Practice Questions for Recursion | Set 7 | GeeksforGeeks

Question 2: Predict the output of the following program.

void fun(int n)

{

    if(n > 0)

    {

        fun(n-1);

        printf("%d ", n);

        fun(n-1);

    }

}

int main()

{

    fun(4);

    return 0;

}

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]

Question 1 What does the following fun() do in general?
Answer: 8

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;

}

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 prgram) doesn’t matter. Following recursion tree shows all steps from 1 to 10, for exceution 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

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