# Homework 4: Sequences, Mutability, Object-Oriented Programming hw04.zip

Due by 11:59pm on Thursday, July 21

## Instructions

Download hw04.zip. Inside the archive, you will find a file called hw04.py, along with a copy of the `ok` autograder.

Submission: When you are done, submit with ```python3 ok --submit```. You may submit more than once before the deadline; only the final submission will be scored. Check that you have successfully submitted your code on okpy.org. See Lab 0 for more instructions on submitting assignments.

Using Ok: If you have any questions about using Ok, please refer to this guide.

Readings: You might find the following references useful:

Grading: Homework is graded based on correctness. Each incorrect problem will decrease the total score by one point. There is a homework recovery policy as stated in the syllabus. This homework is out of 2 points.

# Required questions

## Getting Started Videos

These videos may provide some helpful direction for tackling the coding problems on this assignment.

To see these videos, you should be logged into your berkeley.edu email.

## Code Writing Questions

### Q1: Merge

Write a function `merge` that takes 2 sorted lists `lst1` and `lst2`, and returns a new list that contains all the elements in the two lists in sorted order.

Note: Try to solve this question using recursion instead of iteration.

``````def merge(lst1, lst2):
"""Merges two sorted lists.

>>> s1 = [1, 3, 5]
>>> s2 = [2, 4, 6]
>>> merge(s1, s2)
[1, 2, 3, 4, 5, 6]
>>> s1
[1, 3, 5]
>>> s2
[2, 4, 6]
>>> merge([], [2, 4, 6])
[2, 4, 6]
>>> merge([1, 2, 3], [])
[1, 2, 3]
>>> merge([5, 7], [2, 4, 6])
[2, 4, 5, 6, 7]
>>> merge([2, 3, 4], [2, 4, 6])
[2, 2, 3, 4, 4, 6]
"""
"*** YOUR CODE HERE ***"
``````

Use Ok to test your code:

``python3 ok -q merge``

### Q2: Remove Odd Indices

Complete the recursive function `remove_odd_indices`, which takes in a list `lst` and a boolean `odd`, and returns a new list with elements removed at certain indices. If `odd` is `True`, then the function should remove elements at odd indices; otherwise if `odd` is False, then the function should remove even indexed items.

Note: Remember that lists are zero-indexed; thus, the first element is at index 0, the second element is at index 1, etc. Also remember that you can put `not` in front of a boolean in order to get its opposite value.

Important: Use recursion; the tests will fail if you use any loops (for, while).

``````def remove_odd_indices(lst, odd):
"""Remove elements of lst that have odd indices. Use recursion!

>>> s = [1, 2, 3, 4]
>>> t = remove_odd_indices(s, True)
>>> s
[1, 2, 3, 4]
>>> t
[1, 3]
>>> l = [5, 6, 7, 8]
>>> m = remove_odd_indices(l, False)
>>> m
[6, 8]
>>> remove_odd_indices([9, 8, 7, 6, 5, 4, 3], False)
[8, 6, 4]
>>> remove_odd_indices([2], False)
[]
>>> # Do not use while/for loops!
>>> from construct_check import check
>>> # ban iteration
>>> check(HW_SOURCE_FILE, 'remove_odd_indices',
...       ['While', 'For'])
True
"""
"*** YOUR CODE HERE ***"
``````

Use Ok to test your code:

``python3 ok -q remove_odd_indices``

### Q3: Smart Fridge

The `SmartFridge` class is used by smart refrigerators to track which items are in the fridge and let owners know when an item has run out.

The class internally uses a dictionary to store items, where each key is the item name and the value is the current quantity. The `add_item` method should add the given quantity of the given item and report the current quantity. You can assume that the `use_item` method will only be called on items that are already in the fridge, and it should use up the given quantity of the given item. If the quantity would fall to or below zero, it should only use up to the remaining quantity, and remind the owner to buy more of that item.

Finish implementing the `SmartFridge` class definition so that its `add_item` and `use_item` methods work as expected.

``````class SmartFridge:
""""
>>> fridgey = SmartFridge()
'I now have 1 Mayo'
'I now have 3 Mayo'
>>> fridgey.use_item('Mayo', 2.5)
'I have 0.5 Mayo left'
>>> fridgey.use_item('Mayo', 0.5)
'Oh no, we need more Mayo!'
'I now have 12 Eggs'
>>> fridgey.use_item('Eggs', 15)
'Oh no, we need more Eggs!'
'I now have 1 Eggs'
"""
def __init__(self):
self.items = {}
def add_item(self, item, quantity):
"*** YOUR CODE HERE ***"
def use_item(self, item, quantity):
"*** YOUR CODE HERE ***"
``````

You may find Python's formatted string literals, or f-strings useful. A quick example:

``````>>> feeling = 'love'
>>> course = '61A!'
>>> f'I {feeling} {course}'
'I love 61A!'``````

Use Ok to test your code:

``python3 ok -q SmartFridge``

If you're curious about alternate methods of string formatting, you can also check out an older method of Python string formatting. A quick example:

``````>>> ten, twenty, thirty = 10, 'twenty', [30]
>>> '{0} plus {1} is {2}'.format(ten, twenty, thirty)
'10 plus twenty is [30]'``````

### Q4: Vending Machine

In this question you'll create a vending machine that only outputs a single product and provides change when needed.

Create a class called `VendingMachine` that represents a vending machine for some product. A `VendingMachine` object returns strings describing its interactions. Remember to match exactly the strings in the doctests -- including punctuation and spacing!

Fill in the `VendingMachine` class, adding attributes and methods as appropriate, such that its behavior matches the following doctests:

``````class VendingMachine:
"""A vending machine that vends some product for some price.

>>> v = VendingMachine('candy', 10)
>>> v.vend()
'Nothing left to vend. Please restock.'
'Nothing left to vend. Please restock. Here is your \$15.'
>>> v.restock(2)
'Current candy stock: 2'
>>> v.vend()
'Please update your balance with \$10 more funds.'
'Current balance: \$7'
>>> v.vend()
'Please update your balance with \$3 more funds.'
'Current balance: \$12'
>>> v.vend()
'Here is your candy and \$2 change.'
'Current balance: \$10'
>>> v.vend()
'Here is your candy.'
'Nothing left to vend. Please restock. Here is your \$15.'

>>> w = VendingMachine('soda', 2)
>>> w.restock(3)
'Current soda stock: 3'
>>> w.restock(3)
'Current soda stock: 6'
'Current balance: \$2'
>>> w.vend()
'Here is your soda.'
"""
"*** YOUR CODE HERE ***"
``````

Again, you may find it useful to use f-strings.

Use Ok to test your code:

``python3 ok -q VendingMachine``

## Submit

Make sure to submit this assignment by running:

``python3 ok --submit``