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Dec-01-18 13:08:51
Error in extrapolating data

Hello everyone,
I wrote this function in order to interpolate the value between 2 points of an n-points vector and extrapolate (linear extrapolation) the value over the last point of the vector (and previous the first).

Code:


function InterpolateVector
  input Real x                                   "Independent variable";
  input Real vectorPoints[:, 2]          "points of the function";
  output Real y                                 "Dependent variable";
protected
  Integer i;
  Integer n = size(vectorPoints, 1)    "Number of interpolation points";
  Real p;
  Real y1;
  Real y2;
  Real x1;
  Real x2;
  Real q;

algorithm

if x < vectorPoints[1, 1] then
    p := (x - vectorPoints[1, 1]) / (vectorPoints[2, 1] - vectorPoints[1, 1]);
    y := p * vectorPoints[2, 2] + (1 - p) * vectorPoints[1, 2];
   
elseif x > vectorPoints[n, 1] then
    p := (x - vectorPoints[n, 1]) / (vectorPoints[n-1, 1] - vectorPoints[n, 1]);
    y := p * vectorPoints[n-1, 2] + (1 - p) * vectorPoints[n, 2];
   
else
    i := 1;
    while x >= vectorPoints[i + 1, 1] loop
      i := i + 1;
    end while;
    p := (x - vectorPoints[i, 1]) / (vectorPoints[i + 1, 1] - vectorPoints[i, 1]);
    y := p * vectorPoints[i + 1, 2] + (1 - p) * vectorPoints[i, 2];
  end if;
 
end InterpolateVector;

The extrapolated value (y) became a NaN when the input value (x) coincides with the last value of the array.
I can't figure out of this problem, could you please help me?

Thanks,
Best Regards

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