Collatz Conjecture c++ weird output with big numbers - c++

Question

So, I was bored, and decided to make something completely random, then I came across the collatz conjecture: start with any positive number, and if it is even, devide by two. If it is odd, multiply by three and add one. When repeating this, you will end at the number one. So I made this code:
//Collatz Conjecture by Lucas Knook
#include <iostream>
#include <iomanip>
using namespace std;
int main() {
long long n;
cin >> n;
cout << setw(5) << n;
while(true){
if(n % 2 == 0 && n != 1){
//is even
cout << " E" << endl;
n /=2;
cout << setw(5) << n;
}
else if(n != 1){
//is odd
cout << " O" << endl;
n = n * 3 + 1;
cout << setw(5) << n;
}
else break;
}
cout << " O" << endl << endl << "end loop";
return 0;
}
I am still learning c++ (I just completed the sololearn course, and I am about to get the "C++ for dummies all-in-one" book), but I think this is a good start for me, and it works.
There is just one little problem: If I use big numbers, it does stop at one, and gets the odd and even right, but...
Look at this first part of the output when entering
"1000000000000000000000000" :
9223372036854775807 O
9223372036854775806 E
4611686018427387903 O
-4611686018427387906 E
-2305843009213693953 O
-6917529027641081858 E
-3458764513820540929 O
8070450532247928830 E
4035225266123964415 O
-6341068275337658370 E
-3170534137668829185 O
8935141660703064062 E
4467570830351532031 O
-5044031582654955522 E
-2522015791327477761 O
-7566047373982433282 E
-3783023686991216641 O
7097673012735901694 E
3548836506367950847 O
-7800234554605699074 E
Ehm, that's a bit weird, isn't it? (don't look at the line breaks, it is perfectly stacked, it just doesn't show here, because I copied the output)
Can someone please explain me why this is happening and how to fix it?

Answer 1

You've exceeded the limits of long long integer values. The first value in your output is truncated to be much smaller than your input, then when you get to 4611686018427387903 it's multiplied by 3, since it isn't even. That's when it overflows the type and wraps around into negative values. You'll need to use a BigInteger library, like TTMath.

Answer 2

Basically as other people pointed out you don't check for overflow. But I would like to point out different thing, your code is still not valid as you don't check if you got into endless loop, when multiplication and division sequence generates the same numbers, your while will never end.
This is correct code with simple overflow detection:
#include <iostream>
#include <unordered_set>
bool colatazConjecture(int n) {
std::unordered_set<int> localComputed;
int newN = 0;
while (n != 1) {
if (!localComputed.emplace(n).second) {
throw "Cycle detected";
}
newN = n;
if (n & 0x1) {
newN = 3*n + 1;
if (newN <= n) {
throw std::overflow_error("Overflow for " + std::to_string(newN));
}
} else {
newN >>= 1;
}
n = newN;
}
}