Is it a rule that discrete problems are NP-hard and continuous problems are not? In my computer science education, I increasingly notice that most discrete problems are NP-complete (at least), whereas optimizing continuous problems is almost always easily achievable, usually through gradient techniques. Are there exceptions to this?
An example that I love is the problem where, given distinct $a_1, a_2, \ldots, a_n \in \mathbb{N}$, decide if: $$\int_{-\pi}^{\pi} \cos(a_1 z) \cos(a_2 z) \ldots \cos(a_n z) \, dz \ne 0$$

This at first seems like a continuous problem to evaluate this integral, however it is easy to show that this integral is not zero iff there exists a balanced partition of the set $\{a_1, \ldots, a_n\}$, so this integral problem is actually NP-complete.

Of course, I encourage playing around with some numerical tools to convince yourself that most (if not all) numerical tricks to evaluate this integral are doomed to failure once $n$ gets large enough.