5.45. Group Work: Functions with Sets and Dictionaries

It is best to use a POGIL approach with the following. In POGIL students work in groups on activities and each member has an assigned role. For more information see https://cspogil.org/Home.

Note

If you work in a group, have only one member of the group fill in the answers on this page. You will be able to share your answers with the group at the bottom of the page.

Learning Objectives

Students will know and be able to do the following.

Content Objectives:

• Explain the difference between a set and a list.

• Explain the difference between a dictionary and a list.

• Predict the output of functions with sets and dictionaries.

• Use .get to prevent key errors in dictionaries.

Process Objectives:

• Predict the output of functions with sets and dictionaries.

5.45.1. What is a Set?

A set is like a list in that holds items, but it does not hold items in an particular order. You can add an item to a set and remove an item from a set, just like you can do with a list. You can also check if an item is in a set.

Run this code to see what it prints.

# initialize a set s1 = {1, 4, 8} print(s1) # add an item to a set s1.add(3) print(s1) # remove an item from a set s1.remove(8) print(s1) # check if an item is in a set print(1 in s1) print(9 in s1)

5.45.2. Sets only Contain Immutable Items

Run this code to see what happens. Then remove any code that causes and error and run it again.

# create an empty set s1 = set() # add an integer s1.add(1) print(s1) # add a string s1.add("hi") print(s1) # add a tuple s1.add((1, 2)) print(s1) # add a list s1.add([1, 3, 2]) print(s1)

Q-4: Which of the following types can be added to a set?

• Strings

• Yes, Strings are immutable.

• Lists

• No, Lists can change.

• Tuples

• Yes, Tuples are immutable.

• Integers

• Yes, Integers are immutable.

Note

Sets can only store immutable items, even though sets themselves are mutable (can change).

5.45.3. Sets Do Not Allow Duplicates

Q-5: What values will be printed last when the code below is run?

• Nothing will print. You will get an error since you are trying to add a duplicate item to a set.

• No, it will actually ignore the duplicate item.

• 1, 2, 3

• Yes, it ignores the duplicate item.

• 1, 2, 3, 1

• No, sets do not allow duplicate items.

Before you keep reading...

Runestone Academy can only continue if we get support from individuals like you. As a student you are well aware of the high cost of textbooks. Our mission is to provide great books to you for free, but we ask that you consider a $10 donation, more if you can or less if $10 is a burden.

Support Runestone Academy Today

Before you keep reading...

Making great stuff takes time and $$. If you appreciate the book you are reading now and want to keep quality materials free for other students please consider a donation to Runestone Academy. We ask that you consider a $10 donation, but if you can give more thats great, if $10 is too much for your budget we would be happy with whatever you can afford as a show of support.

Support Runestone Academy Today

Run this code to see what it prints.

# create a set s1 = {1, 2, 3} print(s1) # add an integer s1.add(1) print(s1)

Note

Sets only store unique items. If you try to add a duplicate item to a set it will be ignored.

5.45.4. Adding Mutliple Items to a Set

Run this code to see what it prints. You will get an error. Remove the code that causes the error and run it again.

# create the sets s1 = {8, 3, 5} s2 = {1, 2, 3} # try to add the sets together s1.add(s2) print(s1) print(s2) # use update to add two sets together s1.update(s2) print(s1) print(s2)

Note

The set add method only adds one item to a set. To add several items use the set update method.

5.45.5. Set Methods

You can use the following methods with sets.

• set1.union(other_set) - returns a new set with all the items from set1 and other_set.

• set1.intersection(other_set) - returns a new set with just the items that are in both set1 and other_set.

• set1.difference(other_set) - returns a new set with the items in set1 that are not in other_set.

• set1.issubset(other_set) - returns True if set1 is a subset (has some or all of the same elements and no other elements) of other_set.

• set1.issuperset(other_set) - returns True if set1 is a superset (has all the same elments and may have other elements) of other_set.

• set1.semmantic_difference(other_set) - returns a new set with the items that are in either set1 or other_set, but not both. This is also known as an exclusive or (XOR).

Run this code to see what it prints.

# define two sets s1 = {1, 2, 3} s2 = {3, 4, 5} print(s1.union(s2)) print(s1 | s2) print(s1.intersection(s2)) print(s1 & s2)

Run this code to see what it prints.

# define the sets s1 = {1, 2, 3} s2 = {3, 4, 5} s3 = {1, 3} print(s1.difference(s2)) print(s1 - s2) print(s2 - s1) print(s1.issubset(s2)) print(s1 <= s2) print(s3.issubset(s1))

Run this code to see what it prints.

# define two sets s1 = {1, 2, 3} s2 = {3, 4, 5} s3 = {1, 3} print(s1.issuperset(s3)) print(s1 >= s3) print(s1.symmetric_difference(s2)) print(s1 ^ s2)

Q-14: Drag each symbol to the method it is equivalent to. Read this page and try again.
• &
• intersection
• <=
• issubset
• >=
• issuperset
• -
• difference
• ^
• symmetric_difference

5.45.6. Creating Sets

You can innitialize a set with a string, list, or tuple. The set will only contain the unique items.

Run this code to see what it prints.

# create a set from a string s1 = set("hello") print(s1) # create a set from a list s2 = set([1, 2, 3, 2, 5]) print(s2) # create a set from a tuple s3 = set((6, 6, 9)) print(s3)

5.45.7. Sorting Sets

Run this code to see what it prints.

# initialize a set s1 = {3, 2, 1} print(s1) l1 = list(s1) print(l1) print(sorted(s1)) # initalize another set s2 = {"way", "back", "home"} print(s2) print(sorted(s2)) print(s2)

Note

The function sorted(set) will return a new list sorted in ascending order.

Drag the blocks from the left and put them in the correct order on the right to define a function unique_characters(strings) that takes a list of strings and returns a sorted list of all the distinct characters in the strings.

        def unique_characters(strings):
---
    # Init an empty set
    unique_chars = set()
---
    # Loop through the strings
    for string in strings:
---
        # Add the chars in string
        unique_chars.update(string)
---
        # Add the chars in string
        unique_chars.add(string) #paired
---
    # Convert to a sorted list
    sorted_chars = sorted(unique_chars)
---
    # Convert to a sorted list
    sorted_chars = sort(unique_chars) #paired
---
    # return the sorted list
    return sorted_chars
        

5.45.8. Dictionaries

A dictionary stores a value for a key. The keys must be immutable and unique.

Run this code to see what it prints.

# function definition def count_chars(str): letter_dict = dict() for char in str: if char in letter_dict: letter_dict[char] = letter_dict[char] + 1 else: letter_dict[char] = 1 return letter_dict d1 = count_chars("This is a test") print(d1['t']) d1 = count_chars("Run, run as fast as you can") print(d1['a']) print(d1) print(type(d1))

Q-20: Look at the Python code below. What do you think will happen when you run the following code?

Run this code to see what it prints.

d = {} d['north'] = "Michigan" d['south'] = "Kentucky" print(d['east'])

There is another way to update the value for a key that works even if the key isn’t in the dictionary already.

Run this code to see what it prints.

# function definition def count_chars(str): letter_dict = dict() for char in str: # use .get to stop key errors but if not there # use 0 and then add 1 to the returned value letter_dict[char] = letter_dict.get(char,0) + 1 return letter_dict d1 = count_chars("This is a test") print(d1['t']) d1 = count_chars("Run, run as fast as you can") print(d1['a']) print(d1)

Note

The better way to increment a count at a key is to use dict[key] = dict.get(key,0) + 1. This will avoid a key error if the key isn’t in the dictionary and the code is shorter.

Q-23: Drag each object to its type. Read this page and try again.
• ["a", "b"]
• List
• ("a", "b")
• Tuple
• "ab"
• String
• {1, 2, 3}
• Set
• {"a": 5, "b": 2}
• Dictionary

Q-24: Which of the following types are immutable (don’t change)?

• Strings

• Yes, Strings are immutable.

• Lists

• No, Lists can change.

• Tuples

• Yes, Tuples are immutable.

• Sets

• No, sets can change.

• Dictionaries

• No, Dictionaries can change.

If you worked in a group, you can copy the answers from this page to the other group members. Select the group members below and click the button to share the answers.

The Submit Group button will submit the answer for each each question on this page for each member of your group. It also logs you as the official group submitter.

You have attempted 1 of 26 activities on this page