spider.m

一些常被用于教学或者参考的概率论的实例的源代码

%spider.m/created by PJNahin for "Dueling Idiots"(1/9/99)
%This m-file simulates 10,000 two-dimensional walks of a spider
%on a user-defined web (in the code below, the web is that
%of Figure 19.1).
%
%
rand('state',100*sum(clock))        %new seed for generator;
t1=[3;0;3;1;2;2;3;4;2];             %input web as COLUMN vectors;
t2=[4;0;2;1;3;5;1;1;2];
t3=[7;0;5;0;6;7;6;3;8];
t4=[8;0;8;0;0;0;8;7;0];
t5=[0;0;0;0;0;0;0;9;0];
web=[t1 t2 t3 t4 t5];               %construct web from column vectors;
d=zeros(1,250);                     %initialize walk duration vector;
for walks=1:10000
   position=1;                      %put spider at starting position;
   steps=0;                         %initialize number of steps;
   while position~=2                %if fly not yet reached, then ...
      choices=web(position,1);      %how many choices for next step?
      choice=floor(choices*rand)+2; %make a random choice;
      position=web(position,choice);%move to new position;
      steps=steps+1;                %up-date number of steps;
   end
   d(steps)=d(steps)+1;             %fly reached, so up-date
end                                 %duration vector;
average=0;                          %compute the average number of
for i=1:50                          %steps to reach the fly;
   average=average+i*d(i);
end
average/10000
plot(d)                             %plot duration vector;
title('Fig.19.3-Distribution of Walk Durations on the Web (10,000 walks)')
xlabel('number of steps')
ylabel('number of walks')
figure(1)