12_data_structures.md

Data Structures

• Dictionaries
• Sets
• Tuples

Dictionaries

• Dictionaries contain key/value pairs in a mutable, unordered collection.
• Keys must be immutable and unique.
• Dictionaries are declared:
  • Using curly braces {}
  • dict() constructor
• Syntax: {key1 : value1, key2: value2, key3 : value3}

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Accessing and Updating Key Value Pairs

• Access values by using the key as an index:
  • d[key]
  • This statement will throw an error if the key is not in the dictionary.
• Keys can be added to dictionaries by:
  • d[new_key] = new_value
  • Note that if the key already exists in the dictionary, this syntax will reassign the value to this new_value

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Dictionary Methods

• The keys method will return a list of the keys in a dictionary
  • dct.keys()
• The values method will return a list of the values in a dictionary
  • dct.values()
• The items method will return a list of tuples that contain the key/ value pairs
  • dct.items()

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Membership

• Membership for dictionaries
  • k in dct
  • k in dct.keys()
  • v in dct.values()

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Traversing a Dictionary

• Creating a for loop allows us to iterate through a dictionary’s keys

for key in dictionary:
    print(key)

• Combining a for loop with the items method allows us to iterate through both the keys and the values of a dictionary:

for key, value in dictionary.items():
    print(key, value)

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Deleting Key/Value Pairs

• In order to remove key/value pairs, you can use the pop method or the del keyword
  • d.pop('some_key')
  • del d['some_key']
• Create a copy of the original dictionary before changing it by adding and deleting key/value pairs
  • dictionary.copy()

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.get()

• To check and see if a specific key is in a dictionary, you can use the get method
  • Syntax: dct.get(key, default=None)

states_caps_dict = {'Georgia': 'Atlanta', 'Colorado': 'Denver', 'Indiana': 'Indianapolis'}
d.get(‘Washington’, ‘Capital not found’)

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Copying a dictionary

To make a copy of a dictionary you need to use the .copy() method

• Syntax: dct.copy()
• Err on the side of making copies of dictionaries

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BREAKOUT (5 minutes)

• Make a dictionary called restaurant_types that has the following associated key-value pairs:

keys (restaurant name)	values (restaurant type)
Red Lobster	Seafood
Burger King	Fast Food
Safeway	Grocery Store

• How do you find the restaurant type for 'Burger King'?
• What if you don't know whether or not 'Outback Steakhouse' is in the restaurant_types dictionary - how would you go about trying to get it's restaurant type and make sure that you won't get an error?
• Add to it the key-value pair, ('Outback Steakhouse', 'Delicious!').
• What if we want to change the restaurant type of 'Safeway' to just 'Grocery' - how would you make that change?

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BREAKOUT SOLUTION

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BREAKOUT (5 MINUTES)

Write a function that creates a dictionary where the keys are all the positive integers that are less than 20 and the values are the cube of the integer.

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BREAKOUT SOLUTION

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The set() datatype

• Sets are unordered, mutable collections
• Sets will only contain unique elements
• Sets can be declared:
  • Using the set constructor set()
• Be careful when declaring empty sets. {} defaults to dictionaries.

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Removing Duplicates

• Sets only hold unique elements.
  • This property is useful for removing duplicates from lists and tuples
  • Do this by casting the list or tuple to a set

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Union and Intersection

• The union and intersection methods in sets are similar to their mathematical analogues.
• The union is a set of all elements in two sets
  • Syntax: set1.union(set2)
• The intersection is a set of all the elements that two sets have in common Syntax: set1.intersection(set2)

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Set Difference

• The difference of two sets A - B contains all the elements of set A that are not contained in set B
• Syntax: set1.difference(set2)
• set1.difference(set2) is different from set2.difference(set1)

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BREAKOUT (3 minutes)

l1 = [1, 4, 7, 0, 2, 5, 8]
l2 = [1, 2, 3, 4, 9]

• What does the intersection of these two lists return?
• What does the the union of these two lists return?
• What does the difference of l1 - l2 return?
• What about l2 - l1?

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BREAKOUT SOLUTION

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The tuple() datatype

• Tuples are ordered collections
• Tuples are very similar to list with two key differences:
  • Tuples are immutable.
  • Tuples are declared using parenthesis.
• We can index and slice tuples because they are ordered
• Tuples have two methods associated with them: count and index

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Declaring Tuples

• Tuples can be declared in three ways:
  • tup = (1, 2, 3)
  • tup = tuple([1, 2, 3])
  • tup = 1, 2, 3
• For single elements tuples:
  • tup = (1,)
  • tup = tuple([1])
  • tup = 1,

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Tuple Immutability

• Once a tuple is declared, it generally can’t be changed in anyway
  • However, if an element of a tuple is mutable, the element can be changed
• Tuples hold references to all the objects they contain, rather than the objects themselves.

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BREAKOUT (4 minutes)

Write a function that has two arguments that are both tuples. Return a single tuple that is the combination of the two original tuples that skips every other element in reverse.

• Say tuple1 = (12, 14, 16, 18) and tuple2 = (3, 5, 7, 9):
• The result would be (9, 5, 18, 14)

def function_name(tuple1, tuple2):
   pass

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BREAKOUT SOLUTION