Nov 05: Prac5, Lists of Lists

Learning Objectives

After today's class, you should be able to:

• Write code that reads or writes a file using a with statement.
• Explain the syntax and meaning of a list/set/dict comprehension.

Announcements

• Code: PA 2
  • Part A due TODAY!
  • Part B due Tue, Nov 11
• Read Week 12 – ideally:
  • 1st half before Friday
  • 2nd half before Monday

File I/O Debrief

[5 min]

Refer to Friday's activity

• Question 3: Write a file

  with open("lines.txt", "w") as file:
      for i in range(1, 101):
          file.write(f"Line #{i}\n")
  

• Question 14: Read a file

  with open("names.txt") as file:
      for line in file:
          words = line.split()
          print(words[1])
  

Comprehensions

[10 min]

A concise way to build a new list, set, or dictionary

List comprehension:

new_list = [expression for item in iterable if condition]

Set comprehension:

new_set = {expression for item in iterable if condition}

Dict comprehension:

new_dict = {key_exp: value_exp for item in iterable if condition}

Example 1

• Building a simple list:

  squares = []
  for x in range(10):
      squares.append(x ** 2)
  

• Can be rewritten as:

  squares = [x ** 2 for x in range(10)]
  

Example 2

• Adding an if statement:

  squares = []
  for x in range(10):
      if x % 2 == 0:
          squares.append(x ** 2)
  

• Can be rewritten as:

  [x ** 2 for x in range(10) if x % 2 == 0]
  

Example 3

• Practice Quiz 4:

  def limit_letters(counts: dict, limit: int) -> set:
      """Get letters that don't exceed a count limit."""
  
      return {letter for letter in counts if counts[letter] <= limit}
  

• Actual Quiz 4:

  def big_cats(data: dict, heavy: int) -> list:
      """Get cats whose weight is greater than heavy."""
  
      return [cat for cat in data if data[cat] > heavy]
  

Lists of Lists

[10 min]

Data types from PA 2:

• A "cell" is a list of strings:

  Cell = list[str]
  

• A "grid" is a list of cells:

  Grid = list[Cell]
  

In other words, a grid is a list of list of strings.

• Standard type hint: list[list[str]]
• See test_read_file() in test_file_utils.py

2D lists have two indexes:

• grid[n][0] means "letter of n^th cell"
• grid[n][1] means "color of n^th cell"

Practice Quiz 5

[25 min]

Topics: Strings (PA1-B) and File I/O (PA2-A)

• 1st sheet: "written potion"
  • Turn off monitor during written portion
• 2nd sheet: "coding potion"
  • You must use Thonny (not VS Code)
  • Do not turn off recording; just log out