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Bug introduced in 9.0 and fixed in 11.0.0

WhenEvent is new in 9.0.

This is an example from the docs, slightly modified (action is wrapped in { }), working as expected.

NDSolve[{x'[t] == x[t], x[0] == 1, 
  WhenEvent[Mod[t, 1] == 0, {x[t] -> x[t] + (-1)^t}]}, x, {t, 0, 3}]

  {{x -> InterpolatingFunction[{{0., 3.}}, <>]}}

When the action is deliberately removed (because for example the action is programmatically generated for a list of variables that can be empty), it crashes the kernel.

Warning: the following line might crash the kernel. Save your work.

NDSolve[{x'[t] == x[t], x[0] == 1, 
  WhenEvent[Mod[t, 1] == 0, {(*x[t] -> x[t] + (-1)^t*)}]}, x, {t, 0, 3}]

Can anyone reproduce this?

(Mathematica 10.4.0 for Microsoft Windows (Win7, 64-bit))

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    $\begingroup$ Confirmed on OS X 10.11.4, M 10.4.1, M 10.3.1, M 10.0.2 and M 9.0.1. All of them crash. $\endgroup$
    – Szabolcs
    May 4, 2016 at 12:06
  • 7
    $\begingroup$ Thanks a lot @Szabolcs, submitting to TechSupport. I've found more bugs in the last month than in the last 10 years of using Mathematica. It's like crossing a threshold after which one cannot really be productive due to seeing bugs everyewhere. They are swarming... $\endgroup$
    – István Zachar
    May 4, 2016 at 12:16

1 Answer 1

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TechSupport acknowledged and proposed a simple workaround by putting inert expression, e.g. empty string, inside the { }:

NDSolve[{x'[t] == x[t], x[0] == 1, WhenEvent[Mod[t, 1] == 0, {""}]}, x, {t, 0, 3}]
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