In most languages that use it, it represents the modulus operation.

In APEX, you use Math.Mod()

if (Math.Mod(x, 2) == 0) {
    system.debug(x + ' is even');
}

APEX Expression Operators Reference shows that there is no definition of a % operator.

As of Spring 19 (and all prior versions), we only have mod(Integer, Integer) and mod(Long, Long). There is no known information of when, or if, we might get a built in for Decimal or Double values.

SOQL won't let you do any calculations. Check docs here and here:

You must supply a native value—other field names or calculations are not permitted

Generally speaking it's fieldName = value or datefieldname > YESTERDAY (few special literals for date handling).

Easiest would be to create a formula field in the record (think about them like calculated columns in views in normal databases) with your logic. It could be of text type, call it "weird price" ;) Here's the complete formula functions reference - you have MOD(), IF(condition, true, false) etc goodies.

The only caveat is that you can't GROUP BY formula (there also tricks to bypass that ;))

So yeah - if it's one time thing with poor reusability - filter with code. If you can accept the low cost (formulas don't use storage but you might need them for other purposes) - create a field.

I see 3 problems:

1. The if is wrong. You check if the loop counter is odd, even, odd, even... What you need is if(Math.mod(arr[i], 2) == 0). Use the value at that index, not just the index itself.

2. The first two system.debugs - I think you intended them to display the odd/even variable, not the loop counter i. Probably copy-paste error?

   System.debug('even numbers -- ' + even); should work better?

3. Not terribly evil but weird - in bottom system.debugs you want to just display even, not even++. As it's end of the function it doesn't matter much but if you'd be returning this value and using it further in the program - subtle, nasty error.