1. 控制语句

• 24.8.28 10:39 created

1.1. if

• Comparison Operator: <, <=, > and so on
• Logical Operator: not, in, and, or
• Truthy and Falsy Values: [] and so on

if True:
    print("This is so true!!")
else:
    print("This is false.")

age = 15
if age < 8:
    print("Movie is free for you!!")
elif 8 <= age < 65:
    print("You need to pay $300!")
else
    print("You only need to pay $150!")

if 5 > 3 and []:  # empty collection is falsy
    print("the condition is true")
else:
    print("the condition is false")

k = True
# if k == True:
if k:
    print("Variable k is true")
else:
    print("Variable k is false")

example:

# program asks user`s name
# cash
# Y/N
# program checks if the user as more than or equal to $30
name = input("Enter your name: ")  # excute `python ./try.py` in terminal
money = input("Enter your cash amount: ")
hungry = input("Are you hungry? Y/N")

if hungry == "Y":
    if int(money) >= 30:
        print(f"{name} should go eat breakfast.")
    else:
        print(f"{name} is hungry but might not have enough money to buy breakfast!")
elif hungry == "N":
    if int(money) >= 30:
        print(f"{name} has budget but doesn`t want to eat breakfast.")
    else:
        print(f"{name} has no money but is not hungry...")
else:
    print("Please make sure that you enter either Y or N.")

如果VSCode中的terminal还是2的版本，需要修改为3的版本：https://blog.csdn.net/weixin_43659913/article/details/103830210

1.2. switch

python 3.10 news：https://docs.python.org/3.10/whatsnew/3.10.html

match subject:
    case <pattern_1>:
        <action_1>
    case <pattern_2>:
        <action_2>
    case <pattern_3>:
        <action_3>
    case _:
        <action_wildcard>

example:

lang = input("你希望学什么程式语言？")
match lang:
    case "JavaScript":
        print("你会成为网页前端开发人员")
    case "PHP":
        print("你会成为网页后端开发人员")
    case "Python":
        print("你会成为资料科学家")
    case "Kotlin":
        print("你会成为Android应用开发人员")
    case "Swift":
        print("你会成为iOS应用开发人员")
    case _:
        print("你会成为其他开发人员")

example2:

day = input("今天星期几？")

match day:
    case "星期日" | "星期一":
        print("今日公休")
    case "星期六":
        print("今日营业半天")
    case _:
        print("今日正常营业")

example3:

command = input("Where do your wanna go?")
print(command.split(" "))
match command.split(" "):
    case ["Go", "home"]:
        print("You wanna go home")
    case _:
        prnt("The system cannot determine where you wanna go.")

example4:

name = input("Enter your name: ")  # excute `python ./try.py` in terminal
money = input("Enter your cash amount: ")
hungry = input("Are you hungry? Y/N")

match hungry:
    case "Y":
        if int(money) >= 30:
            print(f"{name} should go eat breakfast.")
        else:
            print(
                f"{name} is hungry but might not have enough money to buy breakfast!")
    case "N":
        if int(money) >= 30:
            print(f"{name} has budget but doesn`t want to eat breakfast.")
        else:
            print(f"{name} has no money but is not hungry...")
    case _:
        print("Please make sure that you enter either Y or N.")

1.3. for and while loop

for variable in iterable:
    do someting here

example:

for letter in "Hello World":
    if (letter == letter.upper()):
        print(letter)

example2:

myList = [1, 3, 5, 7, 9]
for num in myList:
    print(num)

example3:

for a, b in [(1, 2), (3, 5), (5, 7)]:
    print(a + b)

example4:

myDictionary = {"name": "Wilson", "age": 25}
for item in myDictionary:
    # name
    # age
    print(item)

example5:

myDictionary = {"name": "Wilson", "age": 25}
for key, value in myDictionary.items():
    print(f"The key is {key}")
    print(f"The value is {value}")

while loop example:

x = 0
while x < 5:
    print(x)
    x += 1

nested loop example:

counter = 0
for i in "1234":
    for j in "abcdefg":
        print(i, j)
        counter += 1
print(f"counter is {counter}")

pass example:

for i in "How are you":
    # 解决语法上的问题
    pass

if、function都可以使用pass

break example:

print("Before the for loop")
for i in "123456789":
    if i == "5":
        break
    else:
        print(i)
print("After the for loop")

break in nested loop example:

for i in "123456789":
    for j in "abcdefg":
        if j == "c":
            break
        print(i, j)

If the break statement is present in the nested loop, it terminates only those loops containing the break statement.

continue example:

for i in "abcd":
    if i == "a":
        continue
    print(i)

1.4. range function:

range(start, stop, step)

• start，可选，默认0
• stop，必须，不包含stop位置的值
• step，可选，默认1

example:

for i in range(5):
    # 0 1 2 3 4
    print(i)

example2:

for i in range(10, 15):
    # 10 11 12 13 14
    print(i)

example3:

for i in range(0, 100, 10):
    # 0 10 20 30 40 50 60 70 80 90
    print(i)

example4:

for i in range(20, 15, -1):
    # 20 19 18 17 16
    print(i)

typecasting to list:

# 不建议这么使用，range是为了节省空间，放进list就会马上占用空间
myList = list(range(0, 15, 2))
print(myList)

1.5. enumerate

enumerate example:

for counter, char in enumerate("How are you today?"):
    if counter < 10:
        print(char)

1.6. zip

x = [1, 2, 3]
y = ['A', 'B', 'C']
z = ['a', 'b', 'c', 'c']
for tuple in zip(x, y, z):
    # (1, 'A', 'a')
    # (2, 'B', 'b')
    # (3, 'C', 'c')
    print(tuple)

1.7. comprehensions

no use comprehensions:

x = [1, 2, 3, 4]
squared_x = []
for item in x:
    squared_x.append(item ** 2)
# [1, 4, 9, 16]
print(squared_x)

1.7.1. list comprehensions

new_list = [operation for variable in original_list if condition]

example:

x = [1, 2, 3, 4]
squared_x = [item ** 2 for item in x if item > 2]
print(squared_x)

1.7.2. dictionary comprehensions

new_dict = {key: value(operatin) for variable in original_dict if condition}

example:

x = [1, 2, 3, 4]
x_squared_dict = {item: item ** 2 for item in x if item > 2}
# {3: 9, 4: 16}
print(x_squared_dict)

1.7.3. set comprehensions

{operation for variable in original_set if condition}

example:

x = [1, 2, 3, 4]
x_squared_set = {item ** 2 for item in x if item > 2}
print(x_squared_set)

1.7.4. generator

x = [1, 2, 3, 4]
x_squared_generator = (item ** 2 for item in x)
for i in x_squared_generator:
    print(i)

1.8. word count

准备一个myfile.txt，根py文件同一个目录：

roses are red
sky is blue
syntax error in line 32

实现wc myfile.txt功能：

2      11      49 myfile.txt

准备try.py文件：

from sys import argv

if len(argv) < 2:
    print("Please provide a filename")
else:
    file = open(argv[1])  # argv[0] is try.py
    lines = file.read()

    lines = lines.split("\n")
    word_count = 0
    letter_count = 0

    for line in lines:
        words = line.split(" ")
        word_count += len(words)
        letter_count += len(line)

    # ['roses are red', 'sky is blue', 'syntax error in line 32']
    print(lines)
    line_count = len(lines)
    # The line count is 2
    print(f"The line count is {line_count}")
    # The word count is 11
    print(f"The word count is {word_count}")
    # The letter count is 47
    print(f"The letter count is {letter_count}")

执行try.py:

python try.py myfile.txt

• 24.8.28 16:21 updated