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Until this point, our functions have only printed values to the terminal. Although useful, functions are a lot more powerful when they are able to pass values out of a function using a return statement. Passing a value from a function allows us to use it in other functions or save it in a variable until the need for it arises.

If we intend for a function to return a value, we must specify that value’s return type in the function definition. Take a look at the following code:

func funcName() -> Int {
 return 5
}

The function above specifies that it returns a value of type, Int in its definition. Within the body, the return statement includes 5 which matches the return type.

Note: Not specifying a return type or omitting a return statement in a function that expects a return will result in an error.

Let’s see a return statement in action. The following program determines a user’s age given their birth year and the current year:

let birthYear = 1994
var currentYear = 2021

func findAge() -> Int {
  return currentYear - birthYear
}

The findAge() function returns a value of the type, Int.
Within the body of findAge(), a return statement is used to return the difference between the currentYear and birthYear from the function.
The return statement ends the execution of a function.

To see the output of this function, we can:

1. Wrap the function call directly in a print() statement:

print(findAge()) // Prints: 27

2. Save the function call in a variable and then use a print() statement to output the value of the variable:

var age = findAge()

print(age) // Prints: 27

We’ll practice with both strategies going forward.

Instructions

Earlier in the course, we created a BMI calculator that determined a person’s Body Mass Index given their height and weight. We’ll improve upon this program by making it reusable with a function.

First, declare a function, findBMI() in BMI.swift that returns a Double value.

Note: You will see an error in the terminal on the right, but it will go away in the next step when we populate the body of the function with code. Why do you think there’s an error? (Check your answer in the hint!)

Within the findBMI() function, we’ll use the following formula as a guide to set up an expression that calculates the BMI using weight and height:

 $BMI = \frac{weight}{height^2}$

Translate the right side of the formula into code, and return it from the function.

Call the function and wrap it in a print() statement to see its output.

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