5. sets (python).md

SETS

• like dictionaries, but with keys only (no values)
• properties: unordered, iterable, mutable, can contain multiple data types
• made up of unique elements (strings, numbers, or tuples)

create an empty set

empty_set = set()

create a set

languages = {'python', 'r', 'java'}         # create a set directly
snakes = set(['cobra', 'viper', 'python'])  # create a set from a list

examine a set

len(languages)              # returns 3
'python' in languages       # returns True

set operations

languages & snakes          # returns intersection: {'python'}
languages | snakes          # returns union: {'cobra', 'r', 'java', 'viper', 'python'}
languages - snakes          # returns set difference: {'r', 'java'}
snakes - languages          # returns set difference: {'cobra', 'viper'}

modify a set (does not return the set)

languages.add('sql')        # add a new element
languages.add('r')          # try to add an existing element (ignored, no error)
languages.remove('java')    # remove an element
languages.remove('c')       # try to remove a non-existing element (throws an error)
languages.discard('c')      # removes an element if present, but ignored otherwise
languages.pop()             # removes and returns an arbitrary element
languages.clear()           # removes all elements
languages.update('go', 'spark') # add multiple elements (can also pass a list or set)

get a sorted list of unique elements from a list

sorted(set([9, 0, 2, 1, 0]))    # returns [0, 1, 2, 9]

set comprehension

fruits = ['apple', 'banana', 'cherry']
unique_lengths = {len(fruit) for fruit in fruits}   # {5, 6}