Recursion in C++

The process in which a function calls itself is known as recursion and the corresponding function is called the recursive function. The popular example to understand the recursion is factorial function.
Factorial function: f(n) = n*f(n-1), base condition: if n<=1 then f(n) = 1. Don’t worry we wil discuss what is base condition and why it is important.
In the following diagram. I have shown that how the factorial function is calling itself until the function reaches to the base condition.
Lets solve the problem using C++ program.

C++ recursion example: Factorial

#include <iostream>
using namespace std;
//Factorial function
int f(int n){
   /* This is called the base condition, it is
    * very important to specify the base condition
    * in recursion, otherwise your program will throw
    * stack overflow error.
    */
   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


Base condition

In the above program, you can see that I have provided a base condition in the recursive function. The condition is:
if (n <= 1)
        return 1;
The purpose of recursion is to divide the problem into smaller problems till the base condition is reached. For example in the above factorial program I am solving the factorial function f(n) by calling a smaller factorial function f(n-1), this happens repeatedly until the n value reaches base condition(f(1)=1). If you do not define the base condition in the recursive function then you will get stack overflow error.

Direct recursion vs indirect recursion

Direct recursion: When function calls itself, it is called direct recursion, the example we have seen above is a direct recursion example.
Indirect recursion: When function calls another function and that function calls the calling function, then this is called indirect recursion. For example: function A calls function B and Function B calls function A.

Indirect Recursion Example in C++

#include <iostream>
using namespace std;
int fa(int);
int fb(int);
int fa(int n){
   if(n<=1)
      return 1;
   else
      return n*fb(n-1);
}
int fb(int n){
   if(n<=1)
      return 1;
   else
      return n*fa(n-1);
}
int main(){
   int num=5;
   cout<<fa(num);
   return 0;
}
Output:
120

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