CS50x 2024 - Week 6 Python

2024-11-18 14:48:51   2024-11-18 14:48:51
Categories CS50x 2024
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1.1k Words   |   6 Minutes

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时隔一个月重拾 CS50x 的学习啦！其实我觉得记语法的笔记意义不是特别大，所以这里就偷懒了2333，还是多练习比较管用。

ALL THE COURSES TAKE IN CS50X NOW ARE USING PYTHON 3.

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Compare with C

• There is no POINTERS or char * in Python.

• Automatically adjust the data types so you needn’t to initialize a variable.

• '' "" would be exactly the same. You can just kind of alternate back and forth between using single quotes on the outside, double quotes on the inside.

• Use or and instead of || && .

• elif

• Comment: # or ''' """

• i++ is unavailable.

• Indentation is important while C didn’t care it (From left to right).

• Access the string: text[i] .

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Dot Notation

strip()

text = input("...")
text.strip()

• Take out beginning and trailing whitespace characters.

lower() upper() capitalize()

text.lower()      # All to lowercase.
text.capitalize() # First character only to uppercase while others to lowercase.

• Remember to reassign: text = text.lower() .

Object-Oriented Programming

• str itself is a object that has its own attributes and methods.

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Loops

for

• 遍历 str

  for c in text:
      print(c)

• 遍历 list

  words = text.split() // 拆分为单词，获得一个列表。
  for word in words:
      print(word)

• 步长

  for x in range(0, 100, 2)
  # From 0 to 99 step by 2.

• Iterable

• 思考一下当需要隔行遍历的时候， for i in range(n) 和 for (int i = 0; i < n; i++) 区别在哪？

  # sentimental-credit/credit.py
  
  from cs50 import get_string
  
  n = get_string("Number: ")
  
  sum = 0
  
  """
  for i in range(len(n)):
      i += 1
      digit = int(n[len(n) - 1 - i]) * 2
      if digit > 9:
          digit = 1 + digit - 10
      sum += digit
  
  for i in range(len(n)):
      sum += int(n[len(n) - 1 - i])
      i += 1
  """
  # Think why it doesn't work?
  
  for i in range(len(n)):
      digit = int(n[len(n) - 1 - i])
      if i % 2 == 1:
          digit *= 2
          if digit > 9:
              digit = 1 + digit - 10
          sum += digit
      else:
          sum += int(n[len(n) - 1 - i])
  
  if sum % 10 == 0:
      if n[0] == "5" and (n[1] == "1" or n[1] == "2" or n[1] == "3" or n[1] == "4" or n[1] == "5") and len(n) == 16:
          print("MASTERCARD")
      elif n[0] == "4" and (len(n) == 13 or len(n) == 16):
          print("VISA")
      elif n[0] == "3" and (n[1] == "4" or n[1] == "7") and len(n) == 15:
          print("AMEX")
      else:
          print("INVALID")
  else:
      print("INVALID")

• [2:4]

  for word in text.split()[2:4]: # 列表第2-3个的元素，第4个不包含。
      print(word)

while

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print()

print(text, end="")

• end set \n as default.

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Conditionals if

if "g" in word:
    print(word)

• The Ternary Operator

  letters_only = True if input().isalpha() else False

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Dictionaries

• Python also has built in support for dictionaries, allowing you to specify list indices with words or phrases (keys), instead of integers, which you were restricted to in C.

  pizzas = {
      "cheese": 9,
      "pepperoni": 10,
      "vegetable": 11,
      "buffalo chicken": 12
  }

• Key / Value

  book = dict()
  book["title"] = "Corduroy"
  book["author"] = "Don Freeman"
  print(book["title"])
  
  # What if...
  '''
  print(book["Corduroy"])
  '''
  # KeyError: 'Corduroy'
  
  book = {
      "title": "Corduroy"
      "author": "Don Freeman"
  }

• Iterate

  for pie in pizzas:
      # use pie in here as a stand-in for "i"
  
  for pie, price in pizzas.items():
  # Convert the dict into a list but the order would be disturbed.
      print("A whole {} pizza costs ${}".format(pie, price))
      print("A whole " + pie + " pizza costs $" + str(price))

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Lists

• Not fixed in size.

  book = [{"title": "Goodnight Moon", ...}, 
          {"title": "Corduroy", ...},
          {"title": "Curious George", ...}]

• 实现：

  books = []
  # or books = list()
  
  for i in range(3):
      book = dict()
      book["author"] = input("Enter an author: ")
      book["title"] = input("Enter a title: ")
      books.append(book)
  
  for book in books:
      print(f"{book['author']} wrote {book['title']}.")
  
  # {Key: Value, ...}
  # {Key: Value, ...}
  # {Key: Value, ...}

• 只返回 Keys

  print(book.keys())
  # dict_keys(['author', 'title'])

Explicitly Created List

nums = [x for x in range(500)]

append() insert()

nums.append(5)

nums.insert(4, 5)
# In the forth position insert 5.

nums[len(nums):] = [5]

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Tuples aka 元组

• Tupless are ordered, immutable sets of data; they are great for associating collections of data, sort of like a struct in C, but where those values are unlikely to change.

  presidents = [
      ("George Washington", 1789),
      ("John Adams", 1797),
      ("Thomas Jefferson", 1801),
      ("James Madison", 1809)
  ]

• Iterable

  for prez, year in presidents:
      print("In {1}, {0} took office".format(prez, year))

  In 1789, George Washington took office
  In 1797, John Adams took office
  In 1801, Thomas Jefferson took office
  In 1809, James Madison took office

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Libraries and Modules

.csv

• 全部导入

  import csv
  csv.DictReader(...)

• 导入特定函数

  from csv import DictReader
  DictReader(...)

File Reading and Writing

with open(FILENAME) as file:
# with open(FILENAME, "r") as file: different modes.
    text = file.read() # Now `text` is a str including all the content of `FILENAME`.
    file_reader = csv.DictReader(file) # Now `file_reader` is a ITERATOR.

    for row in file_reader: # Now `row` is a dict.
        ...

• with would automatically close the file outside of the sentence.

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Functions

• If you wish to define main nonetheless (and you might want to!), you must at the very end of your code have:

  if __name__ == "__main__":
      main()

def

def square(x):
    return x ** 2
    # x ^ 2

print(square(5))

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Objects

~~function(object);~~
object.method()

class

class Student():
    def __init__(self, name, id):
        self.name = name
        self.id = id
    def changeID(self, id):
        self.id = id
    def print(self):
        print("{} – {}".format(self.name, self.id))

    jane = Student("Jane", 10)
    jane.print()
    jane.changeID(11)
    jane.print()

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Including Files

import cs50

cs50.get_int()
cs50.get_float()
cs50.get_string()

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…Wired?

• You can also make your programs look a lot more like C programs when they execute by adding a shebang to the top of your Python files, which automatically finds and executes the interpreter for you.

  #!/usr/bin/env python3

• If you do this, you need to change the permissions on your file as well using the Linux command chmod as follows:

  chmod a+x file

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