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I've got a problem where WhenEvent successfully detects an event, but then performs only some of the actions.

In this minimal example, x[t] can not exceed 1 (driven by If). If x[t] equals 1 in the time window from 0.1 to 0.9, it should be reduced to 0.2, toggle a DiscreteVariable, and print the time.

sol = NDSolve[{x'[t] == If[x[t] < 1, 4 x[t], 0],
  WhenEvent[0.1 < t < 0.9 && x[t] >= 1,
    {Print[t], x[t] -> 0.2, dv[t] -> 1 - dv[t]}],
  x[0] == 0.9, dv[0] == 0}, {x, dv}, {t, 0, 1},
  DiscreteVariables -> {dv}][[1]];
(* 0.1 *)
(* 0.502359 *)

The two printed times show that the WhenEvent was correctly triggered twice -- once when the time window opens at t == 0.1 and another time later. But plotting x[t] shows that it was not correctly reduced to 0.2 at the second event at t = 0.502359.

Plot[Evaluate[x[t] /. sol], {t, 0, 1}]

Mathematica graphics

However the DiscreteVariable dv[t] was successfully toggled twice:

Plot[Evaluate[dv[t] /. sol], {t, 0, 1}]

Mathematica graphics

This seems to be an unfortunate interaction with the If statement, because changing x'[t] == 4 x[t] causes it to work fine.

Bug or just my improper use of WhenEvent? Any ideas for a workaround?

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A slightly change of the dgl makes your program working:

{X, DV} = 
NDSolveValue[{x'[t] == Boole[x[t] < 1.01] 10 x[t] , 
WhenEvent[0.1 < t < 0.9 && x[t] >= 1, {Print[t], x[t] -> 0.2, dv[t] -> 1 - dv[t]}], x[0] == 0.9, dv[0] == 0}, {x, dv} , {t, 0, 1}, DiscreteVariables -> {dv}] ;
Plot[{X[t], DV[t]} , {t, 0, 1}, GridLines -> {None, {1}}]

enter image description here

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  • $\begingroup$ Thanks! Seems like the secret sauce in your answer is the 1.01, not the Boole, because changing my original code to If[x[t] < 1.01 works the same way. So does x[t] < 1.00000000001 which is an easy fix and close enough. $\endgroup$ – Chris K May 29 '19 at 19:16
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I don't know enough about WhenEvent to say whether the behavior is a bug or not. As a workaround, you can introduce an auxiliary function, and modify the WhenEvent condition:

f[t_] = Piecewise[{{1, .1<t<.9}}, 2];
sol = NDSolveValue[
    {
    x'[t] == If[x[t] < 1, 4 x[t], 0], x[0] == .9,
    h[t] == x[t], h[0] == x[0],
    dv[0] == 0,
    WhenEvent[x[t]>=f[t], {Print[t], h[t] -> .2, dv[t] -> 1 - dv[t]}]
    },
    {x, dv},
    {t, 0, 1},
    DiscreteVariables -> {dv}
];

0.1

0.502359

Visualization:

Plot[First @ Through @ sol[t], {t, 0, 1}]

enter image description here

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  • $\begingroup$ Interesting approach! I wonder if this forces NDSolve to use a DAE solver. $\endgroup$ – Chris K May 29 '19 at 19:26

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