Dictionaries

• A list maps consecutive integers to data
  • ['Bob', 'Dave', 'Tom'] maps 0 to Bob, 1 to Dave, and 2 to Tom.
• A dictionary can map keys of any type to a value.
  • (Kind of like a mathematical function)
• grades = {'John': 'A', 'Dave': 'B', 'George': 'C'}
• Can access much like a list:
  • grades['John']
• Dictionaries are mutable:
  • grades['Susan'] = 'A'
• Multiple ways to create a dictionary:
  • grades = {'John': 'A', 'Dave': 'B', 'George': 'C'}
  • grades = dict(John='A', Dave='B', George='C')
  • grades = dict([('John', 'A'), ('Dave', 'B'), ('George', 'C')])
• Like an array, key can be a variable:
  • student = input(); grades[student]
• Can delete a key with del
• Use in to test for membership
  • if 'Scott' in grades

sales_tax = {
   "Alabama": 0.04,
    "Alaska": 0.0,  # Alaska has no state-level sales tax
    "Arizona": 0.056,
    "Arkansas": 0.065,
    "California": 0.0725,
    "Colorado": 0.029,
    "Connecticut": 0.0635,
    "Delaware": 0.0,  # Delaware has no state-level sales tax
    "Florida": 0.06,
    "Georgia": 0.04,
    "Hawaii": 0.04,
    "Idaho": 0.06,
  'Michigan': 0.06,
  'Indiana': 0.07,
  'Illinois': 0.0625
}

Common methods

• .clear
• .get(key, default) (use default if key isn’t in the dictionary)
• .update(dict2) merges two dictionaries
• .pop(key, default) return and remove

Iteration

• .items(): (key, value) tuples
• .keys(): return the keys
• .values(): return the values
• Order is not guaranteed!

Misc

• Dictionaries can be nested

  grades = {
    'John Ponting': {
        'Homeworks': [79, 80, 74],
        'Midterm': 85,
        'Final': 92
    },
    'Jacques Kallis': {
        'Homeworks': [90, 92, 65],
        'Midterm': 87,
        'Final': 75
    },
    'Ricky Bobby': {
        'Homeworks': [50, 52, 78],
        'Midterm': 40,
        'Final': 65
    },
  }
  
  print(grades['John Ponting']['Homeworks][1])