Day 2 — Solutions

Control flow, loops, lists, tuples, sets

Exercise 1 — Nested conditionals

is_registered = True
at_correct_precinct = False

if is_registered:
    if at_correct_precinct:
        print("Allow to vote")
    else:
        print("Redirect to correct precinct")
else:
    print("Provisional ballot")

Redirect to correct precinct

Alternative using a single chain of conditions:

if is_registered and at_correct_precinct:
    print("Allow to vote")
elif is_registered:
    print("Redirect to correct precinct")
else:
    print("Provisional ballot")

Exercise 2 — `for` loop with a list

presidents = ["Washington", "Adams", "Jefferson", "Madison", "Monroe"]

# Part 1
for name in presidents:
    print(f"President {name}")

# Part 2
count = 1
for name in presidents:
    print(f"#{count}: {name}")
    count += 1

President Washington
President Adams
President Jefferson
President Madison
President Monroe
#1: Washington
#2: Adams
#3: Jefferson
#4: Madison
#5: Monroe

Exercise 3 — `while` loop with a condition

n = 101
while not (n % 7 == 0 and n % 11 == 0):
    n += 1
print(n)  # 154

7 * 11 = 77, so any multiple of both is a multiple of 77. The first one above 100 is 154.

Exercise 4 — Nested `while` loops

year = 2020
while year <= 2030:
    count = 0
    y = year
    while y % 4 != 0:
        count += 1
        y += 1
    print(f"{year} reaches a presidential election year ({y}) in {count} years.")
    year += 1

2020 reaches a presidential election year (2020) in 0 years.
2021 reaches a presidential election year (2024) in 3 years.
2022 reaches a presidential election year (2024) in 2 years.
2023 reaches a presidential election year (2024) in 1 years.
2024 reaches a presidential election year (2024) in 0 years.
2025 reaches a presidential election year (2028) in 3 years.
2026 reaches a presidential election year (2028) in 2 years.
2027 reaches a presidential election year (2028) in 1 years.
2028 reaches a presidential election year (2028) in 0 years.
2029 reaches a presidential election year (2032) in 3 years.
2030 reaches a presidential election year (2032) in 2 years.

Same pattern as the multiples-of-13 example from the slides, the outer loop walks through values, inner loop performs a counting computation per value. Years already divisible by 4 (2020, 2024, 2028) print 0 years because the inner loop’s condition (y % 4 != 0) is False before any iteration. Note y = year and count = 0 reset inside the outer loop so each year starts fresh.

Exercise 5 — List operations

states = ["Ohio", "Florida", "Pennsylvania", "Wisconsin", "Michigan"]

states.append("Arizona")
states.insert(2, "Georgia")
states.remove("Florida")
states.sort()

print(states)
print(len(states))

['Arizona', 'Georgia', 'Michigan', 'Ohio', 'Pennsylvania', 'Wisconsin']
6

append adds to the end, insert adds at a position, remove deletes by value, sort reorders in place.

Exercise 6 — Iterating over two lists

senators = ["Sanders", "Warren", "Cruz", "Cornyn", "Schumer"]
years = [34, 12, 13, 23, 26]

for i in range(len(senators)):
    if years[i] > 15:
        print(f"{senators[i]}: {years[i]} years")

Sanders: 34 years
Cornyn: 23 years
Schumer: 26 years

range(len(senators)) produces indices 0, 1, 2, ... up to len(senators) - 1. We then access each list with [i]. This works, but it’s clunky. Tomorrow we’ll see zip(), which produces matched pairs directly, plus a one-line version using a list comprehension.

Exercise 7 — Sets

bill_a_sponsors = ["Sanders", "Warren", "Klobuchar", "Booker", "Brown"]
bill_b_sponsors = ["Warren", "Booker", "Markey", "Whitehouse", "Klobuchar"]

a = set(bill_a_sponsors)
b = set(bill_b_sponsors)

print("Both:", a.intersection(b))
print("A only:", a.difference(b))
print("Total unique:", len(a.union(b)))

Both: {'Booker', 'Warren', 'Klobuchar'}
A only: {'Sanders', 'Brown'}
Total unique: 7

Once you’ve converted both lists to sets, use set methods `.intersection()` returns elements in both, `.difference()` returns elements in the first but not the second, and `.union()` returns all elements combined (with duplicates removed).

Exercise 8 — Putting it together

vote_counts = [4250, 3980, -1, 5120, 2890, -1, 6400, 4100]

total = 0
valid_count = 0
missing_count = 0

for v in vote_counts:
    if v == -1:
        missing_count += 1
    else:
        total += v
        valid_count += 1

mean = round(total / valid_count, 1)
print(f"Mean: {mean} ({missing_count} missing values dropped)")

Mean: 4456.7 (2 missing values dropped)

Tracking three counters in one pass is the common pattern. Tomorrow we’ll see how numpy and pandas handle missing values more gracefully (NaN).

Exercise 1 — Nested conditionals

Exercise 2 — for loop with a list

Exercise 3 — while loop with a condition

Exercise 4 — Nested while loops