Recursion Stack Overflow C++

Question

I am new to C++ but thought that working on some project Euler problems would familiarize me with the language.

When attempting Project Euler Problem 14: Longest Collatz sequence

I could not manage to get my C++ solution to work, but had no problem with my python solution...

import time 
start = time.time()
memo = {1:1,2:2}
longest_chain, longest_starting_key = 2, 2

def rec(a):
    global longest_chain, longest_starting_key

    if a in memo.keys():
        return memo[a]    
    if a % 2 == 0:
        memo[a] = rec(a // 2) + 1
    else:
        memo[a] = rec(3 * a + 1) + 1
    if memo[a] > longest_chain:
        longest_chain = memo[a]
        longest_starting_key = a
    return memo[a]    

for i in range(1000000,3,-1): rec(i)

print("starting key", longest_starting_key , ": has length", longest_chain)
print((time.time() - start), "seconds")

and got

starting key 837799 has length 525
1.399820327758789 seconds

My C++ attempt... (that I thought was equivalent...)

#include <iostream>
#include <unordered_map>
std::unordered_map<int, int> mem = { {1,1},{2,2} };
int longest_chain{ 2 }, best_starting_num{ 2 };

bool is_in(const int& i){
    auto search = mem.find(i);
    if(search == mem.end())
        return false;
    else
        return true;
}

int f(int n){
    if(is_in(n))
        return mem[n];
    if(n % 2)
        mem[n] = f(3 * n + 1) + 1;
    else
        mem[n] = f(n / 2) + 1;
    if(mem[n] > longest_chain)
        longest_chain = mem[n];
    return mem[n];
}

int main(){
    for(int i = 3; i < 1000000; i++){
        f(i);
    }
    std::cout << longest_chain << std::endl;
}

When running this in Visual Studio in debug mode I get the following error

Unhandled exception at 0x00007FF75A64EB70 in
0014_longest)collatz_sequence_cpp.exe: 0xC00000FD: Stack overflow
(parameters: 0x0000000000000001, 0x000000DC81493FE0).

Someone told me that I should allocate on the heap using pointers, but have very limited experience with working with pointers...

The other thing I don't understand is... when I run it with 100'000 instead of 1'000'000 in the main body loop, it works but is very slow.

Answer 1

Hint : What is the invariant on the range of n that the Collatz sequence provides (mathematically), which your Python code satisfies but your C++ code doesn't? Hint to the hint: what are the last few values of n before your stack overflow actually occurs?