Practice Questions for Recursion | Set 7

Question 1 Predict the output of the following program. What does the following fun() do in general?

C++

#include <iostream> 
using namespace std; 
  
int fun ( int n, int *fp ) 
{ 
    int t, f; 
  
    if ( n <= 1 ) 
    { 
        *fp = 1; 
        return 1; 
    } 
    t = fun ( n - 1, fp ); 
    f = t + *fp; 
    *fp = t; 
    return f; 
} 
  
int main() 
{ 
    int x = 15; 
    cout << fun(5, &x) << endl; 
  
    return 0; 
} 
  
// This code is contributed by shubhamsingh10 

C

#include <stdio.h> 
  
int fun ( int n, int *fp ) 
{ 
    int t, f; 
  
    if ( n <= 1 ) 
    { 
        *fp = 1; 
        return 1; 
    } 
    t = fun ( n-1, fp ); 
    f = t + *fp; 
    *fp = t; 
    return f; 
} 
  
int main() 
{ 
    int x = 15; 
    printf("%d\n",fun(5, &x)); 
  
    return 0; 
} 

Java

import java.io.*; 
  
class GFG { 
    static int fp = 15; 
    static int fun ( int n) 
    { 
        int t, f; 
       
        if ( n <= 1 ) 
        { 
            fp = 1; 
            return 1; 
        } 
        t = fun ( n - 1); 
        f = t + fp; 
        fp = t; 
        return f; 
    } 
     public static void main (String[] args)  
    { 
        System.out.println(fun(5)); 
    } 
} 
// This code is contributed by shubhamsingh10 

Python3

fp = 15
def fun ( n ): 
    global fp 
    if ( n <= 1 ): 
        fp = 1
        return 1
      
    t = fun ( n - 1 ) 
    f = t + fp 
    fp = t 
    return f 
  
# Driver code 
  
print(fun(5)) 
  
# This code is contributed by shubhamsingh10 

C#

using System; 
   
class GFG{ 
    static int fp = 15; 
    static int fun ( int n) 
    { 
        int t, f; 
      
        if ( n <= 1 ) 
        { 
            fp = 1; 
            return 1; 
        } 
        t = fun ( n - 1 ); 
        f = t + fp; 
        fp = t; 
        return f; 
    } 
      
    static public void Main () 
    { 
        Console.Write(fun(5)); 
    } 
} 
// This code is contributed by shubhamsingh10 

Output:

The program calculates nth Fibonacci Number. The statement t = fun ( n-1, fp ) gives the (n-1)th Fibonacci number and *fp is used to store the (n-2)th Fibonacci Number. Initial value of *fp (which is 15 in the above program) doesn’t matter. Following recursion tree shows all steps from 1 to 10, for execution of fun(5, &x).

                              (1) fun(5, fp)
                              /           \
                         (2) fun(4, fp)  (10) t = 5, f = 8, *fp = 5
                         /          \
                   (3) fun(3, fp)  (9) t = 3, f = 5, *fp = 3
                  /            \
           (4) fun(2, fp)      (8) t = 2, f = 3, *fp = 2
          /          \
   (5) fun(1, fp)   (7) t = 1, f = 2, *fp = 1
   /
(6) *fp = 1

Question 2: Predict the output of the following program.

C++

#include <iostream> 
using namespace std; 
void fun(int n)  
{  
    if(n > 0)  
    {  
        fun(n - 1);  
        cout << n <<" ";  
        fun(n - 1);  
    }  
}  
  
int main()  
{  
    fun(4);  
    return 0;  
}  
  
// This code is contributed by shubhamsingh10 

C

#include <stdio.h> 
  
void fun(int n) 
{ 
    if(n > 0) 
    { 
        fun(n-1); 
        printf("%d ", n); 
        fun(n-1); 
    } 
} 
  
int main() 
{ 
    fun(4); 
    return 0; 
} 

Java

import java.util.*; 
   
class GFG{ 
static void fun(int n)  
{  
    if(n > 0)  
    {  
        fun(n - 1);  
        System.out.print(n+" ");  
        fun(n - 1);  
    }  
} 
  
public static void main(String[] args) 
{  
    fun(4); 
} 
} 
// This code is contributed by Shubhamsingh10 

Python3

def fun(n): 
      
    if(n > 0): 
        fun(n - 1)  
        print(n,end=" ") 
        fun(n - 1)  
  
# driver code 
  
fun(4)  
  
# This code is contributed by shubhamsingh10 

C#

using System; 
  
class GFG{ 
    static void fun(int n)  
    {  
        if(n > 0)  
        {  
            fun(n - 1);  
            Console.Write(n+" "); 
            fun(n - 1);  
        }  
    }  
        
    static public void Main () 
    {   
        fun(4);    
    }  
} 
  
// This code is contributed by shubhamsingh10 

Output

1 2 1 3 1 2 1 4 1 2 1 3 1 2 1

                     fun(4)
                   /
                fun(3), print(4), fun(3) [fun(3) prints 1 2 1 3 1 2 1]
               /
           fun(2), print(3), fun(2) [fun(2) prints 1 2 1]
           /
       fun(1), print(2), fun(1) [fun(1) prints 1]
       /
    fun(0), print(1), fun(0) [fun(0) does nothing]

Please write comments if you find any of the answers/codes incorrect, or you want to share more information/questions about the topics discussed above.

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My Personal Notes

C++

C

Java

Python3

C#

C++

C

Java

Python3

C#

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