Support for fix() or abs()

[Ardnived]

In GURPS you should be able to "spend" negative values of points to reduce your stats. However, since floor() always rounds to the lower number, the calculation gets messed up.

For example, if the stat has a base value of 10, and costs 20 points the raise/lower. I'd use a formula like this.

10 + floor(@{points} / 20)

For positive values this works fine, but for most negative values the output is 1 lower than it should be.

10 + floor(24 / 20) = 11
10 + floor(-24 / 20) = 8

Could you implement a fix() function, which always rounds the value towards 0.

Or alternatively add an absolute value function. In which case I could emulate fix() like so:

@{points}/abs(@{points}) * floor(abs(@{points}) / 20)

[Lithl]

floor becomes round if you add 0.5 to the value. For example: floor((-24 / 20) + 0.5) = floor(-1.2 + 0.5) = floor(-0.7) = 1. The same thing happens with ceil if you subtract 0.5.

I also seem to recall Riley including a round function on the dev server, but I can't find his post about that, so I'm not sure.

[Ardnived]

I thought about that, but it doesn't always work.

floor((30 / 20) + 0.5) = floor(1.5 + 0.5) = 2
fix((30 / 20)) = fix(1.5) = 1

To be clear, I'm looking for a function that always rounds towards 0. Not a function that rounds naturally.
ie. 1.9 should become 1, 1.5 should become 1, -2.1 should become -2.

[Lithl]

Ah, yeah; if you want "towards 0," that wouldn't work. Adding half and flooring the number is round, not towards 0.

An option other than fix (which I've honestly never heard of being a standard implementation) would be sign. sign(x) = 1 if x >= 0 and sign(x) = -1 if x < 0. Thus, x * sign(x) = abs(x), and func(x * sign(x)) * sign(x) maintains the sign of x for the final result.

[Ardnived]

Yeah, what you pointed out with sign() is essentially what I was suggesting could be done using abs().

func(x * sign(x)) * sign(x)   vs   func(abs(x)) * x/abs(x)

sign(x) and x/abs(x) would have the same result.
Unfortunately I don't think Roll20 offers the sign() function either.

I've also never heard of fix being regularly available, I just googled to see what such a function is usually called, and "fix" is what came up.

[victor]

Math.sign(x) is only in the candidate stages of the specification, and thus it is not available although some browsers do implement it. But as you know, it can easily be substituted for equivalent expressions.

I think the fix function name comes from MATLAB.
Anyway, FWIW, I'd also like this this functionality to be available, to be able to calculate degrees of success in Deathwatch rolls

[Ardnived]

Another solution would be to add a tertiary operator (ie. x == 0 ? y : z which translates to if x is 0 then y else z). Then we could do x > 0 ? floor(x) : ceil(y)

Which would probably be somewhat more complicated to implement, though also more powerful for those developing character sheets.

[rileydutton]

Support for abs() was added today: https://app.roll20.net/forum/post/827711/#post-827711