Algorithms & Testing — Lesson 2: Break a problem into helper functions

New skill: Decomposition, pure functions
Previously learned: Units 01–09

Why this matters

Large apps become easier to debug when their calculation rules are separated into reusable pieces.

Step 1 — Explore working Python

Read the program below. Before running it, write down the result you expect.

def nonnegative(value):
    return max(0, value)

def total_points(scores):
    total = 0
    for score in scores:
        total += nonnegative(score)
    return total

print(total_points([8, -2, 5]))

Step 2 — Trace the code

1. Read the helper’s contract: negative numbers become zero.
2. Trace how the main function calls the helper for each item.
3. Combine helper results into a total rather than writing one giant expression.

Step 3 — Check your understanding

What does total_points([-2, -1, 7]) return?

Step 4 — Watch for this mistake

Do not forget the return statement in either function. A missing return produces None.

Practice: Break a problem into helper functions

Complete total_with_bonus(scores, bonus) to sum only positive scores, then add bonus. Zero and negative scores contribute nothing. You may create helper functions.

Function contract: total_with_bonus(scores, bonus)
Starter file: main.py

Work in the editor, using Run to explore and Check to compare against the published examples. Your submitted code, not the public check, determines the recorded score.

Hint 1 — Start

Start a total at zero.

Hint 2 — Build

Only add a score when it is greater than zero.

Hint 3 — Test

Return the total plus the bonus after the loop.

Reflect

In 3–5 sentences, explain the algorithm you chose, one test case you predicted, and how you would detect an incorrect answer.

Optional reading

For another perspective on this skill, see How to Think Like a Computer Scientist (interactive textbook). This Playground lesson, worked example, and exercise are original; no textbook passages were reproduced.