Dictionaries

Calvin (Deutschbein)

W11Mon: 04 Nov

Announcements

• Project 3 tonight.
• Midterm Friday.
  • Practice Exam posted.
  • Tuples
  • Pixels
  • Lists
  • Types

Today

• Dictionaries
  • Dictionaries and Maps
  • Editorializing
  • Keys and Values
  • In and Indices
  • Iteration
  • Methods

Maps and Dictionaries

• A common form of information associates pairs of data values
  • Commonly called a map in computer science
  • Python calls such a structure a dictionary
  • I learned them as "hash tables"

Maps and Dictionaries

• A dictionary associates two different values:
  • A simple value called the key, which is often a string but doesn’t need to be
  • A larger and more complex object called the value

Maps and Dictionaries

• This idea of associating pairs of values is exhibited all over in the real world
  • Actual dictionaries! The words are the keys, the definitions the values.
  • Web addresses! Keys are the urls, the values are the webpage contents.
  • Humans! Keys are names, values are the actual people.

Creating Dictionaries

• Python dictionaries use squiggly brackets {} to enclose their contents

• Can create an empty dictionary by providing no key-value pairs:

  d = {}

• Or via "dict()" (like list(), int(), etc)

  d = dict()
• Separate keys from values with colons, separate key-value pairs from pairs with commas. skewlz = {'Willamette': "Bearcat", "Carolina": "Tarheel", "Chicago:": "Phoenix"}

Creating Dictionaries

• We look up values using a key: >>> skewlz = {'Willamette': "Bearcat", "Carolina": "Tarheel", "Chicago:": "Phoenix"} >>> skewlz["Carolina"] 'Tarheel' >>> skewlz["Willamette"] 'Bearcat'

Today

• Dictionaries
  • ✓ Dictionaries and Maps
  • Editorializing
  • Keys and Values
  • In and Indices
  • Iteration
  • Methods

Editorializing

• Known class-hater Calvin Deutschbein approves of maps.
  • "Map" is a functional idea, the not-class style of things
  • Google, and latter other cloud provides, use maps heavily.
    • That is, in 2024, I expect the preponderance of computing to take place within a map framework.
  • Maps seems very close to human reasoning in my impression.
  • Consider - have you used a map? A dictionary?
  • How a an object instantied from some class?

Editorializing

• My brain works in maps.
  • When I met you in the early weeks of class, I was learning keys.
    • I learned to spell your name, where you commonly sat, etc.
  • Now I've learned more about you as people!
  • Because of who I am as a person, I historically struggled with e.g. names, but...
  • I programmed my brain with maps.
  • Thank you for helping me!

Today

• Dictionaries
  • ✓ Dictionaries and Maps
  • ✓ Editorializing
  • Keys and Values
  • In and Indices
  • Iteration
  • Methods

Keys and Values

• The value of a key-value pair can be any Python object, mutable or immutable
  • This include other dictionaries!
• The key is more restricted:
  • Must be immutable
    • So dictionaries or lists can not work as a key
    • Tuples can though!
  • Must be unique per dictionary
  >>> d = {"Fav Prof":"Calvin", "Fav Prof":"Anyone else"} >>> d {'Fav Prof': 'Anyone else'}

Keys and Values

• Must be unique per dictionary
>>> d = {"Fav Prof":"Calvin", "Fav Prof":"Anyone else"} >>> d {'Fav Prof': 'Anyone else'}
• Setting a key to have some value overwrites the previous value.
  • Same as updating a variable
  • Same as updating an attribute of a class.
  • Same as updating an element of a list at some index.
• Wait... are Python variables... key-value pairs? (yes)

Today

• Dictionaries
  • ✓ Dictionaries and Maps
  • ✓ Editorializing
  • ✓ Keys and Values
  • In and Indices
  • Iteration
  • Methods

Selection

• The fundamental operation on dictionaries is selection, which is indicated with square brackets just like list/string/tuple: []

• Dictionaries though are unordered, so it is not a numeric index

• You instead use the key directly to select corresponding values:

  >>> movie = {"Franchise" : "Alien", "Entry": "Romulus"} >>> movie["Entry"] 'Romulus'

Non-numeric keys

• With strings/lists/tuples we trivially knew what indices existed based on length.
• Dictionaries are more complicated.
• If you attempt to index out a key that doesn’t exist: >>> movie["anything bad"] Traceback (most recent call last): File "<stdin>", line 1, in KeyError: 'anything bad'
• Wow! There is not "anything bad" in Alien: Romulus!

Non-numeric keys

• If in doubt, check for the presence of a key with the special operator 'in'
  • Recall we used "in" many times with strings, like: 'ei' in word and 'cei' not in word
• With dictionaries, we can if keys are 'in': >>> movie = {"Franchise" : "Alien", "Entry": "Romulus"} >>> "anything bad" in movie False >>> "Year" in movie True >>> "Franchise" in movie True

Non-numeric keys

• Look out!
• In strings/list/tuples, "in" checks values - the things that have an index.
• In dictionaries, it checks the keys, not the values! >>> "Alien" in movie False >>> "Franchise" in movie True >>> movie["Franchise"] 'Alien'

Non-numeric keys

• We can update values.
• Some critics consider Alien: Romulus part of the larger "Alien vs Predator" franchise
• They are wrong, but still deserve rights.
• We can falsify our data entry to sate the Predator fanbase >>> movie = {"Franchise" : "Alien", "Entry": "Romulus"} >>> movie["Franchise"] 'Alien' >>> movie["Franchise"] = "AVP" >>> movie["Franchise"] 'AVP'
• Like the xenomorph, dictionaries may mutate.
• (Dictionaries are mutable)

Today

• Dictionaries
  • ✓ Dictionaries and Maps
  • ✓ Editorializing
  • ✓ Keys and Values
  • ✓ In and IndicesSelectors
  • Iteration
  • Methods

Iterating through a Dictionary

• I love for loops.
for key in movie: value = movie[key] print("The", key, "of the movie is", value)
• This would give us, for example: The Franchise of the movie is Alien The Entry of the movie is Romulus

Iterating through a Dictionary

• Like zip() and enumerate(), we have a bundle of tricks to look at indiceskeys and values at the same time. for key, value in movie.items(): print("The", key, "of the movie is", value)
• This would give us, for example: The Franchise of the movie is Alien The Entry of the movie is Romulus

Iterating through a Dictionary

• What is "Alien"? >>> for key, value in movie.items(): ... if value == "Alien": ... print(key) ... Franchise
• Wow, it's "the" franchise. Incredible.
• What a time to be alive!

Today

• Dictionaries
  • ✓ Dictionaries and Maps
  • ✓ Editorializing
  • ✓ Keys and Values
  • ✓ In and IndicesSelectors
  • ✓ Iteration
  • Methods

Common Dictionary Methods

Method call	Description
len(d)	Returns the number of key-value pairs in the dictionary
d.get(key, value)	Returns the value associated with the given key in the dictionary. If the key is not found, returns None
d.pop(key)	Removes the key-value pair corresponding to key and returns the associated value. Will raise an error if the key is not found.
d.clear()	Removes all key-value pairs from the dictionary, leaving it empty.
d.items()	Returns an iterable object that cycles through the successive tuples consisting of a key-value pair. Iterables are weird - either use this within a for loop, or convert it to a list with list()

Editorializing Again

• My take on classes is a minority position.
• Another take: Dictionaries are the "Pythonic" way to do records.
• I find this to be a is a majority position,
• It is held by major language designers and most programers that use the language heavily.
• Looks at this: console = { 'name': 'XBox', 'vers': 360, }
• It is impossible for me to believe that isn't better than writing a class.

Today

• ✓ Dictionaries
  • ✓ Dictionaries and Maps
  • ✓ Editorializing
  • ✓ Keys and Values
  • ✓ In and IndicesSelectors
  • ✓ Iteration
  • ✓ Methods

Announcements

• Project 3 tonight.
• Midterm Friday.
  • Practice Exam posted.
  • Tuples
  • Pixels
  • Lists
  • Types