Task: Numerical integration
C#: using System;
using System.Collections.Generic;
using System.Linq;

public class Interval
{
    public Interval(double leftEndpoint, double size)
    {
        LeftEndpoint = leftEndpoint;
        RightEndpoint = leftEndpoint + size;
    }

    public double LeftEndpoint
    {
        get;
        set;
    }

    public double RightEndpoint
    {
        get;
        set;
    }

    public double Size
    {
        get
        {
            return RightEndpoint - LeftEndpoint;
        }
    }

    public double Center
    {
        get
        {
            return (LeftEndpoint + RightEndpoint) / 2;
        }
    }

    public IEnumerable<Interval> Subdivide(int subintervalCount)
    {
        double subintervalSize = Size / subintervalCount;
        return Enumerable.Range(0, subintervalCount).Select(index => new Interval(LeftEndpoint + index * subintervalSize, subintervalSize));
    }
}

public class DefiniteIntegral
{
    public DefiniteIntegral(Func<double, double> integrand, Interval domain)
    {
        Integrand = integrand;
        Domain = domain;
    }

    public Func<double, double> Integrand
    {
        get;
        set;
    }

    public Interval Domain
    {
        get;
        set;
    }

    public double SampleIntegrand(ApproximationMethod approximationMethod, Interval subdomain)
    {
        switch (approximationMethod)
        {
            case ApproximationMethod.RectangleLeft:
                return Integrand(subdomain.LeftEndpoint);
            case ApproximationMethod.RectangleMidpoint:
                return Integrand(subdomain.Center);
            case ApproximationMethod.RectangleRight:
                return Integrand(subdomain.RightEndpoint);
            case ApproximationMethod.Trapezium:
                return (Integrand(subdomain.LeftEndpoint) + Integrand(subdomain.RightEndpoint)) / 2;
            case ApproximationMethod.Simpson:
                return (Integrand(subdomain.LeftEndpoint) + 4 * Integrand(subdomain.Center) + Integrand(subdomain.RightEndpoint)) / 6;
            default:
                throw new NotImplementedException();
        }
    }

    public double Approximate(ApproximationMethod approximationMethod, int subdomainCount)
    {
        return Domain.Size * Domain.Subdivide(subdomainCount).Sum(subdomain => SampleIntegrand(approximationMethod, subdomain)) / subdomainCount;
    }

    public enum ApproximationMethod
    {
        RectangleLeft,
        RectangleMidpoint,
        RectangleRight,
        Trapezium,
        Simpson
    }
}

public class Program
{
    private static void TestApproximationMethods(DefiniteIntegral integral, int subdomainCount)
    {
        foreach (DefiniteIntegral.ApproximationMethod approximationMethod in Enum.GetValues(typeof(DefiniteIntegral.ApproximationMethod)))
        {
            Console.WriteLine(integral.Approximate(approximationMethod, subdomainCount));
        }
    }

    public static void Main()
    {
        TestApproximationMethods(new DefiniteIntegral(x => x * x * x, new Interval(0, 1)), 10000);
        TestApproximationMethods(new DefiniteIntegral(x => 1 / x, new Interval(1, 99)), 1000);
        TestApproximationMethods(new DefiniteIntegral(x => x, new Interval(0, 5000)), 500000);
        TestApproximationMethods(new DefiniteIntegral(x => x, new Interval(0, 6000)), 6000000);
    }
}

Go: package main

import (
    "fmt"
    "math"
)


type spec struct {
    lower, upper float64               
    n            int                   
    exact        float64               
    fs           string                
    f            func(float64) float64 
}


var data = []spec{
    spec{0, 1, 100, .25, "x^3", func(x float64) float64 { return x * x * x }},
    spec{1, 100, 1000, float64(math.Log(100)), "1/x",
        func(x float64) float64 { return 1 / x }},
    spec{0, 5000, 5e5, 12.5e6, "x", func(x float64) float64 { return x }},
    spec{0, 6000, 6e6, 18e6, "x", func(x float64) float64 { return x }},
}


type method struct {
    name      string
    integrate func(spec) float64
}


var methods = []method{
    method{"Rectangular (left)    ", rectLeft},
    method{"Rectangular (right)   ", rectRight},
    method{"Rectangular (midpoint)", rectMid},
    method{"Trapezium             ", trap},
    method{"Simpson's             ", simpson},
}

func rectLeft(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    x0 := t.lower
    for i := 0; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        
        
        a.add(t.f(x0) * (x1 - x0))
        x0 = x1
    }
    return a.total()
}

func rectRight(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    x0 := t.lower
    for i := 0; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        a.add(t.f(x1) * (x1 - x0))
        x0 = x1
    }
    return a.total()
}

func rectMid(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    
    
    
    
    
    
    x0 := t.lower - .5*r/nf
    for i := 0; i < t.n; i++ {
        x1 := t.lower + (float64(i)+.5)*r/nf
        a.add(t.f(x1) * (x1 - x0))
        x0 = x1
    }
    return a.total()
}

func trap(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    x0 := t.lower
    f0 := t.f(x0)
    for i := 0; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        f1 := t.f(x1)
        a.add((f0 + f1) * .5 * (x1 - x0))
        x0, f0 = x1, f1
    }
    return a.total()
}

func simpson(t spec) float64 {
    var a adder
    r := t.upper - t.lower
    nf := float64(t.n)
    
    
    dx0 := r / nf
    a.add(t.f(t.lower) * dx0)
    a.add(t.f(t.lower+dx0*.5) * dx0 * 4)
    x0 := t.lower + dx0
    for i := 1; i < t.n; i++ {
        x1 := t.lower + float64(i+1)*r/nf
        xmid := (x0 + x1) * .5
        dx := x1 - x0
        a.add(t.f(x0) * dx * 2)
        a.add(t.f(xmid) * dx * 4)
        x0 = x1
    }
    a.add(t.f(t.upper) * dx0)
    return a.total() / 6
}

func sum(v []float64) float64 {
    var a adder
    for _, e := range v {
        a.add(e)
    }
    return a.total()
}

type adder struct {
    sum, e float64
}

func (a *adder) total() float64 {
    return a.sum + a.e
}

func (a *adder) add(x float64) {
    sum := a.sum + x
    e := sum - a.sum
    a.e += a.sum - (sum - e) + (x - e)
    a.sum = sum
}

func main() {
    for _, t := range data {
        fmt.Println("Test case: f(x) =", t.fs)
        fmt.Println("Integration from", t.lower, "to", t.upper,
            "in", t.n, "parts")
        fmt.Printf("Exact result            %.7e     Error\n", t.exact)
        for _, m := range methods {
            a := m.integrate(t)
            e := a - t.exact
            if e < 0 {
                e = -e
            }
            fmt.Printf("%s  %.7e  %.7e\n", m.name, a, e)
        }
        fmt.Println("")
    }
}