Dictionaries

CSCI 1012

Announcements

• Homework 9 (dictionaries) is due Sunday, 30 March 11:55 PM

In-Class Exercises

• File names:
  • ex1.py
  • ex2.py
  • ex3.py
• Turn in on the submit server before the end of lecture for credit
• Turn in one .zip with all the .py files
  • Unlimited attempts during lecture
• We’ll provide solutions – pay attention!

Common Errors

• Incorrect file names:
  • check_even.py vs. checkEven.py
• Incorrect function name
  • def check_even(x): vs. def check_evens(x):
• Spacing errors
  • 1 2 3 vs. 1 2 3

Read your autograder output!

Today

• Introducing a new data structure: dictionaries
• What are they? How to create them?
• How to access and update them?
• Iterating through dictionaries
• Counting using dictionaries

What we’ve seen

• Strings: ordered collection of characters
  • s = 'this is a string'

• Lists: a general ordered collection of elements
  • l = [1, 'happy', 3.4, 5]

• The elements in strings and lists are accessed using indices (0, 1, 2,…)
  • s[0], s[4]
  • l[3]

Warmup: Exercise 1

Write a function ex1 that takes a list as its only argument. Your function will return the boolean True if the elements in the list are strictly ascending (smallest to largest: that is, each subsequent value is at least as large as the preceding one) and return False if not.

• ex1([1,2,2,3,4]) returns True
• ex1([1,2,4,5,4]) returns False

You may assume that all of the elements of the list are numeric. Submit as ex1.py. You may not use built-in sorting functions.

What we’ve seen

• Strings: ordered collection of characters
  • s = 'this is a string'
• Lists: a general ordered collection of elements
  • l = [1, 'happy', 3.4, 5]
• The elements in strings and lists are accessed using indices (0, 1, 2,…)
  • s[0], s[4]
  • l[3]

Dictionaries

• Dictionaries are unordered collections of key and value pairs

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}

• In this example, keys: 'Jill', 'Leo', 'Maria', 'Bruno'
• Values: 100, 94, 85, 90

• You access the values via the keys
  • gradebook['Jill'] gives 100

• Dictionaries provide an easy way to store and access structured information of this sort

• Notice: curly brackets, colons separating keys and values

How to create a dictionary

• Creating an empty dictionary:

gradebook = {}
print(gradebook)

{}

gradebook = dict()
print(gradebook)

{}

• Creating a dictionary with elements already in it

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85}
print(gradebook)

{'Jill': 100, 'Leo': 94, 'Maria': 85}

Adding key-value pairs to an existing dictionary

• Syntax: dictionary_name[key] = value

gradebook = {}
gradebook['Jill'] = 100
print(gradebook)

{'Jill': 100}

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85}
gradebook['Bruno'] = 90
print(gradebook)

{'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}

Accessing elements of a dictionary

• You can access the values via the keys

• Syntax: dictionary_name[key] will give you the value corresponding to the key you query

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
print(gradebook['Leo'])

94

D = {1: 'a', 2: 'b', 3: 'c'}
print(D[1])

a

• Notice: square brackets (even though you use {} to create the dictionary)

Dictionaries are “mutable”

• Just like lists (but unlike strings), dictionary values can be changed in place:

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
gradebook['Leo'] = 95
print(gradebook)

{'Jill': 100, 'Leo': 95, 'Maria': 85, 'Bruno': 90}

S = {}
S['hello'] = 14
S['goodbye'] = 3
S['hello'] = S['hello'] + S['hello'] - 1
print(S)

{'hello': 27, 'goodbye': 3}

Key Errors

• If you try to access a key that’s not in the dictionary, you will get a Key Error

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
gradebook['Anna']

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Anna'

in

• You can use in to check whether a particular key is in the dictionary

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
'Jill' in gradebook
'Anna' in gradebook

True

False

• Important: in only checks whether the element is a key in the dictionary, not whether it’s a value

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
'Leo' in gradebook
94 in gradebook

True

False

in

• in can help avoid the key error we saw previously. If you’re not sure whether a key is in your dictionary, you can check using in:

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
if 'Anna' in gradebook:
  print(gradebook['Anna'])

• Error, if we just do:

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
print(gradebook['Anna'])

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Anna'

Exercise 2

Write a function ex2 that takes two lists (of equal length) as its arguments and creates a dictionary. The elements of the first list will be the keys of your dictionary and the corresponding elements of the second list will be the corresponding values. You can assume no duplicate values in the first list. Your function should return your dictionary.

• ex2(['Jill', 'Leo'], [100, 94]) returns {'Jill': 100, 'Leo': 94}
• ex2(['names'], [['Jill', 'Leo']]) returns {'names': ['Jill', 'Leo']}

Submit as ex2.py.

Modifying your code (no submission)

• What would your code output if there was a duplicate in the first list:
  • ex2([1, 1], [12, 20])

• How would you change your code to accept duplicates in the first list, by only keeping the first instance:
  • ex2modified([1, 1], [12, 20]) returns {1: 12}

Other things to know:

• You can use len() to get the number of key-value pairs in a dictionary

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
print(len(gradebook))

4

D = {'names': ['Holly', 'Benny'], 'ages': [32, 17], 'states': ['MD', 'NJ']}
print(len(D))

3

• You can remove key-value pairs from a dictionary

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
del gradebook['Maria']
print(gradebook)

{'Jill': 100, 'Leo': 94, 'Bruno': 90}

What can be a dictionary key?

• Any immutable data type can be a key
  • strings, ints, floats
  • BUT NOT: lists and dictionaries

A = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
B = {100: 1, 94: 1, 85: 1, 90: 1, 12.5: 7}

A = {[1,2,3]: 'hello'}

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

What can be a value?

• All of the data types we’ve seen can be values
  • strings, floats, ints, lists, dictionaries
• For both the keys and values, you can mix and match types

A = {'Jill': [100], 1: 14.1, 1.5: 'hello'}

Iterating through dictionaries

• You can iterate over keys in dictionaries
• Easiest to use a for-loop
• Syntax: for key in dictionary_name:

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
for student in gradebook:
  print(student, gradebook[student])

Jill 100
Leo 94
Maria 85
Bruno 90

In the visualizer

Iterating through dictionaries

gradebook = {'Jill': 100, 'Leo': 94, 'Maria': 85, 'Bruno': 90}
final_grades = {}
for student in gradebook:
  if gradebook[student] >= 90:
    final_grades[student] = 'A'
  else:
    final_grades[student] = 'B'
print(final_grades)

{'Jill': 'A', 'Leo': 'A', 'Maria': 'B', 'Bruno': 'A'}

In the visualizer

Exercise 3

Write a function ex3 that takes as input a dictionary (its key will be strings and its values will be ints). Your function will get the length of each key (string) and check if it’s at least as big as its corresponding int value. If this is true for all key-value pairs, return True. Otherwise, return False.

• ex3({'hello': 4, 'goodbye': 2}) returns True
• ex3({'hello': 4, 'goodbye': 2, 'hey': 5}) returns False (because 'hey' is 3 characters long, which is less than the value corresponding to 'hey', which is 5)

Submit as ex3.py.

Dictionaries are great for counting things

• Calculate how many of each character occur in a string:
  • e.g., s = "aaabbc"

s = "aaabbc"
counter = {}
for char in s:
  if char in counter:
    counter[char] += 1
  else:
    counter[char] = 1

In the visualizer

Dictionaries are great for counting things

• Calculate how many of each element occur in a list:
  • L = [4, 5, 3, 2, 5, 8, 1, 2] returns {1: 1, 2: 2, 3: 1, 4: 1, 5: 2, 8: 1}

L = [4, 5, 3, 2, 5, 8, 1, 2]
counter = {}
for char in s:
  if char in counter:
    counter[char] += 1
  else:
    counter[char] = 1

In the visualizer

Summary: Dictionaries

• Unordered collection of pairs of keys and values
• Created using {}; Accessed using []
• Dictionaries are mutable: Adding key-value pairs
• Iterating through dictionaries
• Using dictionaries to count elements

Next week: Tuples & File System

• Read Think Python Ch. 12
  • You can skip 12.4 - Variable-length argument tuples