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Add method with option to not allow extrapolation. #995

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Add method with option to not allow extrapolation. #995

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13 changes: 13 additions & 0 deletions src/Numerics.Tests/InterpolationTests/LinearSplineTest.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,7 @@

using MathNet.Numerics.Interpolation;
using NUnit.Framework;
using System;

namespace MathNet.Numerics.Tests.InterpolationTests
{
Expand Down Expand Up @@ -138,5 +139,17 @@ public void FewSamples()
Assert.That(() => LinearSpline.Interpolate(new double[1], new double[1]), Throws.ArgumentException);
Assert.That(LinearSpline.Interpolate(new[] { 1.0, 2.0 }, new[] { 2.0, 2.0 }).Interpolate(1.0), Is.EqualTo(2.0));
}

[Test]
public void DoNotExtrapolate()
{
LinearSpline ip = LinearSpline.Interpolate(_t, _y);

Assert.Throws<ArgumentOutOfRangeException>(() => ip.Interpolate(-2.1, false));
Assert.Throws<ArgumentOutOfRangeException>(() => ip.Interpolate(10.0, false));

Assert.Throws<ArgumentOutOfRangeException>(() => LinearSpline.Interpolate(new[] { 1.0, 2.0 }, new[] { 2.0, 2.0 }).Interpolate(0.9, false));
Assert.Throws<ArgumentOutOfRangeException>(() => LinearSpline.Interpolate(new[] { 1.0, 2.0 }, new[] { 2.0, 2.0 }).Interpolate(3.0, false));
}
}
}
18 changes: 18 additions & 0 deletions src/Numerics/Interpolation/LinearSpline.cs
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Expand Up @@ -130,6 +130,24 @@ public static LinearSpline Interpolate(IEnumerable<double> x, IEnumerable<double
/// <returns>Interpolated value x(t).</returns>
public double Interpolate(double t)
{
return Interpolate(t, true);
}

/// <summary>
/// Interpolate at point t.
/// </summary>
/// <param name="t">Point t to interpolate at.</param>
/// <param name="allowExtrapolate">set to fals if extrapolate should not be allowed</param>
/// <returns>Interpolated value x(t).</returns>
/// <exception cref="ArgumentOutOfRangeException">Thrown when t is outside range and would result in Extrapolation!</exception>
public double Interpolate(double t, bool allowExtrapolate)
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I am not a big fan of bool variables in public APIs which act like toggle switches. I would rather create another public function, something like InterpolateWithPossibleExtrapolation, which probably needs to defined on the interface, so every instance can implement it.

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I understand, it was intended to keep the interface clean and avoid polution of weird functions. Adding an additional function like InterpolateWithoutExtrapolation felt a bit like polluting the clean interface.
Also I didn't want to break any compatibility since the default behavior is currently with possible extrapolation.
To be honest (now I read it again) I can see it isn't making the interface less confusing since the flag name can give the impression that extrapolation is by default not allowed.

As stated in the pull request as well. If you do see potential in the addition of such functionality we can discuss what the most suitable implementation is and how it fits the library best.

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Hi again. I am in favor of having a clean API, and I feel that your proposal is in the right direction.

You are right, the name I suggested should have been InterpolateWithoutExtrapolation

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That all the interpolators extrapolate by default is possibly surprising behaviour, especially given that is not documented (as far as I can tell).
But I wonder if InterpolateWithoutExtrapolation or the proposed implementation (with allowExtrapolate) will resolve this in general.
Presumably users of the other splines would also want this method which would mean adding this method to all classes and to the interface.

Perhaps a cleaner implementation would be to create a decorator class called NonExtrapolatingSpline (i.e., implements IInterpolation and wraps IInterpolation), that handles this in general for all the interpolators.
Also, maybe doc comments could be added to each of the interpolators indicating that they extrapolate by default, but that this new class exists if anyone wants to opt out.

{
if (!allowExtrapolate &&
(t < _x[0] || t > _x[_x.Length - 1]))
{
throw new ArgumentOutOfRangeException("'t' is outside of the constructed array. This would result in Extrapolation!");
}

int k = LeftSegmentIndex(t);
return _c0[k] + (t - _x[k])*_c1[k];
}
Expand Down