Skip to main content

error trapping - Check does not interrupt evaluation of the expression when a Message is emitted



On the surface, Check looks like a good method to use when you need to abort the evaluation of an expression whenever a Message is emitted. Unfortunately, as the following code demonstrates, the evaluation of the expression is completed after a Message has occurred.


(* the test message *)
Test::wrpt = "Negative point at `1`";

dat = RandomReal[1, 500];

(* without Check *)
woCheck = MapIndexed[
If[# < 0,
Message[Test::wrpt, #2[[1]]]; Unevaluated[Sequence[]],

#] &,
{-1}~Join~dat];

(* with Check *)
res = {};
Check[
MapIndexed[
If[# < 0,
Message[Test::wrpt, #2[[1]]]; Unevaluated[Sequence[]],
res = {res, #}; #] &,

{-1}~Join~dat],
$Failed
]
(* Test::wrpt: Negative point at 1
$Failed
*)

But, when comparing the two


woCheck == Flatten@res
(* True *)


This implies, that for long running computations Check is not a good solution to abort a computation if an error Message is emitted. Can this be fixed?



Answer



A simple method for accomplishing this is to have Message Throw an error when it is called, interrupting the current execution. Here is a replacement for Check which does that, with the same calling signature:


ClearAll[InterruptingCheck]
SetAttributes[InterruptingCheck, HoldAll]

InterruptingCheck[expr_, failexpr_, msgs : {___MessageName } : {}] :=
Internal`InheritedBlock[{Message, $msgFlag},
(* Module localizes tag while not polluting the global namespace *)

Module[{ tag },
Unprotect[Message];
(*
Attach hook to message, where failexpr is thrown after the
first Message is raised. msgFlag is used to prevent recursion,
so that when msg is called the original def is used.
*)
msg : Message[m_, ___] /; ! TrueQ[$msgFlag] := Block[{$msgFlag = True},
msg;
If[Length@msgs == 0 || MemberQ[msgs, m], Throw[failexpr, tag]]];

Catch[expr, tag]
]
]

(* With Leonid's suggestion, this now works correctly. *)
InterruptingCheck[expr_, failexpr_, msgGroup_String] :=
Hold[msgGroup] /. $MessageGroups /.
Hold[messages_List] :> InterruptingCheck[expr, failexpr, messages]

\begin{Edit}



My original code for the supplying a message group failed to work correctly. The fix is shown above. It works by replacing msgGroup with the appropriate list of messages, but using Hold to prevent them from being replaced by their string equivalents. (That is part of the reason for the HoldAll attribute being used here, to begin with.) Then, the held messages are extracted from Hold and inserted into InterruptingCheck still held because of HoldAll.


\end{Edit}


When applied to the example in the question,


res2 = {};
InterruptingCheck[
MapIndexed[
If[# < 0,
Message[Test::wrpt, #2[[1]]]; Unevaluated[Sequence[]],
res2 = {res2, #}; #] &,
dat[[;; 10]]~Join~{-1}~Join~dat],

$Failed,
{Test::wrpt}
]
(* Test::wrpt: Negative point at 11
$Failed
*)

and res2 is


{0.288047, 0.026642, 0.361008, 0.28977, 0.573743, 
0.272747, 0.937062, 0.330572, 0.192807, 0.916764}


showing that it did stop execution at after the tenth element in the data.


Comments

Popular posts from this blog

plotting - How to draw lines between specified dots on ListPlot?

I would like to create a plot where I have unconnected dots and some connected. So far, I have figured out how to draw the dots. My code is the following: ListPlot[{{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4,13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full] I have thought using ListLinePlot command, but I don't know how to specify to the command to draw only selected lines between the dots. Do have any suggestions/hints on how to do that? Thank you. Answer One possibility would be to use Epilog with Line : ListPlot[ {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {1, 4}, {2, 5}, {3, 6}, {4, 7}, {1, 7}, {2, 8}, {3, 9}, {4, 10}, {1, 10}, {2, 11}, {3, 12}, {4, 13}, {2.5, 7}}, Ticks -> {{1, 2, 3, 4}, None}, AxesStyle -> Thin, TicksStyle -> Directive[Black, Bold, 12], Mesh -> Full, Epilog -> { Line[ ...

list manipulation - Selecting multiple columns from a matrix?

Sample data: data = { {{2013, 1, 1}, 24.13, 167.67, 231.82}, {{2013, 1, 2}, 32.15, 170.92, 225.99}, {{2013, 1, 3}, 35.43, 172.68, 221.67}, {{2013, 1, 4}, 36.73, 173.05, 218.32}, {{2013, 1, 5}, 58.19, 165.96, 197.05}, {{2013, 1, 6}, 69.99, 163.50, 187.52}, {{2013, 1, 7}, 71.37, 154.21, 175.58}, {{2013, 1, 8}, 72.51, 149.66, 163.25}}; I want a DateListPlot with three graphs, so for a matrix formed by columns 1 and 2, one for columns 1 and 3, and 1 for columns 1 and 4. At the moment I'm using this code: data2 = Transpose[{data[[All, 1]], data[[All, 2]]}]; data3 = Transpose[{data[[All, 1]], data[[All, 3]]}]; data4 = Transpose[{data[[All, 1]], data[[All, 4]]}]; DateListPlot[{data2, data3, data4}, Joined -> True, Filling -> {3 -> {1}}] but I have a hunch that this can be done more efficiently. I don't like the Transpose s in particular. Any ideas? edit (for extra credit) What if I need to multiply the second column by 2, which in my solution is simp...

plotting - How to draw some 2D curves in a 3D plot?

I want to plot several 2D curves ( Sin[x] at y=0 , Cos[x] at y=1 ) in a 3D plot, what function should i use? f[x_, y_] := Piecewise[{{0, y 0}, {Sin[x], y == 0}}] g[x_, y_] := Piecewise[{{0, y 1}, {Cos[x], y == 1}}] Plot3D[{f[x, y], g[x, y]}, {x, 0, 10}, {y, 0, 10}] doesn't work. Any ideas? Answer ParametricPlot3D[{{Sin[u], 0, u}, {Cos[u], 1, u}}, {u, 0, 20}, BoxRatios -> 1, PlotRange -> {{-2, 2}, {-.5, 2}, {0, 20}}] === update - general thoughts === I've seen the comments to this question and a few other approaches to the waterfall (or wire) plot are given in this question: Plotting several functions . Also I cannot not mention one of my favorite Mathematica blogs - Archery with this post on the story of discovery of 1st pulsar and an its infamous graphics - code is given in the post: