Functions challenge: parameters + return values

Learning intentions

In this challenge, you will combine everything from this level into one complete program.

• Use parameters to pass data into functions.
• Use return values to send results back.
• Choose clear function roles so logic is reusable.

You will also practise program structure.

• Build a menu loop with user choices.
• Keep input, calculation, and output in separate functions.

Challenge overview

You are building a small command-line app called Egg Shop.

• The app tracks egg stock and sales.
• The user can add eggs, sell eggs, and view stock summaries.
• All calculations must happen through functions with parameters and return values.

This is one longer task, not separate mini tasks.

• Plan your functions first.
• Then build the full program and test each menu option.

Program requirements

Your program must show this menu repeatedly until the user exits.

==== Egg Shop ====
1. Add eggs
2. Sell eggs
3. Show current stock
4. Show total eggs sold
5. Exit
Choose an option:

Use variables to track stock and sales, for example:

@main
struct SwiftPlayground {
    static func main() {
        var eggsInStock = 0
        var eggsSold = 0
    }
}

Required functions

Create and use functions like these (you may add more):

func menuChoice() -> Int
func addEggs(currentStock: Int, amount: Int) -> Int
func sellEggs(currentStock: Int, amount: Int) -> Int?
func updateSoldCount(currentSold: Int, amount: Int) -> Int
func stockMessage(stock: Int) -> String

Use parameters to pass in data.

• Example: amount should be passed in as a parameter, not hard-coded.

Use return values for calculated results.

• addEggs returns the new stock count.
• sellEggs returns an optional because a sale may fail (can’t be completed).

Behaviour rules

Your app should handle normal and edge cases safely.

• If the user chooses an invalid menu option, show an error and continue.
• If the user enters a quantity less than 1, reject it.
• If the user tries to sell more eggs than available stock, show an error and do not change values.
• Option 3 should print a clear stock message.

Suggested build order

1. Write menuChoice() and the main while loop.
2. Implement addEggs(currentStock:amount:).
3. Implement sellEggs(currentStock:amount:).
4. Implement updateSoldCount(currentSold:amount:).
5. Implement stockMessage(stock:).
6. Connect each menu option and test all paths.

Test checklist

Run your app and verify each case.

• Add eggs at least twice.
• Try one invalid quantity (like 0 or -3).
• Sell eggs and confirm stock decreases.
• Try to sell more than stock and confirm it is blocked.
• Confirm total eggs sold updates correctly.
• Confirm the app exits cleanly when option 5 is selected.

Extension for Super Players!

Improve your app with one extra feature.

• Add an option to reset the shop for a new day.
• Add an option to print a low-stock warning when stock is under a threshold.
• Add a function that returns a daily summary sentence.

Review

After completing the program, explain these in your own words.

• Why is amount better as a parameter?
• Why does sellEggs return an optional?
• Which parts of your program became easier to test because of return values?