Python编程基础教程：初识Python语言

    int main() {
      int a = 1; // initialize variable a to 1
      int b = 2; // initialize variable b to 2
      int sum = a + b; // calculate the sum of a and b
      printf("The sum is %d", sum); // print the result to screen
      return 0; // indicate that program execution has completed successfully
    }

    (define (abs x)
    (if (< x 0)
        (- x)
        x))

    name = "Tom" # valid identifier name
    age_in_years = 30 # valid identifier name with digits in it
    number_of_employees = 500 # valid identifier name with underscore separating words
    message_from_Alice = "Hello, world!" # valid identifier name using an English phrase
    _salary = 12000 # valid identifier name starting with underscore for internal use only

    2num = 10 # invalid since first character cannot be digit or punctuation mark
    4age = 30 # invalid since keyword 'age' is used as an identifier
    my-name = "John" # invalid since hyphen '-' is not allowed in identifiers
    True = False # invalid because 'true' is a reserved word in Python
    for = 10 # invalid because 'for' is a reserved word in Python

    # this is a single line comment
    
    """ This is a 
    multiline comment."""
    
    '''This is also a 
    multiline comment.'''

    >>> print("hello, world")
    hello, world
    >>> print("hello,", "world")
    hello, world
    >>> print("the answer is", 42)
    the answer is 42

    >>> user_input = input("Please enter your name:")
    Please enter your name:John
    >>> print("Your name is:", user_input)
    Your name is: John

    a = 10   # integer
    b = 0xa  # hexadecimal (base 16)
    c = 0o21 # octal (base 8)
    d = 0b10 # binary (base 2)
    e = -30  # negative number

    f = 3.14          # floating point
    g = 6.02e23       # scientific notation
    h = -2.5e-3       # another way to write g
    i = 3.14j         # complex number (real part 3.14, imaginary part 0)

    j = 2+3j     # complex number
    k = 5*1J     # another way to create k
    l = 4.25e-6J # yet another way to create l

    m = int('123')    # convert string to integer
    n = float('-3.14') # convert float string to float
    o = str(20)        # convert integer to string
    p = complex(1, 2)  # convert two integers to a complex number
    q = bool(-2)       # convert nonzero value to True, otherwise False
    r = abs(-5)        # absolute value of a number
    s = divmod(7, 3)   # returns tuple containing quotient and remainder
    t = round(3.75)    # rounds off to nearest whole number
    u = ord('A')       # returns Unicode code point of given character
    v = chr(65)        # converts Unicode code point to corresponding character
    w = hex(255)       # returns hexadecimal representation of a number
    x = bin(255)       # returns binary representation of a number
    y = max(3, 5)      # returns maximum of two numbers
    z = min(3, 5)      # returns minimum of two numbers

    s = 'hello, world!' # string enclosed within single quotes
    t = "I'm learning Python." # string enclosed within double quotes
    u = r'\n'            # raw string notation (escapes backslash but does not interpret any other escape sequences)
    v = "The quick brown fox jumps over the lazy dog.\nA paragraph can have multiple lines."
    w = len(v)           # length of a string
    x = v[0]             # access first element of a string by index
    y = v[-1]            # access last element of a string by index
    z = v[:5]            # slice from start to position 5 (not included)
    aa = v[::2]          # step through the string every second character
    bb = v.upper()       # make all characters uppercase
    cc = v.lower()       # make all characters lowercase
    dd = v.strip()       # remove whitespace at beginning and end of string
    ee = v.replace('dog', 'cat') # replace one substring with another
    ff =''.join(['apple', 'banana']) # join strings with separator space
    gg = ','.join(['apple', 'banana']) # join strings with separator comma
    hh = v.split(' ')    # split the string into substrings based on delimiter

    colors = ['red', 'green', 'blue'] # list of three elements
    fruits = [['apple', 'orange'], ['grape']] # nested lists
    numbers = [1, 2, 3, 4, 5]
    squares = [num * num for num in numbers if num > 3] # squares greater than 3
    names = ["Alice", "Bob", "Charlie"]
    sorted_names = sorted(names)              # sort names alphabetically
    reversed_names = reversed(sorted_names)  # reverse order of sorted names

    coordinates = (3, 4)                   # tuple of two coordinates
    coordinates = 3, 4                     # alternate syntax for creating tuples
    numbers = (1, 2, 3, 4, 5)
    sum_squared = sum([num ** 2 for num in numbers]) # compute sum of squares of numbers

    person = {'name': 'John', 'age': 30, 'city': 'New York'} # dictionary of person details
    phonebook = {'Alice': '555-1234', 'Bob': '555-5678'}
    grades = {'Math': 85, 'Science': 90, 'English': 80}
    contacts = dict({'Alice': '555-1234'}, Bob='555-5678') # alternative syntax for creating dictionaries
    employee = dict(name="John Doe", age=30, city="New York") # key-value pairs separated by commas

    unique_numbers = set([1, 2, 3, 4, 4, 5, 5, 6]) # create unique set of numbers
    union = {1, 2, 3}.union({2, 3, 4})             # find union of sets
    intersection = {1, 2, 3}.intersection({2, 3, 4}) # find intersection of sets
    difference = {1, 2, 3}.difference({2, 3, 4})     # find difference between sets

    is_student = True                # boolean flag indicating student status
    status = ""                      # empty string means false
    has_income = None                # null value means false
    is_adult = age >= 18             # check age against threshold to determine adulthood
    can_vote = age >= 18 and income > 5000 # check age and income to determine eligibility for vote

    result = 2 + 3 # addition operator
    result = 2 - 3 # subtraction operator
    result = 2 * 3 # multiplication operator
    result = 2 / 3 # division operator
    result = 2 % 3 # modulo operator
    result = 2 // 3 # floor division operator
    result = 2 ** 3 # exponentiation operator
    result = matrix1 @ matrix2 # matrix multiplication operator

    result = 2 == 3 # equal to operator
    result = 2!= 3 # not equal to operator
    result = 2 < 3  # less than operator
    result = 2 <= 3 # less than or equal to operator
    result = 2 > 3  # greater than operator
    result = 2 >= 3 # greater than or equal to operator

    result = True and True    # logical AND operator
    result = True or False    # logical OR operator
    result = not True         # logical NOT operator

    result = 0b1010 & 0b1100 # bitwise AND operator
    result = 0b1010 | 0b1100 # bitwise OR operator
    result = 0b1010 ^ 0b1100 # bitwise XOR operator
    result = ~ 0b1010        # bitwise NOT operator
    result = 0b1010 << 2     # left shift operator
    result = 0b1010 >> 2     # right shift operator

    fruits = ['apple', 'orange', 'banana']
    result = 'apple' in fruits # check if apple is present in fruit list
    result = 'kiwi' not in fruits # check if kiwi is absent from fruit list

    a = 10   # allocate memory for new object with value 10
    b = 10   # assign same address to b as a initially refers to
    result = a is b # test whether a and b refer to the same object
    result = a is not b # test whether a and b do not refer to the same object

    a = 10
    b = 20
    if a < b:
       print("a is smaller than b")
    elif a == b:
       print("a is equal to b")
    else:
       print("a is larger than b")

    fruits = ['apple', 'banana', 'orange']
    for fruit in fruits:
    print(fruit)
    
    for i in range(1, 10):
    print(i)
    
    s = "Hello World"
    for char in s:
    print(char)

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

    try:
    age = int(input("Enter your age:"))
    if age < 18:
        raise ValueError("You are too young!")
    elif age > 100:
        raise ValueError("Age out of bounds!")
    except ValueError as error:
    print(error)

    with open('filename.txt') as file:
    contents = file.read()
    process_contents(contents)
    
    data = read_data_from_database()
    save_to_file(data)