Project: Grade calculator

Milestone 1: the grade chain

Start with a fixed score and an if / elif / else chain. Remember lesson 2.2: only the first true branch runs, so the order matters.

score = 90
# Print the grade: A, B, C, or F

score = 90
if score >= 85:
    print("Grade: A")
elif score >= 70:
    print("Grade: B")
elif score >= 50:
    print("Grade: C")
else:
    print("Grade: F")

Before you move on, break it on purpose: move the score >= 50 branch to the top and run it with 90. You get a C, the bug from lesson 2.2. Now you'll recognize it when it happens by accident.

Milestone 2: asking, and saying no

Now ask for the score. A real user will make typos, so check the score is possible before grading it. A score below 0 or above 100 gets a clear message instead of a grade. It can be the first branch of the chain:

score = 90
if score >= 85:
    print("Grade: A")
elif score >= 70:
    print("Grade: B")
elif score >= 50:
    print("Grade: C")
else:
    print("Grade: F")

score = int(input("Score (0-100): "))
if score < 0 or score > 100:
    print("Please enter a score from 0 to 100.")
elif score >= 85:
    print("Grade: A")
elif score >= 70:
    print("Grade: B")
elif score >= 50:
    print("Grade: C")
else:
    print("Grade: F")

Why must the check go first? Predict what this version prints for 101.

score = 101
if score >= 85:
    print("Grade: A")
elif score < 0 or score > 100:
    print("Please enter a score from 0 to 100.")

Grade: A

One kind of bad input still gets through. If someone types a word, int() can't convert it (lesson 1.5):

Milestone 3: grade points

Now print the points too. Instead of printing in every branch, give two names, grade and points, a value in each branch, and print once at the end. That print only makes sense for a valid score, so the grade chain moves inside the else. (This else holds more than an if, so it can't be flattened into elif.)

score = int(input("Score (0-100): "))
if score < 0 or score > 100:
    print("Please enter a score from 0 to 100.")
else:
    # Give grade and points a value in each branch, then print them
    pass

score = int(input("Score (0-100): "))
if score < 0 or score > 100:
    print("Please enter a score from 0 to 100.")
else:
    if score >= 85:
        grade = "A"
        points = 4.0
    elif score >= 70:
        grade = "B"
        points = 3.0
    elif score >= 50:
        grade = "C"
        points = 2.0
    else:
        grade = "F"
        points = 0.0
    print(f"Grade: {grade} ({points} points)")

pass in the starter means "do nothing": a block can't be empty, so it holds the place until you write the real lines. Delete it when you do.

Test every boundary

A grade calculator is only right if it's right at the edges. Type each score in the Input box and check your output against this table, character by character. Every row is a place where a > instead of >=, or a wrong order, would show up.

Milestone 4: a weighted GPA (stretch)

A GPA weighs each course by its credit hours: multiply each course's points by its credit hours, add them up, and divide by the total credit hours. A 3-credit A counts more than a 2-credit C.

Turning three scores into points would mean writing the grade chain three times. In Module 3 you'll learn to repeat code without copying it, so for now, ask for each course's points directly (your milestone 3 program tells you them).

points1 = float(input("Course 1 points: "))
credits1 = int(input("Course 1 credit hours: "))
# Ask for courses 2 and 3, then work out and print the GPA

points1 = float(input("Course 1 points: "))
credits1 = int(input("Course 1 credit hours: "))
points2 = float(input("Course 2 points: "))
credits2 = int(input("Course 2 credit hours: "))
points3 = float(input("Course 3 points: "))
credits3 = int(input("Course 3 credit hours: "))
weighted = points1 * credits1 + points2 * credits2 + points3 * credits3
total_credits = credits1 + credits2 + credits3
print(f"GPA: {weighted / total_credits:.2f}")

The plain average of 4.0, 3.0, and 2.0 would be 3.00. The weighted GPA is 3.11, because the A carries three credits and the C only two.