Prerequisite: The learner must know how to write a simple function in any language with parameters.

Now, after understanding the basics of recursion, the recursion tree, and the base case of recursion we can solve some basic problems of recursion which would strengthen our concepts and make us understand how recursion functions at its core. In this article, we’ll print integers from N to 1 without using any Global Variable but by using function parameters. 

Problem: Print from N to 1 using Recursion

Since in this problem, there is no global variable that can be decremented each time we call a function, how can we keep a track of the number of integers being printed on the output screen?

For this problem, we’re going to be using a function along with parameters that get decremented with each function call through which we can keep track of the integer count while printing. To understand this problem better, let us see the pseudocode below for this : 

void func( i, n )
{
   if(i<1) return;
 print(i);
   f( i-1,N );
}
main()
{
  int n;
  input(n);
  f(n,n);
}

We can clearly see in this pseudocode that we first call the function when the value of i is n and then print the value of i and decrement i by 1 inside the parameter of the function and make a call again. But, we know that this will go on forever as i will be decreasing continuously after every function call. So, to avoid this we put a base condition that if i is less than 1, then simply terminate the current recursive call and return to the previous call.

In this way, all the integers from N to 1 would get printed and as soon as the count becomes less than 1, the function terminates.

Recursion Tree for the following problem can be represented as follows : 

Code

#include<bits/stdc++.h>
using namespace std;

void func(int i, int n){
   
   // Base Condition.
   if(i<1) return;
   cout<<i<<endl;

   // Function call to print i till i decrements to 1.
   func(i-1,n);

}

int main(){
  
  // Here, let’s take the value of n to be 4.
  int n = 4;
  func(n,n);
  return 0;

}

class Recursion {
    
    static void func(int i, int n){
        
            // Base Condition.
            if(i<1) return;
            System.out.println(i);

            // Function call to print i till i decrements to 1.
            func(i-1,n);

    }
    public static void main(String[] args) {

       // Here, let’s take the value of n to be 4.
       int n = 4;
       func(n,n);
    }
}

Alternate Approach for printing integers from N to 1 (using Backtracking)

This is an alternative approach for printing the integers from N to 1 using recursion. In the previous approach, we used forward recursion but in this approach, we will be using backward recursion. The only change from the previous approach here will be that the print line would be kept after the function call inside the recursive function contrary to the previous approach. The function would be called for printing (i+1) integers and the ith integer would be printed.

To get a clear understanding of what this approach is like let’s see the pseudocode for this : 

void func( i, n )
{
   if(i>n) return;
 f( i+1,N );
print(i);

}
main()
{
  int n;
  input(n);
  f(1,n);
}

We can clearly see in this pseudocode that we first call the function when the value of i is N and then make a call again inside this function for printing (n-1) integers and after this, we print N. But, we know that this will go on forever as i will be decreasing continuously after every function call. So, to avoid this we put a base condition that if i is less than n, then simply terminate the current recursive call and return to the previous call.

In this way, all the integers from 1 to N would get printed and as soon as i becomes less than n, the function call terminates.

Recursion Tree for the following approach can be represented as follows : 

Code ( Alternate Approach )

#include<bits/stdc++.h>
using namespace std;

void func(int i, int n){
   
   // Base Condition.
   if(i>n) return;
   
   // Function call to print (i+1) integers.
   func(i+1,n);
   cout<<i<<endl;

}

int main(){
  
  // Here, let’s take the value of n to be 4.
  int n = 4;
  func(1,n);
  return 0;

}

class Recursion {
    
    static void func(int i, int n){
        
            // Base Condition.
            if(i>n) return;
            
            // Function call to print(i+1) integers.
            func(i+1,n);
            System.out.println(i);

    }
    public static void main(String[] args) {

       // Here, let’s take the value of n to be 4.
       int n = 4;
       func(1,n);
    }
}

Special thanks to Priyanshi Goel for contributing to this article on takeUforward. If you also wish to share your knowledge with the takeUforward fam, please check out this article. If you want to suggest any improvement/correction in this article please mail us at [email protected]