python學習筆記（基礎三：if else流程判斷、while循環、for循環）

if else流程判斷

 getpass在pycharm中無法使用，在命令行窗口中進入python環境可以使用。

import getpass
username = input("username:")
password = getpass.getpass("password:")
 
print(username,password)

 

python中縮進錯誤：

為什么python中強制縮進，因為python中不需要定義結束符。省去了結束符，子代碼強制縮進讓結構變得更清晰。

最外層代碼必須頂格寫，不然就會報縮進錯誤。

 

if else基礎程序舉例：

實例一：判斷用戶名密碼是否正確

_username = 'alex'
_password = 'abc123'
username = input("username:")
password = input("password:")


if _username == username and _password == password:
    print("Welcom user {name} login...".format(name=username))
else:
    print("Ivalid username or password")

實例二：猜年齡

# 猜年齡
age_of_oldboy = 56
guess_age = int(input("guess age:"))
if guess_age == age_of_oldboy:
    print("you got it!")
elif guess_age < age_of_oldboy:
    print("think bigger...")
else:
    print("think smaller...")

 

while循環

 

#最簡單的while循環程序舉例
count = 0
while True:
    print("count:",count)
    count = count+1 #相當於count +=1

 

實例一：猜年齡

#猜年齡，共猜3次，如果3次內猜對也會結束程序
age_of_oldboy = 56

count = 0
while True:
    if count == 3:
        break
    guess_age = int(input("guess age:"))
    if guess_age == age_of_oldboy:
        print("you got it!")
        break
    elif guess_age < age_of_oldboy:
        print("think bigger...")
    else:
        print("think smaller...")
    count +=1

實例二：對實例一代碼的優化

#猜年齡，共猜3次，如果3次內猜對也會結束程序
age_of_oldboy = 56

count = 0
while count < 3:
    guess_age = int(input("guess age:"))
    if guess_age == age_of_oldboy:
        print("you got it!")
        break
    elif guess_age < age_of_oldboy:
        print("think bigger...")
    else:
        print("think smaller...")
    count +=1

實例三：增加人性化提示，輸入3次錯誤密碼后會得到提示：嘗試太多次了。

#猜年齡，共猜3次，如果3次內猜對也會結束程序，嘗試3次后得到提示：你嘗試的次數過多。
age_of_oldboy = 56

count = 0
while count < 3:
    guess_age = int(input("guess age:"))
    if guess_age == age_of_oldboy:
        print("you got it!")
        break
    elif guess_age < age_of_oldboy:
        print("think bigger...")
    else:
        print("think smaller...")
    count +=1
if count == 3:
    print("you have tried too many times...")

實例四：對實例三程序的優化。提示代碼的判斷可以直接用else。

#猜年齡，共猜3次，如果3次內猜對也會結束程序，嘗試3次后得到提示：你嘗試的次數過多。
age_of_oldboy = 56

count = 0
while count < 3:
    guess_age = int(input("guess age:"))
    if guess_age == age_of_oldboy:
        print("you got it!")
        break
    elif guess_age < age_of_oldboy:
        print("think bigger...")
    else:
        print("think smaller...")
    count +=1
else:
    print("you have tried too many times...")

 

for循環

實例一，最簡單的for循環程序

for i in range(10):
    print("loop",i)

等於以下：

等於以下：
for i in range(0,10,1): #步長默認為1
    print("loop",i)

i,臨時變量

range，相當於定義了（0,1,2,3,4,5,6,7,8,9） 每循環一次i按順序取值一次。

 

實例二：上節課中的while循環實例改為for循環：

#猜年齡，共猜3次，如果3次內猜對也會結束程序，嘗試3次后得到提示：你嘗試的次數過多。
age_of_oldboy = 56

for i in range(3):
    guess_age = int(input("guess age:"))
    if guess_age == age_of_oldboy:
        print("you got it!")
        break
    elif guess_age < age_of_oldboy:
        print("think bigger...")
    else:
        print("think smaller...")

else:
    print("you have tried too many times...")

實例三，打印10以內的偶數：

for i in range(0,10,2): #2為步長
    print("loop",i)

實例四，優化while猜年齡程序

#猜年齡，共猜3次，嘗試3次后詢問是否繼續，如果回答：n，則結束程序；其他則重新開始程序。
age_of_oldboy = 56

count = 0
while count < 3:
    guess_age = int(input("guess age:"))
    if guess_age == age_of_oldboy:
        print("you got it!")
        break
    elif guess_age < age_of_oldboy:
        print("think bigger...")
    else:
        print("think smaller...")
    count +=1
    if count ==3:
        continue_confirm = input("do you want to keep guessing?")
        if continue_confirm != "n":
            count = 0

break和continue的區別，根據下面2段代碼，使用debug調試功能在pycharm中運行，觀察后得知

代碼一：

# continue的作用是結束本次循環，不會終止for循環
for i in range(0,10):
    if i <3:
        print("loop",i)
    else:
        continue
    print("hehe...")

代碼二：

# break是結束當前循環
for i in range(0,10):
    if i <3:
        print("loop",i)
    else:
        break
    print("hehe...")

 

循環嵌套

for i in range(0,10):
    print("--------",i)
    for j in range(10):
        print(j)
        if j >5:
            break

查看輸出：小循環輸出0-6，大循環輸出0-9，brake只中斷當前循環。