Dictionary

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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

• 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

A = {True: 'Seth', False: 'Jesse'}
B = {'Jill': 13, 'Jack': 12}
C = {(1,2): {'x': 1}}

#X = {{'x': 1, 'y': 2}: 'Shark'}
Y = {[1,3,5]: 'Odd'}
Z = {'A': 13, 'B': 24, 'A': 15}

Selection

• The fundamental operation on dictionaries is selection, which is still indicated with square brackets: []

• Dictionaries though are unordered, so it is not a numeric index that goes inside the [ ]

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

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

Losing your keys

• If you attempt to index out a key that doesn’t exist:

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

  you will get an error!

• 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 of corresponding keys

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

Grade Sheet Example

• Suppose we had a file of student ids and accompanying scores that we wanted to read into a dictionary and then access.

  def read_to_dict(filename):
      dictionary = {}
      with open(filename) as f:
          for line in f:
              ID, score = line.strip().split(',')
              dictionary[ID] = score
      return dictionary
  
  def get_student_score():
      scores = read_to_dict('SampleGrades.txt')
      done = False
      while not done:
          student_id = input('Enter a student id number: ')
          if student_id == "":
              done = True
          else:
              if student_id in scores:
                  print(f"Student got a {scores[student_id]}.")
              else:
                  print(f"Student id {student_id} was not found in classlist")

Understanding Check

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])

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, which has the form of:

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

• You can also use the .items method to grab both key and values together:

  • Returns a tuple with both the key and corresponding 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 have seen increased use of late as structures to store records

• Looks surprisingly close to our original template of:

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

• Allows easy access of attributes without worrying about ordering

  print(boss['name'])