# Cubic Splines in C# QuickStart Sample

Illustrates using natural and clamped cubic splines for interpolation using classes in the Numerics.NET.LinearAlgebra namespace in C#.

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``````using System;

// The CubicSpline class resides in the
// Numerics.NET.Curves namespace.
using Numerics.NET.Curves;

namespace Numerics.NET.QuickStart.CSharp
{
/// <summary>
/// Illustrates creating natural and clamped cubic splines using
/// the CubicSpline class in the Numerics.NET.Curves
/// namespace.
/// </summary>
class CubicSplines
{
static void Main(string[] args)
{
// The license is verified at runtime. We're using
//     https://numerics.net/trial-key

// A cubic spline is a piecewise curve that is made up
// of pieces of cubic polynomials. Its value as well as its first
// derivative are continuous, giving it a smooth appearance.
//
// Cubic splines are implemented by the CubicSpline class,
// which inherits from PiecewiseCurve.
//
// For an example of piecewise constant and piecewise
// linear curves, see the PiecewiseCurves QuickStart
// Sample.
//

//
// Creating Cubic Splines
//

// In order to define a spline curve completely, two extra
// conditions must be imposed.

// 'Natural' splines have zero second derivatives at the
// end points. This is the default.

// The data points are specified as double arrays containing
// the x and y values:
double[] xValues = {1, 2, 3, 4, 5, 6};
double[] yValues = {1, 3, 4, 3, 4, 2};
CubicSpline naturalSpline = new CubicSpline(xValues, yValues);

// There is also a static method that constructs natural splines:
naturalSpline = CubicSpline.CreateNatural(xValues, yValues);

// 'Clamped' splines have a fixed slope or first derivative at the
// leftmost and rightmost points. The slopes are specified as
// two extra parameters in the constructor:
CubicSpline clampedSpline = new CubicSpline(xValues, yValues, -1, 1);
// Here is the factory method:
clampedSpline = CubicSpline.CreateClamped(xValues, yValues, -1, 1);

// 'Akima' splines minimize the oscillations in the interpolating
// curve. The constructor takes an extra argument of type
// CubicSplineKind:
CubicSpline akimaSpline = new CubicSpline(xValues, yValues, CubicSplineKind.Akima);
// The factory method does not require the extra parameter:
akimaSpline = CubicSpline.CreateAkima(xValues, yValues);

// Hermite splines have fixed values for the first derivative at each
// data point. The first derivatives must be supplied as an array
// or list:
double[] yPrimeValues = { 0, 1, -1, 1, 0, -1 };
CubicSpline hermiteSpline = new CubicSpline(xValues, yValues, yPrimeValues);
// Likewise for the factory method:
hermiteSpline = CubicSpline.CreateHermiteInterpolant(xValues, yValues, yPrimeValues);

//
// Curve Parameters
//

// The shape of any curve is determined by a set of parameters.
// These parameters can be retrieved and set through the
// Parameters collection. The number of parameters for a curve
// is given by this collection's Count property.
//
// Cubic splines have 2n+2 parameters, where n is the number of
// data points. The first n parameters are the x-values. The next
// n parameters are the y-values. The last two parameters are
// the values of the derivative at the first and last point. For natural
// splines, these parameters are unused.

Console.WriteLine("naturalSpline.Parameters.Count = {0}",
naturalSpline.Parameters.Count);
// Parameters can easily be retrieved:
Console.WriteLine("naturalSpline.Parameters[0] = {0}",
naturalSpline.Parameters[0]);
// Parameters can also be set:
naturalSpline.Parameters[0] = 1;

//
// Piecewise curve methods and properties
//

// The NumberOfIntervals property returns the number of subintervals
// on which the curve has unique definitions.
Console.WriteLine("Number of intervals: {0}",
naturalSpline.NumberOfIntervals);

// The IndexOf method returns the index of the interval
// that contains a specific value.
Console.WriteLine(\$"naturalSpline.IndexOf(1.4) = {naturalSpline.IndexOf(1.4)}");
// The method returns -1 when the value is smaller than the lower bound
// of the first interval, and NumberOfIntervals if the value is equal to or larger than
// the upper bound of the last interval.

//
// Curve Methods
//

// The ValueAt method returns the y value of the
// curve at the specified x value:
Console.WriteLine(\$"naturalSpline.ValueAt(2.4) = {naturalSpline.ValueAt(2.4)}");

// The SlopeAt method returns the slope of the curve
// a the specified x value:
Console.WriteLine(\$"naturalSpline.SlopeAt(2) = {naturalSpline.SlopeAt(2)}");
// You can verify that the clamped spline has the correct slope at the end points:
Console.WriteLine(\$"clampedSpline.SlopeAt(1) = {clampedSpline.SlopeAt(1)}");
Console.WriteLine(\$"clampedSpline.SlopeAt(6) = {clampedSpline.SlopeAt(6)}");

// Cubic splines do not have a defined derivative. The GetDerivative method
// returns a GeneralCurve:
Curve derivative = naturalSpline.GetDerivative();
Console.WriteLine(\$"Type of derivative: {derivative.GetType().ToString()}");
Console.WriteLine(\$"derivative(2) = {derivative.ValueAt(2)}");

// You can get a Line that is the tangent to a curve
// at a specified x value using the TangentAt method:
Polynomial tangent = clampedSpline.TangentAt(2);
Console.WriteLine("Slope of tangent line at 2 = {0}",
tangent.Parameters[1]);

// The integral of a spline curve can be calculated exactly. This technique is
// often used to approximate the integral of a tabulated function:
Console.WriteLine("Integral of naturalSpline between 1.4 and 4.6 = {0}",
naturalSpline.Integral(1.4, 4.6));

Console.Write("Press Enter key to exit...");