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I tried to plot integer points that fall into a 2D region that's dynamically changed. So I resort to Cases and Select, but Cases with Condition is not working expectedly. Here is the simplifed version of my code and result:

region[u_] := u <= x + y <= u + 2;
intPoints = Flatten[Table[{x, y}, {x, 0, 13}, {y, 0, 13}], 1];
Manipulate[
 Column[{
   {ListPlot[intPoints], 
    RegionPlot[region[u], {x, 0, 13}, {y, 0, 13}]},
   {ListPlot[Cases[intPoints, {x_, y_} /; region[u]]],
    ListPlot[Cases[intPoints, {x_, y_} /; Evaluate[region[u]]]],
    ListPlot[Select[intPoints, region[u] /. {x -> #[[1]], y -> #[[2]]} &]]}}],
 {u, 0, 13}
 ]

enter image description here

I don't understand why inside Cases the point coordinates x and y won't go into region for the Condition to be evaluated. Does it first do the replacement of x and y and then do the evaluation? But why the additional Evaluate operation still doesn't help?

Could someone elaborate me on this? Thanks.

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1 Answer 1

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Cases could not 'see' x and y from the pattern {x_,y_} (Condition has Attribute HoldAll); it could have been resolved using Evaluate around region[u].

In short, use Initialization; allows for better control over the content of Manipulate.

Manipulate[
 (* expr *)
 grid[u],

 (* control *)
 {u, 0, 13},

 (* init *)
 Initialization :> (
   (* points *)
   intPoints = Flatten[Table[{x, y}, {x, 0, 13}, {y, 0, 13}], 1];

   (* redefined region function; making point coordinates explicit *)
   region[u_, x_, y_] := u <= x + y <= u + 2;

   (* list plot *)
   listPlot = ListPlot[intPoints];

   (* region plot *)
   regionPlot[u_] := RegionPlot[region[u, x, y], {x, 0, 13}, {y, 0, 13}, PlotLabel -> RegionPlot];

   (* list plot - Cases *)
   listPlotCases[u_] := ListPlot[Cases[intPoints, {x_, y_} /; region[u, x, y]], PlotLabel -> Cases, PlotRange -> {{0, 13}, {0, 13}}];

   (* list plot - Select *)
   listPlotSelect[u_] := ListPlot[Select[intPoints, region[u, #[[1]], #[[-1]]] &], PlotLabel -> Select, PlotRange -> {{0, 13}, {0, 13}}];

   (* assemble output *)
   grid[u_] := Grid[{
      {listPlot, regionPlot[u]},
      {listPlotCases[u], listPlotSelect[u]},
      {Style[StringTemplate["u=``"][u], Red, Bold, Large], SpanFromLeft}
      }];
   ),
 SynchronousInitialization -> False,
 SynchronousUpdating -> False,
 LocalizeVariables -> True
]

The code above outputs

enter image description here

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2
  • $\begingroup$ Thanks for your solution. I learned some good practice from your code. I still need to understand why Evaluate[region[u]] won't work. And when I set LocalizeVariables -> False, this line ListPlot[Cases[intPoints, {x_, y_} /; Evaluate[region[u]]]] actually works inside Manipulate, just like it works inside Dynamic. Could you help me explain why this is the case? Thanks. $\endgroup$
    – ricecakebear
    Commented Feb 28, 2018 at 14:01
  • $\begingroup$ you're welcome! it's nice to know the answer was helpful; I think it would be instructive to evaluate Manipulate[aHead[{x_, y_} /; region[u]] // FullForm, {u, 0, 13}, LocalizeVariables -> False] with LocalizeVariables set to False/True (defaults to True) and with using/not using Evaluate; notice what happens to the variables $\endgroup$
    – user42582
    Commented Feb 28, 2018 at 15:25

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