Python之流程控制语句

这里介绍Python中的if、for、while语句

条件测试

常见地条件测试如下所示。特别地，Python支持链式比较

print("--------------------- 条件测试 ----------------------------")

# 判断值是否相等、不等
print('"Aaron" == "Aa"+"ron" : ', "Aaron" == "Aa"+"ron")
print('"Aaron" != "Bob" : ', "Aaron" != "Bob")

# 数学比较
print("2 != 8 : ", 2 != 8)
print("2 < 8 : ", 2 < 8)
print("2 <= 8 : ", 2 <= 8)
print("8 <= 8 : ", 8 <= 8)
print("8 > 2 : ", 8 > 2)
print("8 >= 2 : ", 8 >= 2)
print("2 >= 2 : ", 2 >= 2)

# 链式比较
aaron_age = 35
aaron_is_young = 18 <= aaron_age < 60
print("aaron is young ? ", aaron_is_young)


# 逻辑与 and
print("(2==2) and (1<9) : ", (2==2) and (1<9) )
print("(2!=2) and (1<9) : ", (2!=2) and (1<9) )
# 逻辑或 or
print("(2>996) or (11<9) : ", (2>996) or (11<9) )
print("(2!=2) or (1<9) : ", (2!=2) or (1<9) )
# 逻辑非 not
print("not (3>1) : ", not(3>1) )
print("not (3>9) : ", not(3>9) )

# in、not in 成员运算符：判断指定值是否 在/不在 容器中
names = ("Aaron", "Tony")
print("Tony in names : ", "Tony" in names)
print("Bob not in names : ", "Bob" not in names)
ages = [28, 25, 37]
print("25 in ages : ", 25 in ages)
print("99 not in ages : ", 99 not in ages)
citys = {"Beijing", "Shanghai"}
print("Beijing in citys : ", "Beijing" in citys)
print("Hangzhou not in citys : ", "Hangzhou" not in citys)

# is、is not 身份运算符：判断两个变量 是否指向 同一个对象/不同对象
a = 4
b = a
print('a(4) is b(4) : ', a is b)
# 也可以通过id()函数获取所指向对象的内存地址进行比较
print('id(a) == id(b) : ', id(a) == id(b))

m = [1,2,3]
n = [1,2,3]
print("m:", m)
print("n:", n)
print('m == n : ', m == n)
print('m is n : ', m is n)
print('m is not n : ', m is not n)
# 也可以通过id()函数获取所指向对象的内存地址进行比较
print('id(m) != id(n) : ', id(m) != id(n))

if语句

Python中的if语句在功能上和其他语言没有什么区别。支持if、if-else、if-elif-else等形式

print("------------------ if语句 --------------------")

# if 语句
if 996 > 995 :
    print("Bye, Aaron")

# if-else 语句
if 996 < 995 :
    print("Hello ", end="")
    print("996 < 995")
else:
    print("Hi ", end="")
    print("996 > 995")
    
# if-elif-else语句
age = 12
if age < 4:
    print("Hi ", end="")
    print("Your cost is $0")
elif age < 18:
    print("Hello ", end="")
    print("Your cost is $11.11")
elif age < 35:
    print("Oh, ", end="")
    print("Your cost is $33")
else:   # 这里 else 子句是可选的，不是必须的
    print("GG ", end="")
    print("Your cost is $996")

特别地，对于if、while语句的条件表达式，对于True/False的判定存在下述规则

• 对于数字0、空值 None、单引号空字符串’’、双引号空字符串””、空列表[]、空元组()、空字典{}、空集合set() 都会被视为False
• 对于非零数字、非空单引号空字符串、非空双引号空字符串、非空列表、非空元组、非空字典、非空集合 都会被视为True

if "" :
    print("Bye, Aaron")
else:
    print("This is a Bad News")

while语句

Python的while语句在功能上与其他语言并无无区别

print("-------------while语句----------------------")

count = 0
while count<15:
    count += 1
    if count==2:
        # continue语句: 提前结束本次循环，继续执行下次循环
        continue
    if count==7:
        # break语句: 直接退出当前循环
        break
    print("count -->>", count)

特别地，Python中的while语句还可以使用else子句，其仅在while循环中没有调用break时才会执行

print("---------------while-else语句-----------------------------")
nums =[1,3,5,7,9,12]
index = 0
while index < len(nums):
    num = nums[index]
    index += 1
    if( num%2==0 ):
        print("[break]: 发现偶数")
        break
else:
    print("[else]: 未发现偶数!!")  
    
print("--------------------------------------------------")

index = 0
nums =[2,4,6,8,10]
while index < len(nums):
    num = nums[index]
    index += 1
    if( num%2==1 ):
        print("[break]: 发现奇数")
        break
else:
    print("[else]: 未发现奇数!!")  

for语句

Python的for语句仅可用于遍历可迭代对象

print("--------------------------for语句--------------------------")

names = ["Aaron", "David", "Bob"]
for name in names:
    print("name ---->>>",name)

for index in range(3):
    print("index : ", index, " --->>> ", names[index])

特别地，Python中的for语句还可以使用else子句，其仅在for循环中没有调用break时才会执行

print("--------------------------for-else语句--------------------------")

nums =[1,3,5,7,9,12]
for num in nums:
    if( num%2==0 ):
        print("[break]: 发现偶数")
        break
else:
    print("[else]: 未发现偶数!!")  
    
print("----------------------------------------")

nums =[2,4,6,8,10]
for num in nums:
    if( num%2==1 ):
        print("[break]: 发现奇数")
        break
else:
    print("[else]: 未发现奇数!!")  

参考文献

1. Python编程·第3版：从入门到实践 Eric Matthes著
2. Python基础教程·第3版 Magnus Lie Hetland著
3. 流畅的Python·第1版 Luciano Ramalho著