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A function returns a reusable calculation

Part F · L21 · Prerequisite: L20 · One modelling idea

Situation, question, and prediction

You have already written the same ordinary D&D check calculation several times. How can one definition answer different DC and bonus questions without copying the formula? Predict what should change between calling a helper and displaying its result.

Build

def defines a function. Its parameters name inputs local to that call. Indented statements form the body; return gives the computed value back to the caller. The helper does not call output: calculations and presentation have separate jobs. Calling it uses parentheses with actual arguments.

def check_chance(bonus, dc):
    roll = d(20) + bonus
    return roll.p_ge(dc)

rows = [("Bonus 2 at DC 15", check_chance(2, 15)),
        ("Bonus 5 at DC 15", check_chance(5, 15)),
        ("Bonus 5 at DC 20", check_chance(5, 20))]
output("Reusable ability-check helper", prob_table(rows))

Reusable ability-check helper · Table

Run, read, and check

The rows reproduce L06: 0.4, 0.55, 0.3. A function packages familiar reasoning; it should not hide a new game rule. Its local variable roll does not need to exist outside the function.

Change one thing

Call check_chance(5, 26). Answer: zero. A helper still needs boundary checks; abstraction does not certify it.

Try it yourself

Write a helper accepting a percentile skill and returning its regular-difficulty chance. Answer shape: define one parameter, return d(100).cdf(skill), and keep scope 1–99. Call it for 40 and 65 to get 0.4 and 0.65.

Rules and model notes

D&D Basic Rules 2014, Using Ability Scores. Ordinary checks only; no attack exceptions, rerolls, or special features.

What you now know / where next

A function returns a reusable calculation is the reusable idea. Follow the generated previous/next links below, or return to the course index. For a complete self-contained application, see dnd2014-ability-check.

Content ID: L21 · Review status: pilot

Prerequisites: L20