intro to sets (slides)

CSc 110 - Intro to Sets

Data Structures in Python

• lists (mutable)
• dictionaries (mutable)
• tuples (immutable)
• sets (mutable)

Lists

• store:
  • ordered items (it keeps the original order)
  • elements of different types
  • repeated elements
• use square brackets ([]) syntax for creating a list
• allow indexing (integers starting at 0) with square brackets ([]) as well
• are mutable

Tuples

• store:
  • ordered items (it keeps the original order)
  • elements of different types
  • repeated elements
• use parentheses (()) syntax for creating a tuple
• allow indexing (integers starting at 0) with square brackets ([])
• are immutable

Dictionaries

• store pairs of items key: value
• keys have to be unique (no repeated keys allowed)
• use curly brackets ({}) with key and value separated by colon (:)
• allow value retrieval through key with square brackets ([])
• are mutable

Sets

• store:

  • do NOT store items in the order you created them
  • CANNOT use square brackets to retrieve an item
  • do NOT allow repeated items to be stored

• use set() to initiate an empty set

• use curly brackets ({}) for creating a set with values

• use for x in set to iterate over the elements in a set

Set methods

• .add(value) adds an element to the set

• .discard(value) discards the specified value

• You can also use in and len() with sets

Converting data structure

• set() to convert a list/tuple into a set

print(set([3, 2, 2, 1]), set((3, 2, 2, 1)))

{1, 2, 3} {1, 2, 3}

• list() to convert a set/tuple into a list

print(list({3, 2, 2, 1}), list((3, 2, 2, 1)))

[1, 2, 3] [3, 2, 2, 1]

• tuple() to convert a list/set into a tuple

print(tuple({3, 2, 2, 1}), tuple([3, 2, 2, 1]))

(1, 2, 3) (3, 2, 2, 1)

Evaluate the code

numbers_list = [2, 1, 2, 1, 3, 4, 1]
numbers_list # evaluate this line

numbers_set = set(numbers_list)
numbers_set # evaluate this line

numbers_set.add(1)
numbers_set.add(2)
numbers_set # evaluate this line

numbers_set.add(5)
numbers_set # evaluate this line

numbers_set.discard(6)
numbers_set.discard(1)
numbers_set # evaluate this line

Evaluate the code

numbers_list = [2, 1, 2, 1, 3, 4, 1]
numbers_list # evaluate this line

[2, 1, 2, 1, 3, 4, 1]

numbers_set = set(numbers_list)
numbers_set # evaluate this line

{1, 2, 3, 4}

numbers_set.add(1)
numbers_set.add(2)
numbers_set # evaluate this line

{1, 2, 3, 4}

numbers_set.add(5)
numbers_set # evaluate this line

{1, 2, 3, 4, 5}

numbers_set.discard(6)
numbers_set.discard(1)
numbers_set # evaluate this line

{2, 3, 4, 5}

Write a function

1. Its name is count_names
2. It opens in read mode a file that contains a name per line, with repeated names
3. It counts how many unique names there are in the file
4. It returns an integer with the count
5. Use a set for this

assert count_names("names.txt") == 11

Write a function – solution

def count_names(file_name):
  f = open(file_name, "r")
  name_set = set()
  for line in f:
    name = line.strip()
    if name != "":
      name_set.add(name)
  f.close()
  return len(name_set)

def main():
  assert count_names("names.txt") == 11
  
main()

Write a function

1. Its name is has_duplicates
2. It takes a variable number of arguments: *values
3. It returns True if there are repeated elements in values, False otherwise

assert has_duplicates(3, 2, 1, 2, 3) == True
assert has_duplicates(3, 20, 1, 2, 13) == False

Write a function – solution 1

def has_duplicates(*values):
  values_set = set(values)
  return len(values) > len(values_set)

def main():
  assert has_duplicates(3, 2, 1, 2, 3) == True
  assert has_duplicates(3, 20, 1, 2, 13) == False
  
main()

Write a function – solution 2

def has_duplicates(*values):
  unique_values = set()
  for v in values:
    if v not in unique_values:
      unique_values.add(v)
    else:
      return True
  return False

def main():
  assert has_duplicates(3, 2, 1, 2, 3) == True
  assert has_duplicates(3, 20, 1, 2, 13) == False
  
main()

Quiz 10

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You have 10 minutes to complete the quiz.