Swift Recursion

In this article, you will learn to create recursive functions. A self-calling function.

Calling itselfFunctionCalled recursive function. And, this technique is called recursion. When creating a recursive function, you must create conditions so that the function does not call itself indefinitely.

How does recursion work in Swift?

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

The following figure shows the way of recursive calls.

In the above flowchart, recursion is executed infinitely. In some cases, you may need to create a recursion that will continue to execute until certain conditions are met.

To prevent infinite recursion, use recursive calls within Swift conditional statements, for exampleif ... else statement.

Example1Print N positive numbers

func　countDownToZero(num:　Int)　{
　　　　print(num)
　　　　if　num　>　0　{
　　　　　　　　countDownToZero(num:　num　-　1)
　　　　}
}
print("Countdown:")
countDownToZero(num:3)

When you run the following program, the output will be:

Countdown:
3
2
1
0

In the above program, the statement print("Countdown:") outputs to the consoleCountdown：This statement calls the function countDownToZero(num: with an Integer type parameter3).

countDownToZero() executes the statements inside the function, if the condition num > 0 is met, countDownToZero() calls the function countDownToZero(num: num again - 1).

If the condition is not met, do not execute the function call and stop the recursion.

Let's take a look at the specific steps

Example2Find the factorial of a number

func factorial(of num: Int)　-> Int
　　　　if num ==　1　{
　　　　　　　　return　1
　　　　}
　　　　　　　　return num　*　factorial(of:num　-　1)
　　　　}
}
let x =　4
let result = factorial(of: x)
print("The factorial of \(x) is \(result)")

When you run the following program, the output will be:

The factorial of　4　is　24

How does this instance work?

Let's look at it step by step

When a solution to a problem can be found in about two steps, recursion is often used as a replacement for iteration.