Skip to main content

plotting - Exporting the solutions of two ODEs sampled over same set of domain values



I have two functions h[r] and c[r] defined by two differentials equation.


operh[h_, c_] := h D[D[h, r, r] + 1/r D[h, r] - 1/lc^2 h, r] + m D[c, r]
operc[h_, c_] := 
  cmt h + ξ ((ϕ - H) (1 - ϕ) )/Sqrt[R^2 - r^2] - 
    1/r D[r h (1 + e^2/1680  h^4  (D[c, r])^2 ) D[c, r], r]

I numerically solve them with NDSolve (with solution called "s" here under). Then I plot them with no problem:



Plot[h[r] /. s, {r, rleft, rright}, PlotRange -> All, AxesOrigin -> {0, 0}]
Plot[c[r] /. s, {r, rleft, rright}, PlotRange -> All, AxesOrigin -> {0, 0}]

With same rright and rleft.


graph for h graph for c


What I want is a .txt or .csv file with 3 columns, containing r, h[r] and c[r]. From what I have read on this site I tried:


ploth = 
  Plot[h[r] /. s, {r, rleft, rright}, PlotRange -> All, AxesOrigin -> {0, 0}]
pts = Cases[ploth, Line[pts_] :> pts, Infinity][[1]]
Export[SystemDialogInput["FileSave", "filename.csv"], pts]


It gave me a list of 157 pairs {r, h[r]}. Then I did the same for c:


plotc = 
  Plot[c[r] /. s, {r, rleft, rright}, PlotRange -> All, AxesOrigin -> {0, 0}]
pts = Cases[plotc, Line[pts_] :> pts, Infinity][[1]]
Export[SystemDialogInput["FileSave", "filename.csv"], pts]

And it gave me a list of 226 pairs {r, c[r]}. This is not the same number of data than for h(r), so I can't merge them.


I tried by plotting the two curves on a single graph but it gave me only one of the two data sets.


I also tried to use Table but it gave me an empty list:



tableh = Table[{r, ploth}, {r, rleft, rright}]


{}



Do you know how to extract h and c data for common r values?


I'm on Mathematica 7.0



Answer



An approach to this issue I like is to rely on the fact that NDSolve found the solution on the same grid points for both functions in the first place:


a simple example:



 sol = First@NDSolve[ {f''[x] + g'[x] + 1 == 0 , g[x] == x^2, f[0] == 0, 
   f'[0] == 1}, {f, g}, {x, 0, 1}];
 Plot[{f[x] /. sol, (g[x] /. sol)}, {x, 0, 1}]

enter image description here


here we do not actually use the interpolation but extract the grid values:


 gridpoints = Flatten[(g /. sol)["Grid"]];
 computeddata = 
    Transpose[{gridpoints, 
              (f /. sol)["ValuesOnGrid"],

              (g /. sol)["ValuesOnGrid"]}];
 ListPlot[{ computeddata[[All, {1, 2}]], computeddata[[All, {1, 3}]]}, 
  Joined -> True, PlotMarkers -> Automatic]

enter image description here


here computeddata is a nice array you can simply export with each row {x,f[x],g[x]}


An advantage to this is the point spacing is naturally refined as was needed by NDSolve


Comments

Post a Comment