Linear interpolation in C#

Readme.md

Calculating linear interpolation in C#

Calculates, or predicts, a future value by using existing values.

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace LinearInterpolation
{
    class Program
    {
        static void Main(string[] args)
        {
            // Input pairs in form of a key(x) - value(y)
            var knownPairsXY = new SortedList<int, int>()
            {
                {0, 0},
                {30, 35},
                {70, 90 },
                {100, 130}
            };
            
            // Input keys(x) for predicting a value(y) pairs
            var forecastKeys = new List<int> { 10, 20, 40, 50, 60, 80, 190 };

            // Ensuring interpolation
            if (forecastKeys.Max() >= knownPairsXY.Keys.Max())
                throw new Exception("The maximum value of the forecast points must be lower!");
            if (forecastKeys.Min() <= knownPairsXY.Keys.Min())
                throw new Exception("The minimum value of the forecast points must be higher!");
            
            var avgX = knownPairsXY.Keys.Average();
            var avgY = knownPairsXY.Values.Average();

            var level = SlopeLevel(knownPairsXY);

            foreach (var forecastKey in forecastKeys)
            {
                // Calculating predicted value
                var predictedValue = (avgY - level * avgX) + level * forecastKey;
                knownPairsXY.Add(forecastKey, (int)Math.Round(predictedValue));
            }

            Console.WriteLine("\t-KEY-\t-VALUE-");
            foreach (var (key, value) in knownPairsXY)
                Console.WriteLine("\t{0}:\t{1}", key, value);
        }

        /// <summary>
        /// Calculates the slope level for given values
        /// </summary>
        /// <param name="knownPairsXY"> The known x and y values </param>
        /// <returns> Slope level </returns>
        private static double SlopeLevel(SortedList<int, int> knownPairsXY)
        {
            var top = 0.0;
            var bottom = 0.0;

            var avgX = knownPairsXY.Keys.Average();
            var avgY = knownPairsXY.Values.Average();

            // Calculating top and bottom bound
            for (var i = 0; i < knownPairsXY.Count; i++)
            {
                top += (knownPairsXY.Keys[i] - avgX) * (knownPairsXY.Values[i] - avgY);
                bottom += Math.Pow((knownPairsXY.Keys[i] - avgX), 2);
            }

            return top / bottom;
        }
    }
}

Hello and thank you for your code
I have a code in Matlab which uses interp1 function:
https://ch.mathworks.com/help/matlab/ref/interp1.html

I need the same function in C#, can you help with that?

Hello and thank you for your code I have a code in Matlab which uses interp1 function: https://ch.mathworks.com/help/matlab/ref/interp1.html

I need the same function in C#, can you help with that?

I assume this is now very old but it shows up in google so this was my solution:

    public static double[] Interp(double[] x, double[] v, double[] xq)
    {
        var stepInterp = new MathNet.Numerics.Interpolation.StepInterpolation(x, v);
        return xq.Select(q => stepInterp.Interpolate(q)).ToArray();
    }

If you want something more complicated there is spline interpolation stuff there as well.