brownian.cpp

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#include <iostream>
#include <cmath>          // for sin, cos, sqrt
#include "randgen.h"
#include "prompt.h"
#include "mathutils.h"    // for PI
#include "point.h"
using namespace std;

// simluate two-dimensional random walk
// Owen Astrachan, 6/20/95, modified 6/29/96, modified 5/1/99

class RandomWalk2D
{
  public:
	 RandomWalk2D(long maxSteps,
                  int size);    // # of steps, size of one step
    void Init();                // take first step of walk
    bool HasMore();             // returns false if walk finished, else true
    void Next();                // take next step of random walk
    void Simulate();            // complete an entire random walk
    
    long  TotalSteps() const;     // total # of steps taken by molecule
    Point Position()   const;     // current position
    Point Current()    const;     // alias for Position

  private:
    void TakeStep();       // simulate one step of walk   
    Point myPosition;      // coordinate of current position
    long  mySteps;         // # of steps taken
    int   myStepSize;      // size of step
    long  myMaxSteps;      // maximum # of steps allowed
};

RandomWalk2D::RandomWalk2D(long maxSteps,int size)
  : myPosition(),
    mySteps(0),
    myStepSize(size),
    myMaxSteps(maxSteps)
// postcondition: walker initialized
{

}

void RandomWalk2D::TakeStep()
// postcondition: one step of random walk taken
{
    RandGen gen;                    // random number generator
    double randDirection = gen.RandReal(0,2*PI);
 
    myPosition.x += myStepSize * cos(randDirection);
    myPosition.y += myStepSize * sin(randDirection);
    mySteps++;
}

void RandomWalk2D::Init()
// postcondition: Init step of random walk taken
{
    mySteps = 0;
    myPosition = Point(0,0);
    TakeStep();
}

bool RandomWalk2D::HasMore()
// postcondition: returns false when random walk is finished
//                i.e., when # of steps taken >= max. # of steps     
//                return true if walk still in progress
{
    return mySteps < myMaxSteps;
}

void RandomWalk2D::Next()
// postcondition: next step in random walk simulated     
{
    TakeStep();
}

void RandomWalk2D::Simulate()
{
    for(Init(); HasMore(); Next())
    {
        // simulation complete using iterator methods
    }
}

long RandomWalk2D::TotalSteps() const
// postcondition: returns number of steps taken by molecule
{
	 return mySteps;
}

Point RandomWalk2D::Position() const
// postcondition: return molecule's position
{
   return myPosition;
}

Point RandomWalk2D::Current() const
// postcondition: return molecule's position
{
   return myPosition;
}

int main()
{
    long numSteps= PromptRange("enter # of random steps",1L,1000000L);
    int stepSize=  PromptRange("size of one step",1,20);    
    int trials=    PromptRange("number of simulated walks",1,1000);
    RandomWalk2D molecule(numSteps,stepSize);

    int k;
    double total = 0.0;
    Point p;
    for(k=0; k < trials; k++)
    {
        molecule.Simulate();
        p = molecule.Position();
        total += p.distanceFrom(Point(0,0));  // total final distance from origin
    }
    cout << "average distance from origin = " << total/trials << endl;
    return 0;
}