Why is this variable returning 32766?

Question

I wrote a very basic evolution algorithm. The way it's supposed to work is that the user types in the desired value, and the amount of generations to try to reach it. Then, the program will run through, taking the nearest value in an array to the goal and mutating it four times (while also leaving the original, in case it's right) to try and get closer to the goal. In theory, it should take roughly |n|/2 generations to reach the value, as mutations happen in either one or two points. Here's the code to demonstrate what I mean:

#include <iostream>
using namespace std;

int gen [5] = {0, 0, 0, 0, 0}; int goal; int gens; int best; int i = 0; int fit;

int dif(int in) {
    return abs(gen[in] - goal);
}

void nextgen() {
    int fit [5] = {dif(1), dif(2), dif(3), dif(4), dif(5)};
    best = *max_element(fit, fit + 6);
    int gen [5] = {best - 2, best - 1, best, best + 1, best + 2};
}

int main() {
    cout << "Goal: "; cin >> goal; cout << "Gens: "; cin >> gens;
    while(i < gens) {
        nextgen(); cout << "Generation " << i + 1 << ": " << best << "\n";
        i = i + 1;
    }
}

It's pretty simple code. However, it seems that the int best bit of the output is returning 32766 every time, no matter what I do. Do you know what I've done wrong?

I've tried outputting the entire generation (which is even worse––a jumbled mess of non user friendly data that appears meaningless), I've reworked the code, I've added varibles and functions to try and pin down exactly where the error is, and I watched the entire code aesthetic youtube channel to make sure this looked good for you guys.

Answer 1

Looks like you're driving C++ without a license or safety belt. Joke aside, please keep trying and learning. But with C/C++ you should always enable compiler warnings. The godbolt link in the comment from @user4581301 is really good, the compiler flags -Wall -Wextra -pedantic -O2 -fsanitize=address,undefined are all best practice. (I would add -Werror.)

Why you got 32766 is possible to analyze with a debugger, but it's not meaningful. A number close to 32768 (=2^15) should trigger all the warning bells (could be an integer overflow). Your code is accessing uninitialized memory (among other issues), leading to what is called undefined behaviour. This means it may produce different output depending on your machine, compiler, optimization flags, OS, standard libraries, etc. - even adding a debug-print could change what it does.

For optimization algorithms (like GAs) it's also super easy to fool yourself into thinking that your implementation is correct, because the optimization will find a way to avoid (or exploit) any bugs. I've had one in my NN implementation that was accessing some data from the previous example by accident, and it took several days until I even noticed there was a problem.

If you want to focus on the algorithms, I suggest to start with a different language (anything except C/C++/Assembly). My advice would be either Python (though it can be 50x slower, it's much easier to learn and write) or Rust (just as fast as C++ and just as complicated, but no undefined behaviour). With Rust, every mistake in your code above would have given you either a warning by default, a compiler error, or a runtime error instead of wrong output. Though C++ with the flags mentioned above does the same for your specific code.