Bug type: Syntax

The def header is missing the colon that must end every function definition line, so Python cannot parse the file. Adding the colon lets the function be defined and called.

def perimeter(length, width):
    return 2 * (length + width)

print(perimeter(8, 5))   # 26

Back to Exercise 18.1.1 · sol_18_1_1.py

Bug type: Syntax

The opening parenthesis after bill * is never closed, leaving an unbalanced bracket that the parser rejects. Closing the parenthesis fixes the expression.

bill = 40.0
tip_rate = 0.15
total = bill + (bill * tip_rate)
print(total)   # 46.0

Back to Exercise 18.1.2 · sol_18_1_2.py

Bug type: Syntax

The condition uses = (assignment) where a comparison == is required, which is a syntax error inside an if. Using >= expresses the intended “score is at least 60” test.

score = 72
if score >= 60:
    print("Pass")
else:
    print("Fail")

Back to Exercise 18.1.3 · sol_18_1_3.py

Bug type: Syntax

There is a missing operator (or comma) between tuesday and wednesday; two names sitting side by side cannot be parsed. Adding the + sums all three days.

monday = 3.2
tuesday = 4.1
wednesday = 2.7
total = monday + tuesday + wednesday
print(total)   # 10.0

Back to Exercise 18.1.4 · sol_18_1_4.py

Bug type: Syntax

The print line is not indented under the for, so Python raises an IndentationError because a loop body is expected. Indenting the line by four spaces puts it inside the loop.

coins = [25, 10, 5, 1]
for coin in coins:
    print(coin)

Back to Exercise 18.1.5 · sol_18_1_5.py

Bug type: Runtime

Calling the function with hours = 0 divides by zero at run time, raising ZeroDivisionError. Guarding against a zero time (or passing a real duration) avoids the crash.

def average_speed(distance, hours):
    if hours == 0:
        return "Time cannot be zero"
    return distance / hours

print(average_speed(120, 2))   # 60.0

Back to Exercise 18.2.1 · sol_18_2_1.py

Bug type: Runtime

The expression references tax, but the variable defined above is tax_rate, so Python raises a NameError. Using the correct name computes the taxed price.

price = 50.0
tax_rate = 0.08
final_price = price + (price * tax_rate)
print(final_price)   # 54.0

Back to Exercise 18.2.2 · sol_18_2_2.py

Bug type: Runtime

cups_of_flour is a string, and multiplying a string by a float raises TypeError. Storing the quantity as a number lets the multiplication work.

cups_of_flour = 2.0
doubled = cups_of_flour * 2.0
print(doubled)   # 4.0

Back to Exercise 18.2.3 · sol_18_2_3.py

Bug type: Runtime

len(scores) is 4, but valid indices run 0 to 3, so scores[4] raises IndexError. The last element is at index len(scores) - 1 (or simply -1).

scores = [88, 91, 79, 95]
last_index = len(scores) - 1
print(scores[last_index])   # 95

Back to Exercise 18.2.4 · sol_18_2_4.py

Bug type: Runtime

The key "mars" is lowercase, but the dictionary key is "Mars"; dictionary lookups are case sensitive, so this raises KeyError. Matching the stored key returns the value.

moons = {"Earth": 1, "Mars": 2, "Venus": 0}
print(moons["Mars"])   # 2

Back to Exercise 18.2.5 · sol_18_2_5.py

Bug type: Logical

The program runs but uses + where area requires multiplication, so it returns 10 instead of 24. Multiplying length by width gives the correct area.

def area(length, width):
    return length * width

print(area(6, 4))   # 24

Back to Exercise 18.3.1 · sol_18_3_1.py

Bug type: Logical

Operator precedence divides only c by 3 before adding, so the result is wrong. Parenthesizing the sum before dividing computes the true average.

def average(a, b, c):
    return (a + b + c) / 3

print(average(80, 90, 100))   # 90.0

Back to Exercise 18.3.2 · sol_18_3_2.py

Bug type: Logical

range(1, 10) stops at 9 because the upper bound is excluded, so 10 is never printed (an off-by-one error). Using range(1, 11) includes 10.

for number in range(1, 11):
    print(number)

Back to Exercise 18.3.3 · sol_18_3_3.py

Bug type: Logical

Multiplying by the discount rate gives the amount taken off, not the price paid, so the result is 20 instead of 60. The customer pays the remaining fraction, 1 - discount_rate.

price = 80.0
discount_rate = 0.25
final_price = price * (1 - discount_rate)
print(final_price)   # 60.0

Back to Exercise 18.3.4 · sol_18_3_4.py

Bug type: Logical

The conversion factor is inverted: Fahrenheit to Celsius multiplies by 5 / 9, not 9 / 5. Swapping the fraction gives the right temperature.

def f_to_c(f):
    return (f - 32) * 5 / 9

print(f_to_c(212))   # 100.0

Back to Exercise 18.3.5 · sol_18_3_5.py

Bug type: Logical

Printing the subtotal reveals it is too low: the milk price is subtracted instead of added. Changing the - to + totals all three items.

def total_cost(apples, bread, milk):
    subtotal = apples + bread + milk
    return subtotal

print(total_cost(3.50, 2.25, 1.75))   # 7.5

Back to Exercise 18.4.1 · sol_18_4_1.py

Bug type: Logical

Printing the returned value shows it is half the expected size: the formula omits the factor of 2 (circumference is 2 * pi * radius). Adding the 2 corrects it.

def circumference(radius):
    pi = 3.14159
    return 2 * pi * radius

print(circumference(5))   # about 31.4

Back to Exercise 18.4.2 · sol_18_4_2.py

Bug type: Logical

Printing total inside the loop shows it only ever holds the latest value: the line replaces the running sum instead of adding to it. Using += accumulates the total.

rainfall = [1.2, 0.8, 2.0, 1.5]
total = 0
for amount in rainfall:
    total += amount
print(total)   # 5.5

Back to Exercise 18.4.3 · sol_18_4_3.py

Bug type: Logical

Printing sale_price shows it equals the discount amount, not the discounted price, so the subtraction yields the wrong savings. The savings are simply price * discount_rate.

def savings(price, discount_rate):
    return price * discount_rate

print(savings(120, 0.30))   # 36.0

Back to Exercise 18.4.4 · sol_18_4_4.py

Bug type: Logical

Printing result reveals the division is inverted: speed is distance divided by time, not time divided by distance. Swapping the operands gives the correct speed.

def speed(distance_km, time_hours):
    result = distance_km / time_hours
    return result

print(speed(150, 3))   # 50.0

Back to Exercise 18.4.5 · sol_18_4_5.py