Recursion closed

Question

#include<iostream>
using namespace std;
int f(int &x, int c)

{

    c = c - 1;

    if (c == 0)
        return 1;

    x = x + 1;

    return f(x, c) * x;
}
int main()
{
    int p = 5;
    cout << f(p,p) << endl;
    return 0;
}

The above given code prints 6561 but while solving i am getting 3024 as answer.I have tried multiple times but my answer is still same.

Answer 1

First thing is the difference you are seeing is because of passing x as reference not by value, if you pass x as value to the above function you will get 3024 since you are passing it by reference it is taking final answer of the recursion which is "9" and multiplying it by itself and hence it prints 6561.

Comments

Why can't x evaluate first?

Answer 2

At statement, return f(x, c) * x;

In the recursive tree, don't fix the value of x initially. First go through the recursive function call till the base case.
 

F(5, 5)

   |c = 4, x = 6

F(6, 4)

   | c = 3, x = 7

F(7, 3)

   |c = 2, x = 8

F(8, 2)

   | c = 1, x = 9

F(9, 1)

   | c = 0, return 1;

 

Now, x = 9;

 

F(9, 1) = 1, Multiply it with x now and return. =>9

F(8, 2) = 9, Multiply it with x now and return => 81

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