Relative Reassignment Operators

Refer to the following code snippet to answer the questions below.
```
    def change_number(number: int, modifier: int) -> int:
        idx: int = 0
        end: int = 3
        while idx < end:
            number __ modifier
            idx += 1
        return number

    print(change_number(16, 3))
```
1.1. What would be the output if we ran this code and __ was replaced with +=?

1.2. What would be the output if we ran this code and __ was replaced with -=?

1.3. What would be the output if we ran this code and __ was replaced with *=?

1.4. What would be the output if we ran this code and __ was replaced with //=?
What would you put in the blank spaces (marked with _’s) in the code snippet below to make this a while loop that prints the powers of three (1, 3, 9, …) less than 1000? (Your answer should fit in the blanks)
```
idx: int = _
end: int = 1000
while idx < end:
    print(idx)
    idx __ _
```

SHOW SOLUTIONS

Answers:

1.1. 25

1.2. 7

1.3. 432

1.4. 0

Here is the completed code snippet:

    idx: int = 1
    end: int = 1000
    while idx < end:
        print(idx)
        idx *= 3

Lists

Explain in words how you modify elements in a list? How would you do this in python? Give an example.
Write the general formula of how you call a method on an object (such as a list or dictionary) using the following. You might not need to use all and can use any multiple times: <method_name>, (), <object_variable>, . , <arg>. Give examples.
Give two ways of instantiating an empty list. What are the components you need and what does each part do? Give an example for each. Your explanation should include the words ‘function’, ‘constructor’, ‘variable’, ‘instantiate’, ‘assign’, and ‘reference’.

SHOW SOLUTIONS

In order to modify elements in a list, you first need to identify the element you want to change. Then you must find where the element is within your object. Once you have access to the element’s position, you then want to assign at that position in the list to the desired value. Lists in Python are ordered collections, which means each element has a specific index that starts from 0. You can access elements in a list using these indices.

To access an element, use the index inside square brackets []. To modify an element, you use the assignment operator =.

For example:
```
    # example
    my_list: list[“bark”, “meow”, “tweet”] # Change “meow” to “moo”
```
In this example, we want to change “meow”, which is at index 1, to “moo”. Using the square brackets for subscription notation, we assign a new value at that index like this:
```
    my_list[1] = “moo”
```
<object_variable>.<method_name>(<arg>)

For more arguments, you’d have

<object_variable>.<method_name>(<arg>, <arg>)

And so on.
```
    # example
    my_list.pop(0)
    my_list.append(“Hello”)
```
Two ways of instantiating an empty list:
- Using the list constructor: The list() function is a constructor that instantiates an empty list object. The constructor belongs to the List class. The constructor doesn’t take any arguments for creating an empty list. You assign the result of this function to a variable, which will reference the newly created empty list.
Example: python empty_list: list[str] = list()
- Components:
  - list(): The constructor function that creates a new list object.
  - list[str]: This is the type of empty_list, it is a list that will contain strs (it could also contain other types of data like ints). Always include a type when declaring a new variable!
  - empty_list: A variable that is assigned the reference to the new list object created by the list() constructor.
- Using square brackets literal: You can instantiate an empty list using a pair of square brackets []. This directly creates and instantiates a new empty list object, which you then assign to a variable.
Example: python empty_list: list[str] = []
- Components:
  - []: This is shorthand syntax for creating and instantiating an empty list object.
  - list[str]: This is the type of empty_list, it is a list that will contain strs (it could also contain other types of data like ints). Always include a type when declaring a new variable!
  - empty_list: A variable that is assigned the reference to the newly instantiated empty list.

`for` loops

Refer to the following code snippet to answer these questions: py stats: list[int] = [7, 8, 9] index: int = 0 total: int = 100 while index < len(stats): total -= stats[index] index += 1

1.1. Rewrite the following code snippet with same functionality using a for ... in loop.

1.2. Rewrite the following code snippet with same functionality using a for ... in range(...) loop.
(Challenge Question) Can you iterate through an object using a for loop while also modifying it (removing or adding elements)?

SHOW SOLUTIONS

Original code copied for reference: py stats: list[int] = [7, 8, 9] index: int = 0 total: int = 100 while index < len(stats): total -= stats[index] index += 1 1.1.

    stats: list[int] = [7, 8, 9]
    total: int = 100
    for elem in stats:
        total -= elem

1.2

    stats: list[int] = [7, 8, 9]
    total: int = 100
    for index in range(0, len(stats)):
        total -= stats[index]

No, generally you cannot safely iterate through an object (like a list or dictionary) while simultaneously modifying it by adding or removing elements during the iteration. Doing so can lead to unexpected behavior or errors like the RuntimeError: dictionary changed size during iteration. When you iterate over an object, Python keeps track of the size and structure of that object. If you modify it (e.g., by adding or removing elements), this can disrupt the iteration process because the underlying data structure changes during traversal.

Removing elements: Can cause the iteration to skip items or crash because the index or key you’re iterating over might no longer exist. Adding elements: Can lead to the same type of issue, as the size of the object changes unexpectedly, leading to errors.

Take for example this code:

    def add_task(todo_list, task):
        task_found: bool = False
        for existing_task in todo_list:
            if existing_task == task:
                task_found = True

            if not task_found:
                todo_list[task] = 'not done'

    def mark_done(todo_list, task):
        for existing_task in todo_list:
            if existing_task == task:
                todo_list[existing_task] = 'done'

    def main():
        todo_list: dict[str, str] = {'Buy groceries': 'not done', 'Read a book': 'done', 'Write report': 'not done', 'Call mom': 'done'}

        add_task(todo_list, 'Finish homework')

        mark_done(todo_list, 'Write report')

        print("Current to-do list:", todo_list)

    if __name__ == "__main__":
        main()

Sets and Dictionaries

Sets Conceptual Questions

Sets in Python are ordered collections of data with values that aren’t necessarily unique (T/F).
Testing if a value is in a set takes the same number of operations as it would in a list, no matter the size (T/F).
Syntax Practice: How would you declare a set named classes of the strings "110", "210", and "311"? After this line, add two lines that add makes the condition classes == {"110", "311", "455"} be True.

SHOW SOLUTIONS

False, sets in Python are unordered and have to be unique.
False, in a set it takes only one “operation”, regardless of size but for a list it depends on the size.

    classes: set[str] = {"110", "210", "311"}
    classes.add("455")
    classes.remove("210")

Dictionaries Conceptual Questions

For the following, make sure that for each question you could justify or provide an example of your answer using either memory diagrams or code in VS code.

Dictionaries in Python can have duplicate keys. (T/F)
Dictionaries in Python can be nested, meaning a dictionary can contain another dictionary as a value. (T/F)
Can you remove only a key without removing a value (or vice versa)?
What act as your indices in a dict?
Explain in words how you modify elements in a dict? How would you do this in python? Give an example.
How do you remove elements in a dict? What does this actually remove?
How do you modify a key in a dict?
Write the general formula of how you remove a key-value pair to a dictionary object using the following. You might not need to use all and can use any multiple times: pop, append, (), <object_variable>, . , <key>, <value>, and []. Give examples.
Write the general formula of how you remove a key-value pair to a dictionary object using the following. You might not need to use all and can use any multiple times: pop, append, (), <object_variable>, . , <key>, <value>, and []. Give examples.

SHOW SOLUTIONS

False. Python dictionaries cannot have duplicate keys. Each key in a dictionary must be unique. If you attempt to create a dictionary with duplicate keys, the latest key-value pair will overwrite the previous one. For example:
```
    my_dict = {"a": 1, "a": 2}
    print(my_dict)
    # Output: {'a': 2}
```
True, Python dictionaries can be nested. This means that a dictionary can hold another dictionary as a value. You can use this feature to create more complex data structures. For example:
```
    nested_dict = {
        "key1": {"nested_key1": 1, "nested_key2": 2},
        "key2": {"nested_key3": 3, "nested_key4": 4}
    }
```
False. No, you cannot remove just a key or just a value in a dictionary. A key-value pair is considered a single entity in a dictionary. If you remove the key, the corresponding value is also removed.
In a Python dictionary, the keys act as the “indices.” Unlike lists, where numerical indices are used to access elements, dictionaries use keys to access values and these keys can be of any type. Each key must be unique, and it serves as the identifier for accessing the corresponding value. For example:
```
    my_dict = {"name": "Alice", "age": 25}
    print(my_dict["name"])  # Output: Alice
```
To modify an element in a Python dictionary, you use the key associated with the value you want to change. You can access the value using the key, and then assign a new value to it. In order to access the value, we use subscription notation, [], on the object that we want to access the value at. This replaces the old value with the new one.
```
    my_dict = {"name": "Alice", "age": 25}
    my_dict["age"] = 26  # Modifying the value associated with the key "age"
    print(my_dict)
    # Output: {"name": "Alice", "age": 26}
```
To remove elements from a dictionary, we use the the pop() method. This method will remove the key and its corresponding value (key-value pair). The pop() method allows you to retrieve the value that was removed (not discussed in lecture but a very cool fact).
```
    my_dict = {"name": "Alice", "age": 25}
    removed_value = my_dict.pop("age")  # Removes the key "age" and returns the value
    print(my_dict)  # Output: {"name": "Alice"}
    print(removed_value)  # Output: 25
```

To modify a key in a Python dictionary, we can follow these steps:

Add a new key with the same value as the old key.
Use pop() to remove the old key-value pair.

    my_dict = {"name": "Alice", "age": 25}

    # Step 1: Create a new key-value pair using the value from the old key
    my_dict["years_old"] = my_dict.pop("age")

    print(my_dict)
    # Output: {"name": "Alice", "years_old": 25}

<object_variable>.pop(<key>)

    my_dict = {"name": "Alice", "age": 25}
    removed_value = my_dict.pop("name")  # Removes the key "name" and returns its value
    print(my_dict)
    # Output: {"age": 25}

<object_variable>[<key>] = <value>

    my_dict = {"name": "Alice", "age": 25}
    my_dict["location"] = "New York"  # Adds a new key-value pair to the dictionary
    print(my_dict)
    # Output: {"name": "Alice", "age": 25, "location": "New York"}

Dictionaries Practice Questions

Create a new dictionary called my_dictionary with str keys and float values and initialize it as an empty dictionary.
Using the following dictionary, msg: dict[str, int] = {"Hello": 1, "Yall": 2}, access the value stored under key “Yall”.
Using the following dictionary, msg: dict[str, int] = {"Hello": 1, "Yall": 2}, increase the value stored under key “Yall” by 3.
Using the following dictionary, ids: dict[int, str] = {100: "Alyssa", 200: "Carmine"}, remove the value “Alyssa”, stored at key 100.
Using the following dictionary, ids: dict[int, str] = {100: "Alyssa", 200: "Carmine"}, write a line of code to get the number of key/value pairs in the dictionary.
Using the following dictionary, inventory: dict[str, int] = {"pens": 10, "notebooks": 5, "erasers": 8}, add a new key-value pair "markers": 15.
Using the following dictionary, prices: dict[str, float] = {"bread": 2.99, "milk": 1.99, "eggs": 3.49}, update the value of "milk" to 2.50.
Using the dictionary scores: dict[str, int] = {"Alice": 85, "Bob": 90, "Charlie": 88}, print out all the keys in the dictionary.
Using the dictionary scores: dict[str, int] = {"Alice": 85, "Bob": 90, "Charlie": 88}, write a line of code that returns the sum of all the values (scores) in the dictionary. Assume that we have a written a sum function that will do this for us when passing in a dict. Store this value in a variable total_score.

9.1. Write the same line of code except using key-word arguments assuming that the only parameter in sum is called inp_dict.
Using the dictionary fruit_count: dict[str, int] = {"apples": 5, "bananas": 8}, iterate over the key-value pairs and print them in the format “key: value”.
Create a new dictionary by combining the two dictionaries. Store this new object by assiging it’s reference to a variable called combo_dict:

    first_dict: dict[str, int] = {"a": 1, "b": 2}
    second_dict: dict[str, int] = {"c": 3, "d": 4}

SHOW SOLUTIONS

my_dictionary: dict[str, float] = {} or my_dictionary: dict[str, float] = dict()
msg["Yall"]
msg["Yall"] += 3 or msg["Yall"] = 5
ids.pop(100)
len(ids)
inventory["markers"] = 15
prices["milk"] = 2.50
Code below:
```
    for x in scores:
        print(x)
```
total_score = sum(scores)

9.1. total_score = sum(inp_dict=scores)

Code below:

    for boo in fruit_count:
        ghost = fruit_count[boo]
        print(f"{boo}: {ghost}")

combo_dict: dict[str, int] = {"a": 1, "b": 2, "c": 3, "d": 4}

Extra Practice (Lists, Dictionaries, `for` loops)

The following questions will refer to the list below: python my_list: list[int] = list() 1.1. Write line(s) of code that would add the number 8, 0, 3, and -1 to the list.

1.2. Write line(s) of code that removes 3 from the list.

1.3. Write line(s) of code that assigns the variable dog to the element at the second index.

1.4. Write line(s) of code that prints the amount of items in the list.

1.5. Change the value 8 to 0.

1.6. We now have a function, sum that adds the elements of my_list and returns this amount. Write a line of code that instantiates a list[int] with the first value returned from calling sum on my_list.
The following questions will refer to the dictionary below:
```
    my_dict: dict[int, str] = {}
```
2.1. Write line(s) of code that would add the following key-value pairs to the dictionary:
- 8: 'eight'
- 0: 'zero'
- 3: 'three'
- -1: 'negative one'
2.2. Write line(s) of code that removes value three.

2.3. Write line(s) of code that assign the value associated with the key 0 in my_dict to a variable called cat.

2.4. Write line(s) of code that print the number of keys in my_dict.

2.5. Write line(s) of code that print the number of values in my_dict.

2.6. Change the value associated with the key 8 to 'zero'.

2.7. Suppose we have a function sum_dict_keys that sums the keys of my_dict and returns this amount. Write a line of code that instantiates a dict[str, int] containing a single value, which is the result of calling sum_dict_keys(my_dict) and a key of “returned_amount”.
The following questions will refer to the dictionary below:
```
    my_dict: dict[int, str] = {0: "dog", 1: "cat", 2: "mouse", 3: "bird", 4: "whale"}
```
3.1. What will print from the following code: python for x in range(0, len(my_dict)): print(my_dict[x])
1. dog, cat, mouse, bird, whale
2. dog, cat, mouse, bird
3. IndexOutOfRange
4. 0, 1, 2, 3, 4
3.2. What will print from the following code: python for x in range(0, len(my_dict)): print(x)
1. 0, 1, 2, 3, 4
2. dog, cat, mouse, bird, whale
3. IndexOutOfRange
4. 1, 2, 3, 4
3.2. What will print from the following code: python for x in my_dict: print(my_dict[x])
1. dog, cat, mouse, bird, whale
2. dog, cat, mouse, bird
3. IndexOutOfRange
4. 0, 1, 2, 3, 4
3.3. What will print from the following code: python for x in my_dict: print(x)
1. 0, 1, 2, 3, 4
2. dog, cat, mouse, bird, whale
3. IndexOutOfRange
4. 1, 2, 3, 4
3.4. What will print from the following code: python x: int = 0 while x < len(my_dict): print(x) x += 1
1. 0, 1, 2, 3, 4
2. dog, cat, mouse, bird, whale
3. IndexOutOfRange
4. 1, 2, 3, 4, 5
3.5. What will print from the following code: python x: int = 0 while x < len(my_dict): print(my_dict[x]) x += 1
1. dog, cat, mouse, bird, whale
2. dog, cat, mouse, bird
3. IndexOutOfRange
4. 0, 1, 2, 3, 4
The following questions will refer to the dictionary below:
```
my_dict: dict[int, str] = {8: "dog", 1: "cat", 10: "mouse", 15: "bird", 0: "whale"}
```
4.1. What will print from the following code: python for x in range(0, len(my_dict)): print(my_dict[x])
1. whale, cat, KeyError
2. IndexOutOfRange
3. dog, cat, mouse
4. bird, whale
4.2. What will print from the following code: python for x in range(0, len(my_dict)): print(x)
1. 0, 1, 2, 3, 4
2. 0, 1, 2
3. IndexOutOfRange
4. 1, 2, 3, 4
4.3. What will print from the following code: python for x in my_dict: print(my_dict[x])
1. dog, cat, mouse, bird, whale
2. whale, cat, mouse, bird, dog
3. IndexOutOfRange
4. 0, 1, 2, 3, 4
4.4. What will print from the following code: python for x in my_dict: print(x)
1. 8, 1, 10, 15, 0
2. dog, cat, mouse, bird, whale
3. IndexOutOfRange
4. 0, 1, 2, 3, 4
4.5. What will print from the following code: python x: int = 0 while x < len(my_dict): print(x) x += 1
1. 0, 1, 2, 3, 4
2. 1, 2, 3, 4
3. IndexOutOfRange
4. 0, 1, 2
4.6. What will print from the following code: python x: int = 0 while x < len(my_dict): print(my_dict[x]) x += 1
1. whale, cat, KeyError
2. IndexOutOfRange
3. dog, cat, mouse
4. bird, whale
The following questions will refer to the dictionary below:
```
my_dict: dict[str, str] = {"cat": "dog", "dog": "cat", "bird": "mouse", "mouse": "bird", "while": "whale"}
```
5.1. What will print from the following code: python for x in range(0, len(my_dict)): print(my_dict[x])
1. IndexOutOfRange
2. dog, cat, mouse
3. KeyError
4. bird, whale
5.2. What will print from the following code: python for x in range(0, len(my_dict)): print(x)
1. 0, 1, 2, 3, 4
2. 1, 2, 3, 4
3. IndexOutOfRange
4. 0, 1, 2
5.3. What will print from the following code: python for x in my_dict: print(my_dict[x])
1. dog, cat, mouse, bird, whale
2. dog, cat, mouse, bird
3. IndexOutOfRange
4. cat, dog, bird
5.4. What will print from the following code: python for x in my_dict: print(x)
1. cat, dog, bird, mouse, while
2. dog, cat, mouse, bird
3. IndexOutOfRange
4. 0, 1, 2, 3, 4
5.5. What will print from the following code: python x: int = 0 while x < len(my_dict): print(x) x += 1
1. 0, 1, 2, 3, 4
2. dog, cat, mouse, bird, whale
3. IndexOutOfRange
4. 1, 2, 3, 4
5.6. What will print from the following code: python x: int = 0 while x < len(my_dict): print(my_dict[x]) x += 1
1. IndexOutOfRange
2. dog, cat, mouse
3. KeyError
4. bird, whale

SHOW SOLUTIONS

List Manipulations

my_list: list[int] = list()

# 1.1. Add the numbers 8, 0, 3, and -1 to the list.
my_list.append(8)
my_list.append(0)
my_list.append(3)
my_list.append(-1)

# 1.2. Remove the number 3 from the list.
my_list.pop(2)

# 1.3. Assign the element at the second index to a variable named 'dog'.
dog = my_list[2]

# 1.4. Print the number of items in the list.
print(len(my_list))

# 1.5. Change the value 8 to 0.
my_list[0] = 0

# 1.6. Instantiate a list[int] with the first value returned from calling sum on my_list.
summed_list = [sum(my_list)]

Dictionary Manipulations

my_dict: dict[int, str] = {}

# 2.1. Add key-value pairs to the dictionary.
my_dict[8] = 'eight'
my_dict[0] = 'zero'
my_dict[3] = 'three'
my_dict[-1] = 'negative one'

# 2.2. Remove the key-value pair where the value is 'three'.
my_dict.pop(3) # recall that pop takes an index and in the case of dictionaries or indicies are essentially our keys

# 2.3. Assign the value associated with the key 0 to a variable called 'cat'.
cat = my_dict[0]

# 2.4. Print the number of keys in the dictionary.
print(len(my_dict))

# 2.5. Print the number of values in the dictionary.
print(len(my_dict))

# 2.6. Change the value associated with the key 8 to 'zero'.
my_dict[8] = 'zero'

# 2.7. Instantiate a dict[str, int] with the key 'returned_amount' and the value from sum_dict_keys(my_dict).
result_dict = {'returned_amount': sum_dict_keys(my_dict)}

Dictionary Looping and Output

3.1: Output = (a) dog, cat, mouse, bird, whale
3.2: Output = (a) 0, 1, 2, 3, 4
3.3: Output = (a) dog, cat, mouse, bird, whale
3.4: Output = (a) 0, 1, 2, 3, 4
3.5: Output = (a) 0, 1, 2, 3, 4
3.6: Output = (a) dog, cat, mouse, bird, whale

Looping with Different Key Values

4.1: Output = (a) whale, cat, KeyError
4.2: Output = (a) 0, 1, 2, 3, 4
4.3: Output = (a) dog, cat, mouse, bird, whale
4.4: Output = (a) 0, 1, 8, 10, 15
4.5: Output = (a) 0, 1, 2, 3, 4
4.6: Output = (a) whale, cat, KeyError

Dictionary with String Keys

5.1: Output = (c) KeyError
5.2: Output = (a) 0, 1, 2, 3, 4
5.3: Output = (a) dog, cat, mouse, bird, whale
5.4: Output = (a) cat, dog, bird, mouse, while
5.5: Output = (a) 0, 1, 2, 3, 4
5.6: Output = (c) KeyError

Quiz 02 Practice Problems

Relative Reassignment Operators

Lists

for loops

Sets and Dictionaries

Sets Conceptual Questions

Dictionaries Conceptual Questions

Dictionaries Practice Questions

Extra Practice (Lists, Dictionaries, for loops)

`for` loops

Extra Practice (Lists, Dictionaries, `for` loops)