Unit testing - How would you test functions without any parameters?

So I am extremely new to unit testing! I know how to create some test if in my program there are functions with parameters. How would I test functions without them? Please excuse my lack of knowledge but I have been searching around and have not found any answers!

I am adding code to show how I would test a function with parameters. How would I create a test with a function that takes in no parameters?

Thanks!

func calculateTip(billAmount: Float?, taxPercentage: Float?, tipPercentage: Float?) -> Float? {

    let bill = billAmount!
    let pTax = taxPercentage!
    let pTip = tipPercentage!

    let tax = bill * pTax
    let total = bill + tax
    let tip = total * pTip

    return tip

}

Unit test--

func testAllZerosShouldReturnZero() {



    let result = calc.calculateTip(0, taxPercentage: 0, tipPercentage: 0)

    XCTAssertEqual(result, 0, "should be zero")


}