can anyone tell me why the fibonacci function does not work? - c++

Question

I want to have it add every once a parameter it has not yet found to the map, but when I try numbers higher than 47 it gives me negative numbers, clearly impossible
#include <map>
using namespace std;
//memoization
map<unsigned int, unsigned int> memo;
map<unsigned int, unsigned int>::iterator it;
int fibonacci(int n)
{
it = memo.find(n);
if (it != memo.end())
{
cout << it->first<<endl;
return memo.at(n);
}
if (n <= 2)
{
return 1;
}
memo.insert({ n, fibonacci(n - 1) + fibonacci(n - 2) });
cout << "----"<<n<<endl;
return memo.at(n);
}
int main()
{
cout<<fibonacci(48);
}

Answer 1

First, let's take care of the negative numbers. Comments above explained that you have an overflow of the 32-bit int.
However, you could stretch your code a bit further would you not convert an unsigned int you calculate to a signed one you return from that function.
The solution is to use larger types, like unsigned long long int, AKA uint64_t
UPDATE
There are a few things sub-optimal in accepted answer.
As I noted in a comment there, the code is searching the map twice: it = memo.find(n); and memo[n];; should just return it->second;
There is no need for the key in that map to be 64-bit wide; 32 bit is enough to overflow 64-bit Fibonacci.
As the order of map keys is not important (you only do insert / lookup), the unordered_map will perform better (constant time instead of logarithmic).
You may notice that the memo is filled in order, and accessed by the index. Much better container here will be a vector with a "free" insert and lookup.
The calling function should NOT fill in memo, as it's not its business.
Here is my version:
#include <vector>
#include <iostream>
//memoization
static std::vector<uint64_t> memo = { 0, 0, 1 };
uint64_t fibonacci(unsigned int n) {
if (n < memo.size())
return memo[n];
memo.push_back(fibonacci(n - 1) + fibonacci(n - 2));
return memo[n];
}
int main() {
std::cout << fibonacci(32'000);
}

Answer 2

but when I try numbers higher than 47 it gives me negative numbers,
clearly impossible
unsigned int size is {0 to 4,294,967,295}, 48th Fibonacci is 4,807,526,976
Fixed code
#include <map>
#include <iostream>
#include <cstdint>
using namespace std;
//memoization
map<int64_t , int64_t> memo;
map<int64_t , int64_t>::iterator it;
int64_t fibonacci(int64_t n) {
it = memo.find(n);
if (it != memo.end()) {
return it->second;;
}
memo.insert({n, fibonacci(n - 1) + fibonacci(n - 2)});
return memo[n];
}
int main() {
memo.insert({0, 0});
memo.insert({1, 1});
memo.insert({2, 1});
cout << fibonacci(50);
}