Swift Nested Function

A function inside the body of another function is called nested function.

Syntax:

func function1() {
    //statements of outer function
    func function2() {
        //statements of inner function
    }
}

Parameter Explanation:

Here, function1 is an outer function because there is another function named function2 (nested function) inside the function1.

Note: The inner function can only be called and used inside the outer function.

Nested Function without Return Values

func wish1(_ message: String) {    
    func wish2() {
        print("Good Morning! \(message)")
    }
    wish2 ()
}
wish1("Ajeet")

Output:

Good Morning! Ajeet

In the above program, the nested function wish2() is being called from the outerfunction wish1().

The statement wish1("Ajeet") calls the outer function and the statement wish2 () inside the outer function calls the method which gives the output Good Morning Ajeet.

You cannot call the function wish2 outside of the function wish1.

Nested function with parameters and Return value

The nested functions can also contain parameters and return values.

Example:

func operate(with symbol:String) -> (Int, Int) -> Int {
    func add(num1:Int, num2:Int) -> Int {
        return num1 + num2
    }
    func subtract(num1:Int, num2:Int) -> Int {
        return num1 - num2
    }
    let operation = (symbol == "+") ?  add : subtract
    return operation
}
let operation = operate(with: "+")
let result = operation(10, 20)
print(result)

Output:

In the above program,

The outer function is operate(), with return value of type Function (Int,Int) -> Int.
The inner (nested) functions are add() and subtract().

The nested function add() and subtract() in a way are being used outside of the enclosing function operate(). This is possible because the outer function returns one of these functions.

We've used the inner function outside the enclosing function operate() as operation(10, 20). The program internally calls add(10, 20) which provides an output 30.

Next TopicSwift Function Parameter and Return Value

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