How to bind the value of a field in a query?

[brendonky18]

Looking at the documentation on fields. It shows the following class with the divisor field:

class SmallInt extends int {
  SmallInt() { this = [1 .. 10] }
}

class DivisibleInt extends SmallInt {
  SmallInt divisor;   // declaration of the field `divisor`
  DivisibleInt() { this % divisor = 0 }

  SmallInt getADivisor() { result = divisor }
}

I wrote the following query, and it produced some unexpected results.

from DivisibleInt i
where i.getADivisor() = 2
select i, i.getADivisor()

I was expecting to get all of the even numbers, with just the divisor 2. Instead it gave me all of the even numbers and each of their possible divisors.

Can someone explain why this is happening, and how I could possibly fix my query/class to get the correct result?

[atorralba]

Note that getADivisor() gets all the SmallInts that give 0 as a result of the modulo operation with this. So what your query is currently doing is selecting tuples of a DivisibleInts that has a divisor of 2, and all its divisors (one row for each).

So, what you actually want is binding the divisor to be the same one you are requiring to be equal to 2. Something like:

from DivisibleInt i, SmallInt divisor
where i.getADivisor() = divisor and divisor = 2
select i, divisor

[brendonky18]

So how would I go about doing this in a child class. Say I wanted to create a child class which only contains the tuples of the element, and the divisor of 2. Would there be a way to accomplish this without overriding the parent's predicate?

[atorralba]

To simply create a class that represents an int that is divisible by 2, you can do:

class DivisibleBy2Int extends DivisibleInt {
  DivisibleBy2Int() { divisor = 2 } // you're saying that this number must have 2 as one of its divisors
}

The getADivisior predicate as-is always gets all the divisors of the number, so you'd need to create a new predicate that does what you want (something like hasDivisor(int i) maybe).

Now, the thing is that you want to select a constant, so you can always do:

from DivisibleInt i
where i.getADivisor() = 2
select i, 2

// or

from DivisibleBy2Int i
select i, 2

(which is a little redundant)