# 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.

```dice
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))
```

## 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](https://www.dndbeyond.com/sources/dnd/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](index.html).
For a complete self-contained application, see [dnd2014-ability-check](../cookbook/dnd2014-ability-check.html).