python学习笔记4---分支、循环、条件与枚举

摘要：表达式表达式是运算符和操作数所构成的序列运算符优先级同级的运算符的优先级还是有区别的比如逻辑运算符里的的优先级大于两个括号同级，左结合出现赋值符号时，右结合优先级在文本文件中编写代码脚本是后缀名为的文件，通过命令行执行推荐的，大型工程适合用

表达式（Expression）是运算符（operator）和操作数（operand）所构成的序列

>>> 1 + 1
2
>>> a = [1,2,3]
>>> 1 + 1 + 1 + 1
4
>>> 1 + 2 * 3
7
>>> 1 * 2 + 3
5
>>> a = 1 + 2 * 3
>>> a = 1
>>> b = 2
>>> c = a and b or c
>>> c = int("1") + 2

同级的运算符的优先级还是有区别的 比如逻辑运算符里的and的优先级大于or

>>> a = 1
>>> b = 2
>>> c = 3
>>> a + b * c
7
>>> 1 or 2
1
>>> 1 and 3
3
>>> a or b and c
1
>>> a or (b and c)
1
>>> a or 3
1
>>> (a or b) and c
3
>>> (a or b) and (c + 1)   //两个括号同级，左结合
4
>>> a = 1
>>> b = 2
>>> c = a + b    //出现赋值符号时，右结合
>>> print(c)    
3
>>> c = a or b
>>> print(c)
1
>>> a = 1
>>> b = 2
>>> c = 2
>>> not a or b + 2 == c
False
>>> ((not a) or ((b + 2) == c))    //优先级：not > and > or
False

python脚本是后缀名为.py的文件，通过命令行“python filename.py”执行

推荐的IDE：PyCharm、vsCode，大型工程适合用PyCharm，学习适合用vsCode，vsCode中推荐的插件:python、Terminal、Vim、vscode-icons

单行注释用#
多行注释用```

主要有条件控制（if else）、循环控制（for while）、分支

# encoding: utf-8

mood = False

if mood :
    print("go to left")
    # print("back away")
# print("back away")
else :
    print("go to right")
    
a = 1
b = 2
c = 2
# if后面不仅可以是布尔值，还可以是表达式
if a or b + 1 == c :
    print("go to left")
    # print("back away")
# print("back away")
else :
    print("go to right")

# encoding: utf-8
"""
    一段小程序
"""
# constant 常量  建议全大写
ACCOUNT = "hughie"
PASSWORD = "123456"
# python变量建议都用小写，用下划线分隔单词，不用驼峰命名
print("please input account")
user_account = input()

print("please input password")
user_password = input()

if ACCOUNT == user_account and PASSWORD == user_password:
    print("success")
else:
    print("fail")

# encoding: utf-8
# snippet 片段

if condition:
    pass
else:
    pass


a = True
if a:
    # pass 空语句/占位语句
    pass
else:
    print("")


if True:
    pass
if False:
    pass

# 嵌套分支
if condition:
    if condition:
        pass
    else:
        pass
else:
    if condition:
        pass
    else:
        pass

# 代码块
if condition:
    code1
        code11
        code22
            code333
            code444
                code5555
                code6666
    code2
    code3
else:
    code1
    code2
    code3

改写为elif

# encoding: utf-8
"""
a = x

a = 1 print("apple")
a = 2 print("orange")
a = 3 print("banana")

print("shopping")
"""

a = input()
print("a is" + a)
if a == 1:
    print("apple")
else:
    if a == 2:
        print("orange")
    else:
        if a == 3:
            print("banana")
        else:
            print("shopping")


# 改写为elif
a = input()
print(type(a))
print("a is " + a)
a = int(a)
if a == 1:
    print("apple")
elif a == 2:
    print("orange")
elif a == 3:
    print("banana")
else:
    print("shopping")

# encoding: utf-8
# 循环

# 循环语句

# while     for
# CONDITION = True
# while CONDITION:
#     print("I am while")

counter = 1
# 递归常用while
while counter <= 10:
    counter += 1
    print(counter)
else:
    print("EOF")

# encoding: utf-8

# 主要是用来遍历/循环 序列或者集合、字典
# a = ["apple", "orange", "banana", "grape"]
# for x in a:
#     print(x)

# a = [["apple", "orange", "banana", "grape"], (1, 2, 3)]
# for x in a:
#     for y in x:
#         # print(y, end="")
#         print(y)
# else:
#     print("fruit is gone")


# a = [1, 2, 3]

# for x in a:
#     if x == 2:
#         # break 遇到x==2的时候终止，打印出1
#         # break
#         # continue 遇到x==2的时候跳过，打印出1，3
#         continue
#     print(x)
# else:
#     print("EOF")


a = [["apple", "orange", "banana", "grape"], (1, 2, 3)]
for x in a:
    # if "banana" in x:
    #     break
    for y in x:
        if y == "orange":
            # 内部循环跳出后，外部循环还在执行
            break
        print(y)
else:
    print("fruit is gone")

# encoding: utf-8
# for (i=0; i<10; i++){}

# 以上的for循环用python实现
# for x in range(0, 10):
#     # range(0,10) 表示从0开始的10个数字，并不包含10
#     print(x)


# for x in range(0, 10, 2):
#     # range(0,10,2) 2表示步长
#     print(x, end=" | ")
#     # 打印结果：0 | 2 | 4 | 6 | 8 |


for x in range(10, 0, -2):
    print(x, end=" | ")
    # 打印结果：10 | 8 | 6 | 4 | 2 |

# encoding: utf-8

a = [1, 2, 3, 4, 5, 6, 7, 8]

# for i in range(0, len(a), 2):
#     print(a[i], end=" | ")
    # 1 | 3 | 5 | 7 | 

b = a[0:len(a):2]
print(b)
    # [1, 3, 5, 7]