C++ Recursion

Recursion

The process in which a function calls itself directly or indirectly is called recursion and the corresponding function is called as recursive function.

Recursion has got a problem-solving tool, where it dives the larger problems into simple tasks and wirking out individually to follow an individual sequence.

Syntax of Recursion Function

The general syntax of the recursion function in C++ is given as:

return type function name([arguments])
{
    Body of the statements;
    function name ([actual arguments])        // recursive function
}

Working of Recursion in C++

void recurse()
{
    ... .. ...
    recurse(); // function calls itself
    ... .. ...
}

int main()
{
    ... .. ...
    recurse();
    ... .. ...
}

Working

Recursion performs repetition on the function calls, and it stops the execution when the base case becomes true.
In Simple words, The recursion continues until some condition is met.
A base case condition should be defined in the recursive function to avoid stack overflow errror messages.
If no base case is defined it leads to infinite recursion.
To prevent infinite recursion, if…else statement (or similar approach) can be used where one branch makes the recursive call and the other doesn’t.

Example 1: Factorial of a Number Using Recursion

// Factorial of n = 1*2*3*...*n

#include <iostream>
using namespace std;

int fact(int);

int main() {
    int num, result;

    cout << "Enter positive number: ";
    cin >> num;

    result = fact(num);
    cout << "Factorial of " << num << " = " << result;
    return 0;
}

int fact(int num) {
    if (num > 1) {
        return num * fact(num - 1);
    } else {
        return 1;
    }
}

Output

Enter a positive number: 5
Factorial of 5 = 120

Working of Factorial Program

As we can see, the fact() function is calling itself. However, during each call, we have decreased the value of num by 1. When num is less than 1, the fact() function ultimately returns the output.

Types of recursion

There are two types of recursion

Direct recursion
Indirect recursion

Direct recursion

Direct recursion : When function calls itself, it is known as direct recursion.

The example, we have seen above is a direct recursion example.

Indirect recursion

Indirect recursion : When function calls another function and that function calls the calling function, then this is known as Indirect recursion.

For example : Function A calls function B and function B calls function A.

Example 2: Find Factorial using Indirect recursion

// Factorial of n = 1*2*3*...*n
#include <iostream>
using namespace std;

int factA(int);
int factB(int);

int factA(int n){
    if (n <= 1) {
        return 1;
    } else {
        return  n * factB(n - 1);
    }
}

int factB(int n){
    if (n <= 1) {
        return 1;
    } else {
        return  n * factA(n - 1);
    }
}

int main() {
    int num;

    cout << "Enter positive number: ";
    cin >> num;

    cout << "Factorial of " << num << " = " << factA(num);
    return 0;
}