Day 3 控制流(Control Flow)

条件语句

• if 语句

if phone_balance < 5 :
    phone_balance += 10
    bank_balance -= 10

冒号后为满足if的条件后要执行的语句

• elif 以及else语句

if season == 'spring':
    print('plant the garden!')
elif season == 'summer':
    print('water the garden!')
elif season == 'fall':
    print('harvest the garden!')
elif season == 'winter':
    print('stay indoors!')
else:
    print('unrecognized season')

python中用缩进封装代码块
注意格式输出语法 message = "Somebody who is {} years old will pay ${} to ride the bus.".format(age, ticket_price)

条件布尔表达式

即在if语句中使用非布尔对象代替布尔表达式，python检查其真假值并判断是否运行缩进代码
python中视为false的内置对象
- 定义为false的常量: None和False
- 任何数字类型的零: 0,0.0,0j,Decimal(0),Fraction(0, 1)
- 空序列和空集合: "",(),[],{},set(),range(0)

errors = 3
if errors:
    print("You have {} errors to fix!".format(errors))
else:
    print("No errors to fix!")

  - 并：`and`,或：`or`

for循环

• for 循环的组成部分

cities = ['new york city', 'mountain view', 'chicago', 'los angeles']

for city in cities:
    print(city.title())

⚠ 标准命名应为一个单数一个复数，.title()作用是首字母大写

• 创建列表 －－>append()

cities = ['new york city', 'mountain view', 'chicago', 'los angeles']
capitalized_cities = []

for city in cities:
    capitalized_cities.append(city.title())

• 创建列表 －－>range()
range(start = 0, stop, step = 1)
  以上为默认值，stop比序列的最后一个数字大一，同list的遍历

cities = ['new york city', 'mountain view', 'chicago', 'los angeles']

for index in range(len(cities)):
    cities[index] = cities[index].title()

⚠ 此种写法并未对names对象本身进行修改，所以没有效果

names = ["Joey Tribbiani", "Monica Geller", "Chandler Bing", "Phoebe Buffay"]

for name in names:
    name = name.lower().replace(" ", "_")

print(names)

也可以使用其重复某个操作

for i in range(3):
    print("Hello!")

附：每日一题

if (x>3) : print("True") else: print("False"),若 x=2,则该段代码执行结果是？
答：False