If we want to pick a random point on the surface of a sphere, it is incorrect to select spherical coordinates \theta and \phi from uniform distributions 0 \leq \phi \leq 2\pi and 0 \leq \theta \leq \pi, since the area element \sin(\theta) d\theta d\phi is a function of \theta, and hence points picked in this way will be “bunched” near the poles (top).

To obtain points such that any small area on the sphere is expected to contain the same number of points, choose r_{1} and r_{2} to be random variates on (0,1). Then
\phi = 2 \pi r_{1}
\theta = arccos(1 - 2 r_{2} )
gives the spherical coordinates for a set of points which are uniformly distributed over S^{2} (bottom).

not uniform distribution

uniform distribution

// Copyleft (c) 2012. Ridlo W. Wibowo
#include<iostream>
#include<stdlib.h>
#include<math.h>
#include<stdio.h>
#include<time.h>
#include<fstream>
#define _USE_MATH_DEFINES
using namespace std;


double unirand(){return (double) rand()/(double) RAND_MAX;}

int main(){
    double l = 0.1, phi, theta;
    double x, y, z;
    int n=10000;

    srand(time(NULL));
    ofstream out("bola.txt");
    for (int i=0;i<n;i++){
        phi = 2.*M_PI*unirand();
        theta = acos(1. - 2.*unirand());
        //theta = M_PI*unirand();

        x = l*sin(theta)*cos(phi);
        y = l*sin(theta)*sin(phi);
        z = l*cos(theta);
        
        out << x << " " << y << " " << z << " " << endl;
    }
    out.close();
    
    return 0;
}
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c / c++, gnuplot, random number, random walk, ubuntu

November 18, 2012

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