HW 2: Loops & lists

CMSC 141: Introduction to Python

Due Sunday, July 5, 11:59pm

A loop lets a program repeat work without you writing the same line over and over. This homework uses both kinds. A for loop runs a set number of times; a while loop runs until some condition stops being true. You will also work with lists, which hold many values in order, and near the end you will call one of your own functions from inside another.

Everything here was covered in class. Stick to for loops, range(), lists and .append(), the % operator, and while loops. You do not need anything we have not seen yet.

How the file works

Run git pull to get the assignment, then cd hw2. All of your work goes in hw2.py, which has six blocks that each begin with the word def, one per exercise. The values in parentheses after each name are what you are given to work with; use those names rather than specific numbers, so your code works for every case the tests try. Whatever the block should produce goes after the word return.

The accumulator pattern

Most of this homework is one idea reused. You start a variable at an empty value, then update it once per loop. To add up numbers you start a total at 0 and add to it. To collect values you start a list at [] and append to it. Here is the shape for a running total:

total = 0
for i in range(4):    # i takes the values 0, 1, 2, 3
    total = total + i
# total is now 0 + 1 + 2 + 3 = 6

Notice that range(4) stops before 4, so it gives you 0, 1, 2, 3. That off-by-one is the most common loop mistake, so keep it in mind.

The exercises

Exercise 1: sum_range(n)

Return 0 + 1 + 2 + ... + n using a for loop. Since range stops just before its argument, range(n + 1) gives you the values 0 through n.

>>> sum_range(5)
15

Exercise 2: count_evens(numbers)

Take a list of whole numbers and return how many of them are even. Loop over the list and use n % 2 == 0 to test whether a number n is even. The % operator gives the remainder, so an even number leaves remainder 0.

>>> count_evens([1, 2, 3, 4, 6])
3

Exercise 3: largest(numbers)

Return the biggest value in a non-empty list without using Python's built-in max(). Start by assuming the first item is the biggest, then loop through and update your guess whenever you find something larger.

>>> largest([3, 9, 2, 9, 1])
9

Exercise 4: squares(n)

Return a list of the squares from 0 up to and including n. This time you are accumulating into a list: start with [] and use .append() to add each square as you go.

>>> squares(4)
[0, 1, 4, 9, 16]

Exercise 5: percent_even(numbers)

Return what percentage of the list is even, rounded to a whole number, and return 0 for an empty list. This is the first exercise where you call a function you already wrote: instead of looping again, call your count_evens(numbers) to get the count, then turn that into a percentage of len(numbers) and round it with round(...). Handle the empty list first, since you cannot divide by zero.

>>> percent_even([1, 2, 3, 4])
50
>>> percent_even([])
0

A loop that stops on a condition

A for loop runs a known number of times. A while loop runs as long as a condition holds, which is what you want when you do not know the count in advance. Python checks the condition before each pass, so the body has to change something the condition looks at, or the loop never ends.

Exercise 6: months_to_goal(monthly, goal)

Suppose you save the same amount every month. Return how many whole months it takes for your running total to reach or pass the goal. Use a while loop: keep a running total and a month count, and as long as the total is below the goal, add one month of savings and add one to the count. You may assume monthly is a positive number.

>>> months_to_goal(100, 950)
10
>>> months_to_goal(100, 0)
0

With $100 a month and a $950 goal, after 9 months you have saved $900, which is not enough, so the loop runs once more and the answer is 10. Make sure the total goes up on every pass; a while loop whose total never changes will run forever.

Testing your work

We use pytest to check the six functions. From the hw2 directory, run all the tests with:

uv run pytest

To focus on a single function while you work, add the flags -xvk followed by its name. The -k keeps only the tests whose name contains what you type, -x stops at the first failing case so you can fix one thing at a time, and -v lists each case as it runs:

uv run pytest -xvk largest

Each case shows up as a pass or a fail, and a failing case prints the value your function returned next to the value the test expected. Fix one function at a time and rerun.

What to turn in

Fill in your answers in hw2.py and answer the short questions in QUESTIONS-02.txt. Then commit and push your work, and submit on Gradescope, which pulls your pushed code from GitHub:

git add hw2.py QUESTIONS-02.txt
git commit -m "Complete HW 2"
git push