Im trying to Write a Recursive funtion that computes F(n) = 2 + 4 + 6 +...+2n using pass by reference.. here is the code I have

#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int F(int &);

int main(){

int n;

         cout <<"Enter n for computing F(n) : ";
         cin >> n;

          cout <<"F(  " << n <<" ) = " << F(n);

return 0;

int F(int &){

          if ( n==0)
                return 0;

          if (n==1)
              return 2;
              return F(n-1) + (2 * n);


I keep getting an error any help would be greatly appreciated.

Why is the argument passed by reference? You are returning the result directly.

Reference parameters in recursive functions are tricky things - every instance of the function may be modifying the same object in memory. And you cannot use the reference parameter in the expression return F( n-1 ).... because the function F() is expecting a variable, not the result of an expression.

Lastly, if you in fact modified the reference parameter, consider what happens to your variable n in main( ). The output statement:

cout <<"F(  " << n <<" ) = " << F(n);

would display the modified value of n in both places. Multiple output statements like that are built up from the right to the left, so the function would execute, modifying n, and that modified n is what would show up in the first display of n as well. Another reason not to use reference parameter in this case.

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