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From: "nawal " <nawalyarey@gmail.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: random walk & random number
Date: Wed, 6 Nov 2013 20:32:08 +0000 (UTC)
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"Roger Stafford" wrote in message <l5e0ii$d19$1@newscl01ah.mathworks.com>...
> "nawal " <nawalyarey@gmail.com> wrote in message <l5dauc$5p$1@newscl01ah.mathworks.com>...
> > i've created a random wak and plotted it but what I’m stuck on is the last bit (the rg2) because if I run the file I get a value for rg2 and if I run it again I get different value. I want to find a way to get the different values of rg2 at once instead of trying to run it every time and with that I need to plot the values. I don’t know how to do this bit and I’m stuck so if anyone knows how to do please tell me.
> > r=[0 0]; 
> > X=[0];Y=[0];
> > for t= 1:1:100 
> >       B=rand(1,1)*4; 
> >     if B<1
> >         new_position=r+[1 0];
> >     elseif B<2
> >         new_position=r+[0 1];
> >     elseif B<3
> >         new_position=r+[-1 0];
> >     else
> >         new_position=r+[0 -1];
> >     end
> >     X=[X new_position(1)];
> >     Y=[Y new_position(2)]; 
> >     r=new_position; 
> > end
> > plot(X,Y)
> > rg2=mean(X.^2)+mean(Y.^2)-mean(X)^2-mean(Y)^2;
> - - - - - - - - - -
>   What you have calculated here in rg2 is a kind of sample variance of the distance from a mean position throughout a hundred-step "random walk".  Naturally this variance will vary from one hundred-step run to another since it is only a sample variance and not the theoretical variance for this stochastic process.  However a simple plot of the rg2 values using matlab's 'plot' function would be rather chaotic and not particularly informative.  I would think it preferable to make a histogram plot of rg2 values over a large number of repeated trials.  To accomplish this it would be wise to enclose your code, including the necessary initializations, in an outer for-loop to do the appropriate repetition and collect the rg2 values in a vector, rather than manually repeating your code.
> 
> Roger Stafford