Why Recursive Function returns “Undefined”?

What is Recursion?

In simple words, recursion is when a function is calling itself directly or indirectly. 

It is used to break down the problem into subproblems which makes us easy to solve. It also reduces the length of the code. It has certain advantages over just iterating through that is discussed here.

The function of recursion looks simply like this:

recursion_function (parameters){
    //  Code segments
    recursion_function(parameters);
}

Example of a recursive function returning “Undefined”:

Sometimes the recursive function returns undefined. Let’s understand it with an example of calculating the factorial of a number n.

Now if we call this recursive method using a value for num, say 5, it should give us 120. But instead, it returns “undefined” [“None” for Python, and error in Java].

C++

#include <iostream>

using namespace std;

int fact(int n, int res) {
  if (n == 1) {
    res;
  }
  // Calling function recursively
  return fact(n - 1, n * res);
}

// Driver code
int main() {
  cout << fact(5, 1) << endl;
  return 0;
}

Java

/*package whatever //do not write package name here */

import java.io.*;

class GFG {

  public static int fact(int n, int res)
  {
    if (n == 1)
      return res;
    fact(n - 1, n * res);
  }

  public static void main(String[] args)
  {
    System.out.println(fact(5,1));
  }
}

// This code is contributed by akashish__

Python3

def fact(n, res = 1):
    if n == 1:
        return res
    fact(n - 1, n * res);

print(fact(5));

C#

using System;

public class GFG{

  public static int fact(int n, int res = 1){
    if( n == 1)
        return res;
    fact(n - 1, n * res);
  }

    static public void Main (){

        Console.WriteLine(fact(5));
    }
}
// This code is contributed by akashish__

Javascript

function fact(n, res = 1) {
 if (n === 1) {
   return res;
 }
 // Calling function
 fact(n - 1, n * res);
}

// Driver code
console.log(fact(5));

Output

undefined

Why the call returns Undefined?

Let us come to the basics of recursion and recall what we need in every recursive method:

1. Base Case
2. Recursive function call
3. Return statement

Now let us compare our above code with the above three points. The code has:

Base case:

if (n == 1)
    return res;

Recursive function call

fact( n – 1, n * res);

But the code doesn’t have a global return statement. As a result, the value is getting calculated every time but it is returning nothing, which leads to the output “undefined“.

Solution: 

Insert a global return statement in the code and it should work fine. This ensures from any path, we always return something.

C++

// C++ Code
#include <iostream>
using namespace std;

int fact(int n, int res)
{
  if (n == 1) {
    return res;
  }

  // Return whatever value we get after calling this
  // function
  return fact(n - 1, n * res);
}

int main()
{
  int res = 1;
  cout<<(fact(5, res))<<endl;
}

// This code is contributed by Aman Kumar.

Java

// Java Code
import java.io.*;
class GFG {

  static int fact(int n, int res)
  {
    if (n == 1) {
      return res;
    }

    // Return whatever value we get after calling this
    // function
    return fact(n - 1, n * res);
  }

  public static void main(String[] args)
  {
    int res = 1;
    System.out.print(fact(5, res));
  }
}

// This code is contributed by lokesh.

Python3

def fact(n, res = 1):
    if n == 1:
        return res
    # Return whatever value we get after
    # calling this function
    return fact(n - 1, n * res)


# Driver code
print(fact(5))

C#

using System;

public class GFG {

  public static int fact(int n, int res)
  {
    if (n == 1) {
      return res;
    }

    // Return whatever value we get after calling this
    // function
    return fact(n - 1, n * res);
  }

  static public void Main()
  {

    int res = 1;
    Console.WriteLine(fact(5, res));
  }
}
// This code is contributed by akashish__

Javascript

function fact(n, res = 1) {
  if (n === 1) {
    return res;
  }
  // Return whatever value we get after
  // calling this function
  return fact(n - 1, n * res);
}

// Driver code
console.log(fact(5));

Output

120

Time Complexity: O(n)
Auxiliary Space: O(1)