Derivative of a 2D Interpolating function

Question

I cannot find a way to obtain the partial derivatives of a 2D Interpolating Function obtained with ListInterpolation. I didn't find anything useful on the web. Any clue?

Answer 1

Another form I have found useful is:

f = ListInterpolation[
  Table[Sin[x y], {x, 0, 1, .25}, {y, 0, 2, .25}], {{0, 1}, {0, 2}}]

Then one can make derivative functions that can be treated as normal function via

dfdx[x_, y_] := Evaluate[D[f[x, y], {x, 1}]]
dfdy[x_, y_] := Evaluate[D[f[x, y], {y, 1}]]

and the second derivatives

d2fdx[x_, y_] := Evaluate[D[f[x, y], {x, 2}]]
d2fdy[x_, y_] := Evaluate[D[f[x, y], {y, 2}]]

the partial with respect to x and then y

dfdydx[x_, y_] := Evaluate[D[f[x, y], x, y]]

or as pointed out by J.M. a more compact and very readable form:

dfdx = Derivative[1, 0][f]
dfdy = Derivative[0, 1][f]
dfdydx = Derivative[1, 1][f]

Answer 2

Using an example from the documentation of ListInterpolation:

f = ListInterpolation[
  Table[Sin[x y], {x, 0, 1, .25}, {y, 0, 2, .25}], {{0, 1}, {0, 2}}]
dfdx[u_, v_] := D[f[x, y], x] /. {x -> u, y -> v}
dfdy[u_, v_] := D[f[x, y], y] /. {x -> u, y -> v}
Manipulate[
 Show[{Plot3D[f[x, y], {x, 0, 1}, {y, 0, 2}], 
   Graphics3D[{Red, PointSize[0.03], 
     Table[Point[{x, y, Sin[x y]}], {x, 0, 1, .25}, {y, 0, 2, .25}], 
     Blue, Thick, 
     Arrow[{{p, q, f[p, q]}, {p, q, f[p, q]} - 
        Normalize[{dfdx[p, q], dfdy[p, q], -1}]}]}]}, 
  BoxRatios -> Automatic, 
  PlotRange -> {{-1, 2}, {-1, 2}, {0, 2}}], {p, 0, 1}, {q, 0, 2}]

Answer 3

Please make your question clear.

But I think you're simply using f'[x,y] and hope that you can get a result?

Try the following code:

f = ListInterpolation[
  Table[Sin[x y], {x, 0, 1, .25}, {y, 0, 2, .25}], {{0, 1}, {0, 2}}]

D[f[x, y], x]

Plot3D[Evaluate@D[f[x, y], x], {x, 0, 1}, {y, 0, 2}]

for higher order:

D[f[x,y],{x,2}]

Will this code help?

Also, you may want a derivatives on a certain direction:

Ddir[f_, {x0_, y0_}] := Normalize[{x0, y0}].{f[x, y]~D~x, f[x, y]~D~y}

Plot3D[Evaluate@Ddir[f, {1, 1}], {x, 0, 1}, {y, 0, 2}]

I hate these simple copy work of documentation. Please check the documentation up next time you ask.