can anyone pinpoint the error in my recursive function? - c++

Question

this function prints all triplets in an array whose product is a number k
input first line number of elements, second array elements, third target product .. passes parameters to recursive function f along with a vector that stores elements whose product might give k
thought process -> for each element we can include or exclude it to get the product k. If p > 24 or number elements multiplied > 3 we backtrack. once the prod = k, we print all numbers from vector v and pop them out and set number of elements count to 0 and product to 1 and continue
for an input of:
9
1 2 3 12 4 7 5 24 9
24
my output looks something like this :
12
2
1
9
9
9
| ->cursor justs stops here ..no further outputs...
naming scheme used :
count -> number of elements multiplied till now whose product is stored in -> p
n-> number of elements in array
k -> target pdt
i -> index of element in array currently at
code :
#include <iostream>
#include <vector>
using namespace std;
// all triplets whose product is a number k
void f(int i, int count, int p, int k, vector<int>&v, int *a, int n)
{
// success condition
if(count == 3 && p == k)
{
for(int i = 2; i >= 0; --i)
{
cout << v[i] << " " << endl;
v.pop_back();
}
p = 1;
count = 0;
}
if(count>=3 || i > n - 1 || p > k)
{
return;
}
v.push_back(a[i]);
f(i + 1, count + 1, p * a[i], k, v, a, n);
v.pop_back();
f(i + 1, count, p, k, v, a, n);
}
int main()
{
int n;
cin >> n;
int *a=new int[n];
for(int i = 0; i < n; ++i)
{
cin >> a[i];
}
int k;
cin >> k;
//int p = 1;
vector<int>v;
f(0, 0, 1, k, v, a, n);
delete[] a;
return 0;
}

Answer 1

Your “reset” of p and count on success is immediately strange: why does the function need to keep looking when its caller will already try the other possibilities? But that’s just a distraction from the real issue: the balanced push_back and pop_back around the first recursive call establish and rely on an invariant where every call leaves v the same length as when it started. But the success path clears the vector and leaves it shorter, so eventually you pop_back when it’s empty and—
The joys of undefined behavior (which happened to give an infinite loop here) aside, the fix is trivial: just return after printing without modifying v at all. (Then you may find some further simplifications.)