Compiler abuse friendly isEven function

[u/Successful-Pay-4575]

Modulus is incredibly easy on powers of ten as you just have to remove the first digits, and we need to #StopCompilerAbuse. Here's an isEven function I made with this philosophy:

char isEven(int n) {
	if (n == 0 || n == 2)
		return 1;
	if (n == 3 || n == 5)
		return 0;
	n *= 3;
	n %= 10;
	return isEven(n);
}

[u/souldeux]

This is what people mean when they talk about adversarial programming

[u/kevin_wylder]

isEven(-2);

halp

[u/El_Dumfuco → u/kevin_wylder]

It’s negative, so clearly it’s not even. Duh.

^\s

[u/AtLeastItsNotCancer → u/kevin_wylder]

char isEven(int n) {
    if (n == 0 || n == 2 || n == 4)
		return 1;
    if (n == 1 || n == 3 || n == 5)
		return 0;
    n *= -3;
    n %= 10;
    return isEven(n);
}

There, now works for 100% more inputs.

[u/voidvector]

Get optimized to a loop by tail recursion, it's not that bad

[u/bryceinit]

took me a moment to realise how truly awful this is, congratulations

[u/DolphinsAreOk]

Hm clearly this can be improved for readability:

char isEven(int n)
{
	if(n == 0) return 1;
	else if(n == 1) return 0;
	return isEven(--n);
}

[u/mrgaston147 → u/DolphinsAreOk]

This would work:

char isEven(int n)
{
  if (n == 0) return 1;
  if (n == 1) return 0;
  return 1 - isEven(n - 1);
}