Can you help me with my recursive function? - c++

Question

I have this simple function that I am calculating correctly, but my output statements are off. I have tried other places were the cout statement is commented, but not working.
int recursiveFunc(int n) {
int val; // value at nth sequence
//cout << "(" << n << ") = " << val << endl;
if (n == 1 ) { // base case 1
val = -1;
// outputs before function call
}
else if (n == 2) { // base case 2
val = -1;
// outputs before function call
}
else { // recursive case
//cout << "(" << n << ") = << val << endl;
val = 2*(recursiveFunc(n-1) + recursiveFunc(n-2));
cout << "(" << n << ") = " << val << endl; // not sure where to put cout statement
}
return val;
}
I am looking for an output like (for example n = 5):
(1) = -1
(2) = -1
(3) = -4
(4) = -10
(5) = -28
currently, my output looks like:
(1) = -1
(2) = -1
(3) = -4
(4) = -10
(3) = -4 // here an nth term is displayed twice
(5) = -28

Answer 1

Move your cout statement to outside the function and it works fine.
int recursiveFunc(int n) {
int val; // value at nth sequence
if (n == 1) { // base case 1
val = -1;
}
else if (n == 2) { // base case 2
val = -1;
}
else { // recursive case
val = 2 * (recursiveFunc(n - 1) + recursiveFunc(n - 2));
}
return val;
}
int main() {
for (int i = 1; i < 10; i++) {
std::cout << "(" << i << ") = " << recursiveFunc(i) << endl;
}
return 0;
}
Output:
(1) = -1
(2) = -1
(3) = -4
(4) = -10
(5) = -28
(6) = -76
(7) = -208
(8) = -568
(9) = -1552

Answer 2

Right off the top-of-my-head, a possible fix is to keep track of the highest n output so-far already and only output when a larger n is encountered. (This is probably not the best solution, but I'm feeling too tired and lazy to think about alternatives)
Don't use a global variable for this (mutable global state is an anti-pattern!) - you'll need to pass it as another parameter.
int output_n_and_val( int n, int val, int& biggestN ) {
if( n > biggestN ) {
cout << "(" << n << ") = " << val << endl;
biggestN = n;
}
return val;
}
int recursiveFuncImpl( int n, int& biggestN ) {
if( n == 1 ) {
return output_n_and_val( n, -1, biggestN );
}
else if( n == 2 ) {
return output_n_and_val( n, -1, biggestN );
}
else {
int val = 2 * ( recursiveFuncImpl( n - 1, biggestN ) + recursiveFuncImpl( n - 2, biggestN ) );
return output_n_and_val( n, val, biggestN );
}
}
// Entrypoint function:
int recursiveFunc( int n ) {
int biggestN = -1;
return recursiveFuncImpl( n, biggestN );
}
int main()
{
recursiveFunc( 10 );
recursiveFunc( 5 );
return 0;
}
The downside to this approach is that because of the n == 2 case-handling is encountered before n == 1 you'll never get the output (1) = -1. Fixing that is an exercise left for the reader.

Answer 3

you should use an array to store values that you have computed, for example
const int N = 1000000;
int values[N]; // remember to memset to 0 first
int recursiveFunc(int n) {
if(n < 0) return 0;
if(values[n] != 0) return values[n];
int val; // value at nth sequence
if (n == 1) { // base case 1
val = -1;
}
else if (n == 2) { // base case 2
val = -1;
}
else { // recursive case
val = 2 * (recursiveFunc(n - 1) + recursiveFunc(n - 2));
}
return values[n] = val;
}
int main() {
memset(values, 0, sizeof(values));
for (int i = 1; i < 10; i++) {
std::cout << "(" << i << ") = " << recursiveFunc(i) << endl;
}
return 0;
}