brutally fumbling the easiest CS quiz

Before I start yapping about anything, I'd like to say, today was NOT a hectic day. I came to campus sometime before 12, attended a CSE111 quiz on Java Arrays, and just went on with my day just moving around campus, meeting people, applying to the famous Mongol-Tori team for a spot in the Controls & Software category, and also finishing my MAT120 class by late afternoon, which was around, 4:30 PM.

For some context, the CSE111 course in my university is on Java OOP, or "Object Oriented Programming".

So, while I was listening to the teacher in my maths class, I realized something which I was supposed to realize much earlier, but better late than never - I kind of messed up the quiz, and the reality was so bad that I was being ashamed of myself in the middle of the class xD kind of an overreaction since the damage had already been done.

Let's get to what I actually did wrong, then.

The question was like this:

"Given an array, modify the array in a manner that each of its elements correspond to the summation of all the other elements."

So, let's say we have an array {5, 2, 3}. In this case, the modified array following the rules of the question would look like: {5, 8, 7}. This is a pretty standard, and easy, introductory question at first glance. You just loop over each element and sum-up the other elements, right? It'd look something like:

for (int i = 0; i < arr.length; i++) {
    int x = 0;

    for (int j = 0; j < arr.length; j++) {
        if (j == i) continue;
        x += arr[j];
    }

    arr[i] = x;
    System.out.println("Index " + i + ": " + arr[i]);
}

Except, this was not the full picture.

Even after doing Rust for a while now, I kinda forgot mid-exam how the array mutates in real-time. Very dumb of me glazing a concept which is supposed to be easy, but now that I've managed to ruin it, let's go through this anyhow:

Picture this. For the first iteration (assuming our array is {5, 2, 3} as mentioned before), index 0 is skipped since j == i is true, and the sum of the other elements is 2 + 3 = 5. So, our new 5 (the sum) replaces arr[i] (in this case, our old 5) in-place.

// the current array
{5, 2, 3}

In the second iteration, it also works out of sheer luck (since our first iteration did not really change anything about the array). The element at index 1 is skipped and the sum of the other two elements becomes 5 + 3 = 8, and gets placed inside arr[i] again. So, the array gets modified like this:

// the now-modified array
{5, 8, 3}

However, finally, in the third iteration, things become apparent as now we don't have a record of our original array anymore! In the original array, the last item would have been 3 as usual and the summation would've been 5 + 2 = 7. But since our array is now changed, the actual summation of the other elements becomes 5 + 8 = 13 which is the wrong ANSWER!

// Expected:
{5, 8, 7}

// Original:
{5, 8, 13}

Regardless, this was no CS-lesson. I just wanted to rant about myself a bit. I don't really know how this happened, but maybe it had to do something with me getting too excited for hearing back from Mongol-Tori after submitting the form.

Thanks for tuning into my blog - long time no see, hopefully I'll see you in the next entry! :3