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Swift Recursion

In this tutorial, we will learn about recursive function in Swift and its working with the help of examples.

A function that calls itself is known as a recursive function. And, this technique is known as recursion.

A physical world example would be to place two parallel mirrors facing each other. Any object in between them would be reflected recursively.


Working of Recursion in Swift

func recurse() {
  ... ...  
  recurse()
  ...  ...     
}

recurse()

Here, the recurse() function is calling itself over and over again. The figure below shows how recursion works.

Swift Recursion
Working of Function Recursion in Swift

Stopping Condition for Recursion

If we don't mention any condition to break the recursive call, the function will keep calling itself infinitely.

We use the if...else statement (or similar approach) to break the recursion.

Normally, a recursive function has two branches:

  • One for recursive calls.
  • Another for breaking the call under certain conditions.

For example,

func recurse() {

  if(condition) {
    // break recursive call
    recurse()
  }

  else {
    // recursive call
    recurse()
  }
}

// function call 
recurse()

Example 1: Swift Function Recursion

// program to count down number to 0

func countDown(number: Int) {

  // display the number
  print(number)

  // condition to break recursion
  if number == 0 {
  print("Countdown Stops")
  }

  // condition for recursion call
  else {
  
    // decrease the number value
    countDown(number: number - 1)
  }
}


print("Countdown:")
countDown(number:3)

Output

Countdown:
3
2
1
0
Countdown Stops

In the above example, we have created a recursive function named countDown(). Here, the function calls itself until the number passed to it becomes 0.

When the number is equal to 0, the if condition breaks the recursive call.

if number == 0 {
print(Countdown Stops)
}

Working of the program

Iteration Function call Print number == 0 ?
1 countDown(3) 3 false
2 countDown(2) 2 false
3 countDown(1) 1 false
4 countDown(0) 0 true(function call stops)

Example: Find factorial of a number

func factorial(num: Int) -> Int {

  // condition to break recursion
  if num == 0 {
    return 1
  } 

  // condition for recursive call
  else {
    return num * factorial(num: num - 1)
  }

}

var number = 3

// function call
var result = factorial(num: number)
print("The factorial of 3 is", result)

Output

The factorial of 3 is 6

In the above example, we have a recursive function named factorial(). Notice the statement

return num * factorial(num: num - 1)

Here, we are recursively calling factorial() by decreasing the value of the num parameter.

  • Initially, the value of num is 3 inside factorial().
  • In the next recursive call, num becomes 2.
  • Similarly, the process continues until num becomes 0.
  • When num is equal to 0, the if condition breaks the recursive call.

Working of the Program

Computing Factorial Using Recursion
Computing Factorial Using Recursion

Advantages and Disadvantages of Function Recursion

Below are the advantages and disadvantages of using recursion in Swift programming.

1. Advantages

  • It makes our code shorter and cleaner.
  • Recursion is required in problems concerning data structures and advanced algorithms, such as Graph and Tree Traversal.

2. Disadvantages

  • It takes a lot of stack space compared to an iterative program.
  • It uses more processor time.
  • It can be more difficult to debug compared to an equivalent iterative program.
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