Dictionary

Happy Wednesday!

Quick Announcements

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
• 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
• 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.

Creating Dictionaries

• Python dictionaries use squiggly brackets {} to enclose their contents
• Can create an empty dictionary by providing no key-value pairs:

empty_dict = {}

• If creating a dictionary with key-value pairs
  • Keys are separated from values with a colon :
  • Pairs are separated by a comma ,

  generic_dict = {'Bob': 21, 0: False, 13: 'Thirteen'}

  ---

Keys and Values

Selection

• The fundamental operation on dictionaries is selection, which is still indicated with square brackets: []
• Dictionaries are unordered, so a numeric index does not go inside the brackets
• Use the key directly to select its corresponding value:

>>> A = {'Jack': 12, 'Jill': 13}
>>> A['Jack']
12
>>> B = {True: 13, 0: 'Why?'}
>>> B[0]
'Why?'

Losing Your Keys

• If you attempt to index a key that doesn’t exist, you will get an error:

A = {'Jack': 12, 'Jill': 13}
print(A['Jil'])

• If in doubt, check for the presence of a key with the in operator:

if 'Jil' in A:
        print(A['Jil'])

Rewriting the Dictionary

• Dictionaries are mutable!
  • We can add new key-value pairs
  • We can change the value associated with a key

  >>> d = {}
  >>> d['A'] = 10
  >>> d['B'] = 12
  >>> print(d)
  {'A': 10, 'B': 12}
  >>> d['A'] = d['B']
  >>> print(d)
  {'A': 12, 'B': 12}

Iterating through a Dictionary

• Frequently we might want to iterate through a dictionary, checking either its values or its keys
• Python supports iteration with the for statement:

for key in dictionary:
        value = dictionary[key]
        # Code to work with that key and value

• You can also use the .items() method to get keys and values together
  • Returns an iterable of tuples containing each key-value pair

  for key, value in dictionary.items():
        # Code to work with that key and value

Finding Students by Grade

def get_students_with_score():
        scores = read_to_dict('SampleGrades.txt')
        done = False
        while not done:
                des_grade = input('Enter a letter grade: ')
                if des_grade == "":
                        done = True
                else:
                        for st_id, grade in scores.items():
                                if grade == des_grade.strip().upper():
                                        print(f"{st_id} got a {grade}")

Common Dictionary Methods

Dictionary Records

• While most commonly used to indicate mappings, dictionaries are also used to store records
• For example:

boss = {
        'name': 'Scrooge',
        'title': 'founder',
        'salary': 1000
}

• Allows easy access to attributes without worrying about ordering: python print(boss['name']) —

Group Problems LO’s

• How to define a dictionary in Python
• How to utilize common dictionary operations and methods
• Reading in and parsing data from a file

Problem 1: Tracing Definitions

What is the printed value of the below code?

A = [
    {'name': 'Jill',  'weight':125, 'height':62},
    {'name': 'Sam',   'height':68},
    {'name': 'Bobby', 'height':72},
]
A.append({'weight':204, 'height':70, 'name':'Jim'})
B= A[1]
B['weight'] = 167
print([d['weight'] for d in A if 'weight' in d])

Problem 2a: Parsing Pairs

• Grab the starter codes from Discord
• This is a coding problem. Work in pairs on a single computer, and discuss your algorithm BEFORE YOU WRITE ANY CODE.
• The file grades.txt has student names (first and last), then a comma, and then their current grade in a class
• Your task is to write a function to read in the data from the file and populate a dictionary with the names being the keys and the grades the corresponding values.

Problem 2b: Grade Lookup

• Swap who is typing!
• Write a function called get_student_grade which prompts the user for a student’s name and then prints out that student’s grade.
• If the typed student does not exist, just print out “Student does not exist”

Problem 2c: Grade Reporting

• Swap who is typing!
• Write another function get_students_with_grade which prompts the user to type in a desired grade.
• Then all the names of students who received that grade should be printed to the screen
• Bonus: Also return all the names as elements of a list

Demo

• Tweets.csv contains tweet data from users
• Our task is to see what hashtags are trending or most popular