List Comprehension

Happy Week 9!

Quick Announcements

• Project Breakout is due today at 10:00pm
  • Update on Breakout Project downloaded starter files with no run_journal.py
    • Check the Discord announcement to download the file and include in your project.
• Problem set 5 will be published today and due next week Monday.

Common Useful List Methods

List functions on Iterators

• The .sort and .reverse methods reorder the list in place and do not return anything
• Commonly, you might want to loop through a list in a particular order, but not change the original list
• Python gives you two matching functions to do this, which return a new ordered version of the list, without changing the original
  • The reversed() function creates a new iterable object that returns its elements in the opposite order
  • The sorted() function creates a new iterable object that returns its elements in ascending order

Deleting elements from list

• Deleting an element from a list is possible with del statement
• However, in looping through the list:
  • attempt to delete an element while looping will result to IndexError
• One technique is to create a new list to store all desired element when looping

list1 =[2,4,6,5,3]
del list1[3]
print(list1)

list1 = [2,4,6,5,3]
list2 =[] #to store ele
for i in range(len(list1)):
      if list1[i]%2==0: #remove odd
            list2.append(list1[i])
            
print(list1)
print(list2)

List building with loops

• Commonly will make lists with a simple:

  even_digits = [ 2, 4, 6, 8 ]

• But in many cases, it is easier to specify the elements of a list using a sequence of values generated by a for loop. For instance

  even_digits = [ ]
  for i in range(0, 10, 2):
      even_digits.append(i)

• Python gives us a shorthand notation to achieve this:

  even_digits = [ i for i in range(0, 10, 2) ]

  • Called list comprehension

Comprehending Lists

• The simplest list comprehension syntax is:

  [ expression iterator ]

  where expression is any Python expression and iterator is a for loop header

• The iterator component can be followed by any number of additional modifiers

  • More for loop headers for nested loops
  • or if statements to select specific values

• Example: all even numbers to 20 not also visible by 3

  [i for i in range(0,20,2) if i % 3 != 0]

Group Problems LO’s

• Correctly trace code with list methods and mutability considerations
• Work with common list methods
• Practice using lists to tabulate values

Problem 1: Tracing Clues

A = [
     'Fox',
     'Giraffe', 
     'Hippo'
    ]
A.append('Iguana')
A[:].reverse()
B = A
for anim in B:
    if anim[1] == 'i':
        B.remove(anim)
print(A)

Problem 2a: Some Cleaning

• This is a coding problem. Work in pairs or trios on a single computer
  • Discuss your algorithm BEFORE YOU WRITE ANY CODE. Make sure everyone understands.
• The file raw_data.py contains the constant DATA, which is a large list of strings
• Many of those strings are numbers, but some are not
• Your first task is to use a loop and append to create a list of only the entries that are numbers
• Your result should be a list of numbers, so you’ll need to convert them from strings

Problem 2b: Cleaning Comprehension

• Rewrite your above code to generate the exact same list, but using a list comprehension
• Should be able to compare the lists you got from each method, and they should have identical contents

Problem 2c: Purging

• Both the previous approaches created a new list in memory with the desired traits
• What if we wanted to prune the “bad” values out of the original list?
• Write an approach that would accomplish this, ensuring that the list id before your code and after you code is still the same!

A Tabulation Aside

• Lists can also be useful when you have a collection of values and you need to count how many values fall into different “bins”
  • This is commonly called tabulation
• You could accomplish this with multiple counting variables, but it is unreasonable to define and track 50+ variables
  • With a list, the number at each index represents a count of something
  • Instead of creating 50+ variables, you just have a list of 50+ counts
• Useful if you can equate bins to indexes directly, but sometimes you just need to define what index corresponds to what category

Problem 2d: Tabulation

• The cleaned data list contains number values ranging from 0 to 25
• Use tabulation to count how many times each number appears in the array
• What are the top three more frequently occurring numbers?