Python lists extended

combining two lists

• can use Python's built-in zip function
  • it will take 0 or more iterables and return a new list of combined tuples
  • example:

    movies = ['Interstellar', 'Inception', 'The Prestige', 'The Dark Knight', 'Batman Begins']
    ratings = [1, 10, 10, 8, 6]
    new_list = []
    for tree in zip(movies, ratings)
        new_list.append(tree)
    print(new_list)

    prints [('Interstellar', 1), ('Inception', 10), ('The Prestige', 10), ('The Dark Knight', 8), ('Batman Begins', 6)]
  • only works when there are the same number of elements in both lists
    • it will only return a list where there are elements in both lists and throw away the rest
      • example: if the ratings list above only had the first 3 entries the zip function would have returned [('Interstellar', 1), ('Inception', 10), ('The Prestige', 10)]

list comprehensions

• allows you to create lists based on criteria applied to existing lists
• general syntax: new_list = [expression(i)] for i in input_list if filter(i)]
  • expression(i)] for i is based on the variable used for each variable in the input string
    • example: c for c - use each item in the list
    • example: price * 3 for price - multiply each item in the list by 3
    • example: word[0] for word - use first letter of each word
  • in input_list specifies the input string or list
  • if filter(i) allows you to add a conditional statement to filter out list items that match specified criteria
    • example: if c.isdigit()
    • example: if n%2 == 0
• example to find last digit in a string:

  input_string = 'Buy 1 get 2 free'
  new_list = [c for c in input_string]  # list comp says to loop through each char in string to create a new list
  print(new_list)

  prints ['B', 'u', 'y', ' ', '1', ' ', 'g', 'e', 't', ' ', '2', ' ', 'f', 'r', 'e', 'e']

  input_string = 'Buy 1 get 2 free'
  new_list = [c for c in input_string if c.digit()]  # adding the if clause only lets digits be added to the list
  print(new_list)
  

  prints ['1', '2']

  input_string = 'Buy 1 get 2 free'
  new_list = [c for c in input_string if c.digit()][-1]  # tack on a negative index to select only the last digit
  print(new_list)
  

  prints 2

find the intersection of two lists

• intersection is the elements that are common to both lists
• example: this week's winning lottery numbers: 2, 43, 48, 62, 64, 28, 3 most common winning numbers: 1, 28, 42, 70, 2, 10, 62, 31, 4, 14

  def return_intersection(list1, list2):
    list3 = [value for value in list1 if value in list2]
    return list3
  
  list1 = [2, 43, 48, 62, 64, 28, 3]
  list2 = [1, 28, 42, 70, 2, 10, 62, 31, 4, 14]
  print(return_intersection(list1, list2))

  prints [2, 62, 28]
• can also use Python's built-in intersection()
  • works for sets not for lists
    • can change lists into sets like set1 = set(list1)
  • syntax: set1.intersection(set2)
  • convert back into list using list function: list(set1.intersection(set2))
  • reworking of above example:

    def return_intersection(list1, list2):
      set1 = set(list1)
      set2 = set(list2)
      return list(set1.intersection(list2))
    
    list1 = [2, 43, 48, 62, 64, 28, 3]
    list2 = [1, 28, 42, 70, 2, 10, 62, 31, 4, 14]
    new_list = return_intersection(list1, list2)
    print(new_list)

    prints [2, 28, 64]
  • can work with more than 2 sets
    • set1.intersection(set2, set3, set4), etc