Skip to main content

numerical integration - NIntegrate piecewise vector function


Is there a way to numerically integrate a vector function defined via Piecewise?


Example:


 test[s_] := Piecewise[{
{{s, s^2, s^3}, s < -2},
{{0, s - 1, Sin[s]}, -2 <= s < 1},
{{1 - s^2, Exp[s], 0},1 <= s}}]
NIntegrate[test[s],{s,-2,2}]


This gives NIntegrate::inum: Integrand … is not numerical at … as error.



Answer



I suppose I dislike mixing structural operations - like Thread - inside algebraic expressions. My objections are two-fold:


1) I prefer to keep Mathematica operations distinct from actual algebra for aesthetic reasons.


2) If an expression is cut and pasted and changed, operations like Thread can all too easily end up chewing into mathematical expressions, and generate nonsense!


IMHO, a better approach is to write a VectorPiecewise function that checks if it is handling the scalar case, and returns it if it is, rejects inconsistent vector expressions, and returns the vector case as a list of scalar Piecewise expressions, e.g.:


VectorPiecewise::syntax = 
"Invalid VectorPiecewise construct - vector expressions are either of \
incompatible lengths or some are not vectors";


VectorPiecewise[expr_] := Module[{vv, hv, vc},
vv = expr[[All, 1]];
hv = Map[(Head[#] === List) &, vv];
If[! Or @@ hv,
Piecewise[expr],
If[(! And @@ hv ) || (! Equal @@ Map[Length, vv]),
Message[VectorPiecwise::syntax];,
vc = expr[[All, 2]];
vvt = Transpose[vv];

Map[Piecewise, MapIndexed[{#1, vc[[#2[[2]]]]} &, vvt, {2}]]
]
]
];

test[s_] :=
VectorPiecewise[{{{s, s^2, s^3},
s < -2}, {{0, s - 1, Sin[s]}, -2 <= s < 1}, {{1 - s^2, Exp[s], 0},
1 <= s}}]


NIntegrate[test[s], {s, -2, 2}]
{-1.33333, 0.170774, -0.956449}

Plot[test[s] // Evaluate, {s, -2, 2}, Filling -> {1 -> {2}, 2 -> {3}}]

Mathematica graphics


If your complete expression contains several such VectorPiecewise expressions, as you describe, the normal operation of the Listable attribute will ensure the expression will work as expected, except that multiplying two vectors together needs you to specify either Dot or Cross - default multiplication of vectors gives you nothing useful.


Comments

Popular posts from this blog

front end - keyboard shortcut to invoke Insert new matrix

I frequently need to type in some matrices, and the menu command Insert > Table/Matrix > New... allows matrices with lines drawn between columns and rows, which is very helpful. I would like to make a keyboard shortcut for it, but cannot find the relevant frontend token command (4209405) for it. Since the FullForm[] and InputForm[] of matrices with lines drawn between rows and columns is the same as those without lines, it's hard to do this via 3rd party system-wide text expanders (e.g. autohotkey or atext on mac). How does one assign a keyboard shortcut for the menu item Insert > Table/Matrix > New... , preferably using only mathematica? Thanks! Answer In the MenuSetup.tr (for linux located in the $InstallationDirectory/SystemFiles/FrontEnd/TextResources/X/ directory), I changed the line MenuItem["&New...", "CreateGridBoxDialog"] to read MenuItem["&New...", "CreateGridBoxDialog", MenuKey["m", Modifiers-...

How to thread a list

I have data in format data = {{a1, a2}, {b1, b2}, {c1, c2}, {d1, d2}} Tableform: I want to thread it to : tdata = {{{a1, b1}, {a2, b2}}, {{a1, c1}, {a2, c2}}, {{a1, d1}, {a2, d2}}} Tableform: And I would like to do better then pseudofunction[n_] := Transpose[{data2[[1]], data2[[n]]}]; SetAttributes[pseudofunction, Listable]; Range[2, 4] // pseudofunction Here is my benchmark data, where data3 is normal sample of real data. data3 = Drop[ExcelWorkBook[[Column1 ;; Column4]], None, 1]; data2 = {a #, b #, c #, d #} & /@ Range[1, 10^5]; data = RandomReal[{0, 1}, {10^6, 4}]; Here is my benchmark code kptnw[list_] := Transpose[{Table[First@#, {Length@# - 1}], Rest@#}, {3, 1, 2}] &@list kptnw2[list_] := Transpose[{ConstantArray[First@#, Length@# - 1], Rest@#}, {3, 1, 2}] &@list OleksandrR[list_] := Flatten[Outer[List, List@First[list], Rest[list], 1], {{2}, {1, 4}}] paradox2[list_] := Partition[Riffle[list[[1]], #], 2] & /@ Drop[list, 1] RM[list_] := FoldList[Transpose[{First@li...

dynamic - How can I make a clickable ArrayPlot that returns input?

I would like to create a dynamic ArrayPlot so that the rectangles, when clicked, provide the input. Can I use ArrayPlot for this? Or is there something else I should have to use? Answer ArrayPlot is much more than just a simple array like Grid : it represents a ranged 2D dataset, and its visualization can be finetuned by options like DataReversed and DataRange . These features make it quite complicated to reproduce the same layout and order with Grid . Here I offer AnnotatedArrayPlot which comes in handy when your dataset is more than just a flat 2D array. The dynamic interface allows highlighting individual cells and possibly interacting with them. AnnotatedArrayPlot works the same way as ArrayPlot and accepts the same options plus Enabled , HighlightCoordinates , HighlightStyle and HighlightElementFunction . data = {{Missing["HasSomeMoreData"], GrayLevel[ 1], {RGBColor[0, 1, 1], RGBColor[0, 0, 1], GrayLevel[1]}, RGBColor[0, 1, 0]}, {GrayLevel[0], GrayLevel...