C++ Recursion

Recursion is a process in which function calls itself repeatedly until a certain condition is met and the corresponding function is called the recursive function. A function said to be recursive if a function calls itself.

Factorial Function

We will discuss factorial function using recursion.

Syntax Of Factorial Functions

fact(int n)
{
   if(n==1) return 1;
   else
   return n*fact(n-1);   // recursive call, fact() calling itself
}

So the above function fact() invokes itself to find the factorial of n - 1.

  Say for the first time n=4.
  1. fact(4) = { (4==1) is false thus return 4*fact(3) }
  2. fact(3) = { (3==1) is false thus return 3*fact(2) }
  3. fact(2) = { (2==1) is false thus return 2*fact(1) }
  4. fact(1) = { (1==1) is true thus return 1 }

The result of fact (1) returning to step 3, fact (2) 2 * 1 then returning to fact (3), which is 3 * 2 * 1 and finally fact (4) returning 4 * 3 * 2 * 1 which Is equal to 24.



Example Of Factorial Functions

#include 
using namespace std;
int f(int n)
{
  if (n <= 1)
    return 1;
  else
    return n*f(n-1);
}

int main()
{
  int num;
  cout<<"Enter a number: ";
  cin>>num;
  cout<<"Factorial of entered number: "<< f(num);
  return 0;
}

Output :
Enter a number: 5
Factorial of entered number: 120


In the example above, the f() function is used to calculate the Factorial of entered number.



Return Function

Return function is used to return the control to calling function. A function terminates either by the return statement or when it reaches to the end of the statments.

Syntax

The general form of return statement is as follows:

Factorialreturn(value);

A void function can do return

void func()
  {
    cout << "A void function can do return";

    // We can write return in void
    return;
  }

int main()
{
   func();
   return 0;
}


Output:

A void function can do return

A void fun() can return another void function

void func()
  {
     cout << "The void function has returned "
     " a void() !!! \n";
  }

  // Driver void() returning void work()
void xyz()
  {
     // Returning void function
     return func();
  }

int main()
{
     // Calling void function
     xyz();
     return 0;
}
  


Output:

The void function has returned a void() !!!

A void() can return a void value

#include<iostream.h>
using namespace std;
void xyz()
  {
     cout << "A void() can return a void value";

     // Returning a void value
     return (void)"Doesn't Print";
  }
int main()
{
     xyz();
     return 0;
}


Output:

A void() can return a void value


Exercise:-

1. In which circumstances recursion function is called?

A. To call the same function again inside the function itself
B. For calling maths function
C. To calling input and output stream
D. All of them

View Answer


2. Recursive call is also known as

A. Complex calls
B. Operator call
C. Recursion step
D. None of them

View Answer



Program :-

Program for Fibonacci numbers

#include<bits/stdc++.h> 
using namespace std; 
  
int fib(int n) 
{ 
    if (n <= 1) 
        return n; 
    return fib(n-1) + fib(n-2); 
} 
  
int main () 
{ 
    int n = 9; 
    cout << fib(n); 
    getchar(); 
    return 0; 
} 


Output :

34





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