Task: Langton's ant
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int w = 0, h = 0;
unsigned char *pix;

void refresh(int x, int y)
{
	int i, j, k;
	printf("\033[H");
	for (i = k = 0; i < h; putchar('\n'), i++)
		for (j = 0; j < w; j++, k++)
			putchar(pix[k] ? '#' : ' ');
}

void walk()
{
	int dx = 0, dy = 1, i, k;
	int x = w / 2, y = h / 2;

	pix = calloc(1, w * h);
	printf("\033[H\033[J");

	while (1) {
		i = (y * w + x);
		if (pix[i]) k = dx, dx = -dy, dy = k;
		else	    k = dy, dy = -dx, dx = k;

		pix[i] = !pix[i];
		printf("\033[%d;%dH%c", y + 1, x + 1, pix[i] ? '#' : ' ');

		x += dx, y += dy;

		k = 0;
		if (x < 0) {
			memmove(pix + 1, pix, w * h - 1);
			for (i = 0; i < w * h; i += w) pix[i] = 0;
			x++, k = 1;
		}
		else if (x >= w) {
			memmove(pix, pix + 1, w * h - 1);
			for (i = w-1; i < w * h; i += w) pix[i] = 0;
			x--, k = 1;
		}

		if (y >= h) {
			memmove(pix, pix + w, w * (h - 1));
			memset(pix + w * (h - 1), 0, w);
			y--, k = 1;
		}
		else if (y < 0) {
			memmove(pix + w, pix, w * (h - 1));
			memset(pix, 0, w);
			y++, k = 1;
		}
		if (k) refresh(x, y);
		printf("\033[%d;%dH\033[31m@\033[m", y + 1, x + 1);

		fflush(stdout);
		usleep(10000);
	}
}

int main(int c, char **v)
{
	if (c > 1) w = atoi(v[1]);
	if (c > 2) h = atoi(v[2]);
	if (w < 40) w = 40;
	if (h < 25) h = 25;

	walk();
	return 0;
}

C#: using System;

namespace LangtonAnt
{
    public struct Point
    {
        public int X;
        public int Y;

        public Point(int x, int y)
        {
            X = x;
            Y = y;
        }
    }

    enum Direction
    {
        North, East, West, South
    }

    public class Langton
    {
        public readonly bool [,] IsBlack;
        private Point _origin;
        private Point _antPosition = new Point(0, 0);
        public bool OutOfBounds { get; set;}

        
        private Direction _antDirection = Direction.East;

        private readonly Direction[] _leftTurn = new[] { Direction.West, Direction.North, Direction.South, Direction.East };
        private readonly Direction[] _rightTurn = new[] { Direction.East, Direction.South, Direction.North, Direction.West };
        private readonly int[] _xInc = new[] { 0, 1,-1, 0};
        private readonly int[] _yInc = new[] {-1, 0, 0, 1};

        public Langton(int width, int height, Point origin)
        {
            _origin = origin;
            IsBlack = new bool[width, height];
            OutOfBounds = false;
        }

        public Langton(int width, int height) : this(width, height, new Point(width / 2, height / 2)) {}

        private void MoveAnt()
        {
            _antPosition.X += _xInc[(int)_antDirection];
            _antPosition.Y += _yInc[(int)_antDirection];
        }

        public Point Step()
        {
            if (OutOfBounds)
            {
                throw new InvalidOperationException("Trying to step after ant is out of bounds");
            }
            Point ptCur = new Point(_antPosition.X + _origin.X, _antPosition.Y + _origin.Y);
            bool leftTurn = IsBlack[ptCur.X, ptCur.Y];
            int iDirection = (int) _antDirection;
            _antDirection = leftTurn ? _leftTurn[iDirection] : _rightTurn[iDirection];
            IsBlack[ptCur.X, ptCur.Y] = !IsBlack[ptCur.X, ptCur.Y];
            MoveAnt();
            ptCur = new Point(_antPosition.X + _origin.X, _antPosition.Y + _origin.Y);
            OutOfBounds = 
                ptCur.X < 0 ||
                ptCur.X >= IsBlack.GetUpperBound(0) ||
                ptCur.Y < 0 ||
                ptCur.Y >= IsBlack.GetUpperBound(1);
            return _antPosition;
        }
    }
    class Program
    {
        static void Main()
        {
            Langton ant = new Langton(100, 100);

            while (!ant.OutOfBounds) ant.Step();

            for (int iRow = 0; iRow < 100; iRow++)
            {
                for (int iCol = 0; iCol < 100; iCol++)
                {
                    Console.Write(ant.IsBlack[iCol, iRow] ? "#" : " ");
                }
                Console.WriteLine();
            }

            Console.ReadKey();
        }
    }
}