
3. 設定開頭、結尾和每一次加多少(step)
如range(5,30,5)
```python=
# 5,10,15,20,25
```
當然數字不一定是要正的，負的也行
### 練習1
輸入:n
輸出:n 到 0
ex:10,9,8,7,6,....0,
hint:
```python=
x = int(input()) #輸入一個數字，並轉為整數的型態
print(x) #輸出一個數字
```
### 練習2
輸入:n
輸出:F1~Fn 的費氏數列
ex:
input:6
output:1,1,2,3,5,8,
input:8
output:1,1,2,3,5,8,13,21,
解答:
```python=
n = int(input())
fb_list=[]
for i in range(n):
if i == 0 or i == 1:
fb_list.append(1)
else :
fb_list.append(fb_list[-1]+fb_list[-2])
print(fb_list)
```
### for 範例2
for 後面接一個list
舉例來說
```python=
list=[2,4,5,8,1,3]
for x in list:
print(x)
# 2,4,5,8,1,3
```
當然，不一定要都是相同的型態
```python=
list=['gawr','gura',1,0,0,'!']
for x in list:
print(x,end='')
```
### 練習3
給定
```python=
list_A=[1,2,3,54,4,4,6,8]
```
輸出:
兩倍的list_A
ex:
[2,4,6,108,8.....]
### 練習3.5
給定
```python=
list_B=[1,2,3,54,4,4,6,8]
```
輸出:
新的list_C=list_B的反過來
ex:
[8,6,4,4,54,3,2,1]
>hint: Using "insert()" to add item in new list
### for 範例3
~~防風林外還有防風林外還有防林~~
for 迴圈中當然也還能有for 迴圈
而要怎麼判斷屬於哪層的for迴圈則是看縮排
```python=
list_G=[['x1','x2','x3'],['y1','y2','y3'],['z1','z2','z3']]
for dim in list_G:
for pos in dim:
print(pos)
```
上面這 第一個 dim指的是 list 中的 list
```python=
a=10
b=20
count=0
for i in range(a):
for j in range(b):
count += 1 #count = count + 1
print(count)
```
猜猜看會輸出多少?
因為內層的 for 執行了20次count+=1
而外層的 for 執行了內層的 for 10次
所以20*10=200
```python=
a=10
b=20
count=0
for i in range(a):
count+=1
for j in range(b):
count+=1
print(count)
```
那這樣是多少呢?
### 印星星
for 搭配 if
```python=
a=int(input())
listA=['gura',1,0,0,'!']
for x in listA:
print(x,end=' ')
print('')
for i in range(a):
if i==(int)(a/1.5):
for j in range(a+i+1):
if j>=a-i and j<=a+i+1:
print('*',end='')
else:
print(' ',end='')
else:
for j in range(a):
if j==a-i:
print('*',end='')
else:
print(' ',end='')
for j in range(i+1):
if j==i:
print('*',end='')
else:
print(' ',end='')
print('')
```
### 練習4 (easy)
輸入 n
印出 n*n的正方形
ex:
input=3
output=
**\*
\*\*\*
\*\*\*
```python=
a=int(input())
for i in range(a):
for j in range(a):
print('*',end='')
print('')
```
### 練習5(hard 題目我抄來的)
輸入 n
印出 所有小於n的質數
ex:
input=15
output=2,3,5,7,11,13
```python=
a=int(input())
count=0
for i in range(a):
for j in range(1,i):
if (i%j)==0:
count+=1
if count==1:
print(i,end=',')
count=0
```
### 練習6(hard)
輸入 n
印出 半徑為n的圓形
you may need [sqrt()](https://www.runoob.com/python/func-number-sqrt.html) and [round()](https://www.runoob.com/python/func-number-round.html)
```python=
import math
r=int(input())
count=0
for i in range(2*r+1):
for j in range(2*r+1):
if (i-r)**2+(j-r)**2<=r**2 and (i-r)**2+(j-r)**2>=r**2 :
print('*',end='')
else:
print(' ',end='')
print('')
```
## while
whlie 通常用於我們不知道做幾次，但是知道甚麼時候該結束的事件
```python=
a=2
while(a<4096):
a*=a
print(a)
```
上面範例就是a一直平方直到a>=4096
```python=
a=1
k=' '
while(k):
k=input()
a+=1
print(a,k)
print('--end--')
```
這段程式碼則是會在k沒有輸入時停止
### break (can also use in for loop)
在執行到一半時，如果我們希望能直接跳出迴圈則使用break
break!
```python=
a=1
k=' '
while(k):
k=input()
if k=='exit':
break
a+=1
print(a,k)
print('--end--')
```
上面程式在k==exit時會直接跳出while loop
### continue (can also use in for loop)
```python=
a=1
k=' '
while(k):
k=input()
if k== 'pass':
continue
a+=1
print(a,k)
if k=='exit':
break
print('--end--')
```
### practice 01
input 整數n ,n>0 請輸出 n 的二進位數
ex:
input:4
output:100
input:15
output:1111
```python=
a=int(input())
output=''
while(a>0):
output=str(int(a%2))+output
a=a-(a%2)
a=a/2
print(output)
```
## 疑難雜症
### for x in list??
舉例來說
```python=
list=[2,4,5,8,1,3]
for x in list:
print(x)
# 2,4,5,8,1,3
```
不過這邊的運算大概是這樣
```python=
list=[2,4,5,8,1,3]
for i in range(len(list)):
x=list[i]
print(x)
```
.
.
.
.
```python=
a=[-1,2,-3,-4,5,6,7,-8,9,-10,11]
for x in a:
x+=10
print(a)
```
對x的操作只影響x
不影響list
.
.
.
```python=
a=[-1,2,-3,-4,5,6,7,-8,9,-10,11]
for x in a:
if x<0:
a.remove(x)
print(a)
```
```python=
a=[-1,2,-3,-4,5,6,7,-8,9,-10,11]
for x in a[:]:
if x<0:
a.remove(x)
print(a)
```
如果對list直接操做要注意是否會影響跌代的結構
<style>hr{display:none;}</style>
# 補充
for 迴圈其實是呼叫一個迭代器
[Python進階技巧 (6) — 迭代那件小事：深入了解 Iteration / Iterable / Iterator / \_\_iter__ / \_\_getitem__ / \_\_next__ / yield](https://medium.com/citycoddee/python%E9%80%B2%E9%9A%8E%E6%8A%80%E5%B7%A7-6-%E8%BF%AD%E4%BB%A3%E9%82%A3%E4%BB%B6%E5%B0%8F%E4%BA%8B-%E6%B7%B1%E5%85%A5%E4%BA%86%E8%A7%A3-iteration-iterable-iterator-iter-getitem-next-fac5b4542cf4)
所以可以透過自訂迭代邏輯來客製化迴圈
ex:
```python=
class MyIterator:
def __init__(self):
self.total_times = 0
self.index = 0
def add_times(self, times):
self.total_times += times
return self
def __iter__(self):
return self
def __next__(self):
self.index += 1
if self.index <= self.total_times:
return self.index
else:
self.index -= 1
raise StopIteration
my_iterator = MyIterator()
for item in my_iterator.add_times(3):
print(item)
for item in my_iterator.add_times(3):
print(item)
for item in my_iterator:
print(item)
```
# 挑戰?
https://leetcode.com/problems/two-sum/
https://leetcode.com/problems/add-two-numbers/
https://leetcode.com/problems/climbing-stairs/
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