How can I get Y value?

[yyyljy]

Hello, I'm new to zk so my question might looks silly, but please answer me.

When I'm creating the witness assignment, I have to manually calculate the value for Y based on X (for example, if X is 3, I calculate that Y is 35). Is there a built-in function in the gnark library that can do this calculation for me?"
= Is there a function to find the corresponding Y value for any X value, not just X=3? If I implement a more complex circuit, what's the method for generating the correct Y for a different X each time?

[ivokub]

gnark doesn't provide such witness computation mechanism and in general it is also impossible. In essence SNARK circuits allow you to show that some witness is valid but it is up to the user to provide it.

For example, let say we have a very simple circuit which proves p*q = n (it is the known RSA factoring problem which is known to be computationally hard) with public input n and private inputs p and q. As factoring is a hard problem, then given n it would be very difficult to compute p and q, unless the user already knows them through RSA key generation, where it first computes p and q and then makes n public.

[ivokub]

For some cases it would be doable (a la y == x^3+x+5), but it is impossible to determine ahead of time when such computation is possible and when not. So it is better not to implement any such mechanisms at all.