Task: N-body problem
C#: using System;
using System.IO;

namespace NBodyProblem {
    class Vector3D {
        public Vector3D(double x, double y, double z) {
            X = x;
            Y = y;
            Z = z;
        }

        public double X { get; }
        public double Y { get; }
        public double Z { get; }

        public double Mod() {
            return Math.Sqrt(X * X + Y * Y + Z * Z);
        }

        public static Vector3D operator +(Vector3D lhs, Vector3D rhs) {
            return new Vector3D(lhs.X + rhs.X, lhs.Y + rhs.Y, lhs.Z + rhs.Z);
        }

        public static Vector3D operator -(Vector3D lhs, Vector3D rhs) {
            return new Vector3D(lhs.X - rhs.X, lhs.Y - rhs.Y, lhs.Z - rhs.Z);
        }

        public static Vector3D operator *(Vector3D lhs, double rhs) {
            return new Vector3D(lhs.X * rhs, lhs.Y * rhs, lhs.Z * rhs);
        }
    }

    class NBody {
        private readonly double gc;
        private readonly int bodies;
        private readonly int timeSteps;
        private readonly double[] masses;
        private readonly Vector3D[] positions;
        private readonly Vector3D[] velocities;
        private readonly Vector3D[] accelerations;

        public NBody(string fileName) {
            string[] lines = File.ReadAllLines(fileName);

            string[] gbt = lines[0].Split();
            gc = double.Parse(gbt[0]);
            bodies = int.Parse(gbt[1]);
            timeSteps = int.Parse(gbt[2]);

            masses = new double[bodies];
            positions = new Vector3D[bodies];
            velocities = new Vector3D[bodies];
            accelerations = new Vector3D[bodies];
            for (int i = 0; i < bodies; ++i) {
                masses[i] = double.Parse(lines[i * 3 + 1]);
                positions[i] = Decompose(lines[i * 3 + 2]);
                velocities[i] = Decompose(lines[i * 3 + 3]);
            }

            Console.WriteLine("Contents of {0}", fileName);
            foreach (string line in lines) {
                Console.WriteLine(line);
            }
            Console.WriteLine();
            Console.Write("Body   :      x          y          z    |");
            Console.WriteLine("     vx         vy         vz");
        }

        public int GetTimeSteps() {
            return timeSteps;
        }

        private Vector3D Decompose(string line) {
            string[] xyz = line.Split();
            double x = double.Parse(xyz[0]);
            double y = double.Parse(xyz[1]);
            double z = double.Parse(xyz[2]);
            return new Vector3D(x, y, z);
        }

        private void ComputeAccelerations() {
            for (int i = 0; i < bodies; ++i) {
                accelerations[i] = new Vector3D(0, 0, 0);
                for (int j = 0; j < bodies; ++j) {
                    if (i != j) {
                        double temp = gc * masses[j] / Math.Pow((positions[i] - positions[j]).Mod(), 3);
                        accelerations[i] = accelerations[i] + (positions[j] - positions[i]) * temp;
                    }
                }
            }
        }

        private void ComputeVelocities() {
            for (int i = 0; i < bodies; ++i) {
                velocities[i] = velocities[i] + accelerations[i];
            }
        }

        private void ComputePositions() {
            for (int i = 0; i < bodies; ++i) {
                positions[i] = positions[i] + velocities[i] + accelerations[i] * 0.5;
            }
        }

        private void ResolveCollisions() {
            for (int i = 0; i < bodies; ++i) {
                for (int j = i + 1; j < bodies; ++j) {
                    if (positions[i].X == positions[j].X
                     && positions[i].Y == positions[j].Y
                     && positions[i].Z == positions[j].Z) {
                        Vector3D temp = velocities[i];
                        velocities[i] = velocities[j];
                        velocities[j] = temp;
                    }
                }
            }
        }

        public void Simulate() {
            ComputeAccelerations();
            ComputePositions();
            ComputeVelocities();
            ResolveCollisions();
        }

        public void PrintResults() {
            for (int i = 0; i < bodies; ++i) {
                Console.WriteLine(
                    "Body {0} : {1,9:F6}  {2,9:F6}  {3,9:F6} | {4,9:F6}  {5,9:F6}  {6,9:F6}",
                    i + 1,
                    positions[i].X, positions[i].Y, positions[i].Z,
                    velocities[i].X, velocities[i].Y, velocities[i].Z
                );
            }
        }
    }

    class Program {
        static void Main(string[] args) {
            NBody nb = new NBody("nbody.txt");

            for (int i = 0; i < nb.GetTimeSteps(); ++i) {
                Console.WriteLine();
                Console.WriteLine("Cycle {0}", i + 1);
                nb.Simulate();
                nb.PrintResults();
            }
        }
    }
}

Go: package main

import (
    "fmt"
    "math"
    "os"
)

type vector struct{ x, y, z float64 }

func (v vector) add(w vector) vector {
    return vector{v.x + w.x, v.y + w.y, v.z + w.z}
}

func (v vector) sub(w vector) vector {
    return vector{v.x - w.x, v.y - w.y, v.z - w.z}
}

func (v vector) scale(m float64) vector {
    return vector{v.x * m, v.y * m, v.z * m}
}

func (v vector) mod() float64 {
    return math.Sqrt(v.x*v.x + v.y*v.y + v.z*v.z)
}

var (
    bodies, timeSteps                    int
    masses                               []float64
    gc                                   float64
    positions, velocities, accelerations []vector
)

func initiateSystem(fileName string) error {
    file, err := os.Open(fileName)
    if err != nil {
        return err
    }
    defer file.Close()
    fmt.Fscanf(file, "%f%d%d", &gc, &bodies, &timeSteps)
    masses = make([]float64, bodies)
    positions = make([]vector, bodies)
    velocities = make([]vector, bodies)
    accelerations = make([]vector, bodies)
    for i := 0; i < bodies; i++ {
        fmt.Fscanf(file, "%f", &masses[i])
        fmt.Fscanf(file, "%f%f%f", &positions[i].x, &positions[i].y, &positions[i].z)
        fmt.Fscanf(file, "%f%f%f", &velocities[i].x, &velocities[i].y, &velocities[i].z)
    }
    return nil
}

func resolveCollisions() {
    for i := 0; i < bodies-1; i++ {
        for j := i + 1; j < bodies; j++ {
            if positions[i] == positions[j] {
                velocities[i], velocities[j] = velocities[j], velocities[i]
            }
        }
    }
}

func computeAccelerations() {
    for i := 0; i < bodies; i++ {
        accelerations[i] = vector{0, 0, 0}
        for j := 0; j < bodies; j++ {
            if i != j {
                temp := gc * masses[j] / math.Pow(positions[i].sub(positions[j]).mod(), 3)
                accelerations[i] = accelerations[i].add(positions[j].sub(positions[i]).scale(temp))
            }
        }
    }
}

func computeVelocities() {
    for i := 0; i < bodies; i++ {
        velocities[i] = velocities[i].add(accelerations[i])
    }
}

func computePositions() {
    for i := 0; i < bodies; i++ {
        positions[i] = positions[i].add(velocities[i].add(accelerations[i].scale(0.5)))
    }
}

func simulate() {
    computeAccelerations()
    computePositions()
    computeVelocities()
    resolveCollisions()
}

func printResults() {
    f := "Body %d : % 8.6f  % 8.6f  % 8.6f | % 8.6f  % 8.6f  % 8.6f\n"
    for i := 0; i < bodies; i++ {
        fmt.Printf(
            f, i+1,
            positions[i].x, positions[i].y, positions[i].z,
            velocities[i].x, velocities[i].y, velocities[i].z,
        )
    }
}

func main() {
    if len(os.Args) != 2 {
        fmt.Printf("Usage : %s <file name containing system configuration data>\n", os.Args[0])
    } else {
        err := initiateSystem(os.Args[1])
        if err != nil {
            fmt.Println(err)
            return
        }
        fmt.Print("Body   :      x          y          z    |")
        fmt.Println("     vx         vy         vz")
        for i := 0; i < timeSteps; i++ {
            fmt.Printf("\nCycle %d\n", i+1)
            simulate()
            printResults()
        }
    }
}