python学习笔记（基础三：if else流程判断、while循环、for循环）

if else流程判断

getpass在pycharm中无法使用，在命令行窗口中进入python环境可以使用。

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

print(username,password)

python中缩进错误：

为什么python中强制缩进，因为python中不需要定义结束符。省去了结束符，子代码强制缩进让结构变得更清晰。

最外层代码必须顶格写，不然就会报缩进错误。

if else基础程序举例：

实例一：判断用户名密码是否正确

_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")

实例二：猜年龄

# 猜年龄
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循环

#最简单的while循环程序举例
count = 0
while True:
    print("count:",count)
    count = count+1 #相当于count +=1

实例一：猜年龄

#猜年龄，共猜3次，如果3次内猜对也会结束程序
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

实例二：对实例一代码的优化

#猜年龄，共猜3次，如果3次内猜对也会结束程序
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

实例三：增加人性化提示，输入3次错误密码后会得到提示：尝试太多次了。

#猜年龄，共猜3次，如果3次内猜对也会结束程序，尝试3次后得到提示：你尝试的次数过多。
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...")

实例四：对实例三程序的优化。提示代码的判断可以直接用else。

#猜年龄，共猜3次，如果3次内猜对也会结束程序，尝试3次后得到提示：你尝试的次数过多。
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循环

实例一，最简单的for循环程序

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

等于以下：

等于以下：
for i in range(0,10,1): #步长默认为1
    print("loop",i)

i,临时变量

range，相当于定义了（0,1,2,3,4,5,6,7,8,9） 每循环一次i按顺序取值一次。

实例二：上节课中的while循环实例改为for循环：

#猜年龄，共猜3次，如果3次内猜对也会结束程序，尝试3次后得到提示：你尝试的次数过多。
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...")

实例三，打印10以内的偶数：

for i in range(0,10,2): #2为步长
    print("loop",i)

实例四，优化while猜年龄程序

#猜年龄，共猜3次，尝试3次后询问是否继续，如果回答：n，则结束程序；其他则重新开始程序。
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的区别，根据下面2段代码，使用debug调试功能在pycharm中运行，观察后得知

代码一：

# continue的作用是结束本次循环，不会终止for循环
for i in range(0,10):
    if i <3:
        print("loop",i)
    else:
        continue
    print("hehe...")

代码二：

# break是结束当前循环
for i in range(0,10):
    if i <3:
        print("loop",i)
    else:
        break
    print("hehe...")

循环嵌套

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

查看输出：小循环输出0-6，大循环输出0-9，brake只中断当前循环。