Does anyone have a useful mnemonic for implementing Comparator?

Question

Every time I need to implement a comparator, I get stuck trying to remember when I should return -1 and when 1, and I have to look it up.

I mean, obviously -1 is less, so it implies that first is less than second. But whenever I say that to myself, I get that nagging "are you sure?" feeling. I suspect part of my confusion comes from implementing it the other way around whenever I need a descending sort.

What do you use to remember which is which?

Answer 1

comparator.compare(a, b) < 0 <==> a < b

Answer 2

I use this simple "substraction" mnemonic:

first - second

So, if first is "less" than second you'll get negative result, otherwise - positive or zero if they are equal.

Answer 3

I am not sure what you mean by mnemonic. However, I have had a very similar cognitive dissonance.

I am very visual, so I use the number line (the one I was taught in grade school). I just visualize the negative numbers as "left", 0 as "center" and positive numbers as "right". That the corresponds to the truth: -1 < 0 < 1

Answer 4

I remember the base integer case (pseudocode):

int comparator(int a, int b) {
   return a-b;
}

So if we give in a small a and a large b which is the first < last we get a negative result.

I have a more visual memory, so remembering the "structure" of that function is easy and natural for me.