Question

I’m trying to use SplineInterpolator

and PolynomialSplineFunction to double a dataset. I think I’m fairly far along the path (I’m probably missing some exception handling):

SplineInterpolator splineInterp;

public double[] doubledArray(double[] y){
    double[] yy = new double[y.length*2];
    // make a double version of y w/ -1 for "null" values
    for(int i = 0; i < yy.length; i++){
        if(i%2 == 0)
            yy[i] = y[i];
        else if(i == yy.length-1)
            yy[i] = yy[0];
        else
            yy[i] = -1;
    }
    // make a corresponding x array to satisfy SplineInterpolator.interpolate
    double[] x = new double[y.length];
    for(int i = 0; i < x.length; i++)
        x[i] = i;
    splineInterp = new SplineInterpolator();
    PolynomialSplineFunction polySplineF = splineInterp.interpolate(x, y);
    for(int i = 0; i < yy.length; i++){
        if(yy[i] == -1){
            yy[i] = polySplineF.value(i);
           // breaks down halfway through polySplineF.value expects and array of y.length
        }
    }
    return yy;
}

But the above is gonna crash in the last for loop at the latest. So, do I have the first part more or less right? After I have my polynomial spline function, how do I use that to create a larger dataset?

Answer 1

In case anybody is following along at home, here is the implementation I came up with for this:

private double[] multiplyArray(double[] y){
    // An array 2 or 4 or N times bigger than the original:
    double[] yy = new double[y.length*arrayMultiplier];
    // An array representing the indices of the original:
    double[] x = new double[y.length];
    for(int i = 0; i < x.length; i++)
        x[i] = i;
    // Get and instance of SplineInterpolator:
    SplineInterpolator splineInterp = new SplineInterpolator();
    // Use that instance's interpolate() function to a PolynomialSplineFunction
    // fitting your data, points y at indices x.
    PolynomialSplineFunction polySplineF = splineInterp.interpolate(x, y);

    // Use the PolynomialSplineFunction to fill in your larger array by supplying
    // index values divided by the arrayMultiplier
    for(int i = 0; i < yy.length; i++){
        yy[i] = polySplineF.value((double)(i/arrayMultiplier));
    }
    return yy;
}

I also figured out how to do the probably more useful fill-in-blanks use if anybody needs it.