Swift Recursion

A function that calls itself repeatedly is known as recursive function and this technique is called recursion. When you create a recursive function, you must create a condition so that the function does not call itself indefinitely.

Example:

func recurse() {
    //statements
    recurse()
}
recurse()

You can see in the above diagram that recursion is executed indefinitely. To get rid of these type of indefinite recursive, we use control statements.

Example: Swift 4 Program to print N positive numbers

func countDownToZero(num: Int) {
    print(num)
    if num > 0 {
        countDownToZero(num: num - 1)
    }
}
print("Countdown:")
countDownToZero(num:6)

Output:

Countdown:
6
5
4
3
2
1
0

You can see in the above program that the statement print("Countdown:") gives the output and the statement countDownToZero(num:3) calls the function that takes a parameter Integer.

The statement inside the function countDownToZero() executes and if the condition num > 0 is fulfilled, the function countDownToZero() is called again as countDownToZero(num: num - 1).

When the condition is not fulfilled, the function call is not done and the recursion stops.

Execution Steps

Steps	Function call	Printed	Is num > 0 ?
1	countDownToZero(6)	6	Yes
2	countDownToZero(5)	5	Yes
3	countDownToZero(4)	4	Yes
4	countDownToZero(3)	3	Yes
5	countDownToZero(2)	2	Yes
6	countDownToZero(1)	1	Yes
7	countDownToZero(0)	0	No (Recursion Ends)

Example2: Swift 4 program to find factorial of a number

func findFactorial(of num: Int) -> Int {
    if num == 1 {
        return 1
    } else {
        return num * findFactorial(of:num - 1)
    }
}

let x = 6
let result = findFactorial(of: x)
print("The factorial of \(x) is \(result)") 

Output:

The factorial of 6 is 720

Execution Steps

Steps	Argument passed	Return statement	Value
1	6	return 6 * findFactorial(of:5)	6 * findFactorial(of:5)
2	5	return 5 * findFactorial(of:4)	6 *5 findFactorial(of:4)
3	4	return 4 * findFactorial(of:3)	6 54 findFactorial(of:3)
4	3	return 3 * findFactorial(of:2)	6 54*3 findFactorial(of:2)
5	2	return 2 * findFactorial(of:1)	65432 findFactorial(of:1)
6	1	return 1	65432*1