CS50P: 2. Loops

control + C 终止循环

while循环

#meow 3 times
i = 0
while i < 3:
    print("meow")
    i += 1		#python中没有i++

for循环

for i in [0, 1, 2]:
    print("meow")

i 初始为 1，依次取 2、3

in 可以让 i 按序取遍 list 中的元素，其中元素可以是 int, dict, str, etc.

for _ in range(3):	
  	print("meow")

range(x) 返回 [0, 1, ..., x-1]

_ 是Pythonic的用法，因为 i 只用来计数，之后不会用到

*

print("meow\n" * 3, end = '')

continue & break

while True:    
    n = int(input("What's n? "))
    if n < 0:
        continue    #继续循环
    else:
        break

如果想让input符合预期，刻意用 while True

当然，还可以写为：

if n > 0:
  	break

list

a datatype

students = ["Hermoine", "Harry", "Ron"]
for student in students:
    print(student) 

这样可以不考虑list的长度

student会初始化为 Hermoine ，然后取 Harry ……

len

len() 返回列表的元素个数

for i in range(len(students)):
    print(students[i])

使用列表中的元素 students[i]

dict (dictionaries)

associate one value with another, keys(words) and values(definition)

定义变量

students = {
    "Hermoine": "Gryffindor", 
    "Harry": "Gryffindor", 
    "Ron": "Gryffindor",
    "Draco": "Slytherin",
}

what do I want to associate Hermoine with? Gryffindor.

keys on the left, values on the right（美观）

引用

想输出 Hermoine 的 house？

方法一

print(stu["Hermoine"])

name of the variable: stu

和 list 的数字下标不同，dict 使用 actual words as index，得到 value

方法二

for student in students:
    print(student)

这样会输出所有的 key

for student in students:
    print(student, students[student])

for student in students: 会使 student 取 "Hermione" 到 "Draco"

如果 student 的名字是 key，那 students[student] 会去取 student 的 value

多个value

a list of dictionaries, a list of 4 students, each of the students is a dictionary

students = [
    {"name": "Hermoine", "house": "Gryffindor", "patronus": "Otter"},
    {"name": "Harry", "house": "Gryffindor", "patronus": "Stag"},
    {"name": "Ron", "house": "Gryffindor", "patronus": "Jack Russell terrier"},
    {"name": "Draco", "house": "Slytherin", "patronus": None}
]

引用多个value

for student in students:
    print(student["name"],student["house"], student["patronus"])

student 是 dict 型变量

None

python关键字，the absence of a value