draw histogramm of X(i)

v=9.91256303526217e-3;
x(1)=3.442619855899;
x(128)=0;
for i=2:128
    a(i)=exp(-0.5*x(i-1)^2)+(v/x(i-1));
    x(i)=sqrt(-2*log(a(i)))
    y(i)=nrmlpdf(x(i))
 for i=2:128
        zigr(i)=(x(i)/x(i-1))
    end 
r=x(1);
for i= i:1:128
    if i==0
  u0 = 2*rand()-1; 
  if(abs(u0)<zigr(i))
      X(i)=u0*x(i);
      if(i==0)
            X(i) = tail( r);
           z(i)=u*x(i);
           f0=exp(-0.5*(x(i)^2)-(x(i)^2));
           f1=exp(-0.5*(x(i+1)^2)-(x(i+1)^2));
           if(f1+rand()*(f0-f1)<1.0)
               X(i)=z(i);
           end 
      end 
  end 
    end 
end 
end 
hist(X)

[the cyclist]

I don't think we can run your code, because nrmlpdf is not a standard MATLAB function.

Also, you did not actually ask a question. What do you need? Be specific.

[the cyclist]

I've edited your code here (mostly lining up the code blocks), and changed your nrmlpdf function into an inline function, for convenience.

nrmlpdf = @(x) exp(-x.^2/2);

v=9.91256303526217e-3;
x(1)=3.442619855899;
x(128)=0;
for i=2:128
    a(i)=exp(-0.5*x(i-1)^2)+(v/x(i-1));
    x(i)=sqrt(-2*log(a(i)));
    y(i)=nrmlpdf(x(i));
    for i=2:128
        zigr(i)=(x(i)/x(i-1));
    end 
    r=x(1);
    for i= i:1:128
        if i==0
          u0 = 2*rand()-1; 
          if(abs(u0)<zigr(i))
              X(i)=u0*x(i);
              if(i==0)
                   X(i) = tail( r);
                   z(i)=u*x(i);
                   f0=exp(-0.5*(x(i)^2)-(x(i)^2));
                   f1=exp(-0.5*(x(i+1)^2)-(x(i+1)^2));
                   if(f1+rand()*(f0-f1)<1.0)
                       X(i)=z(i);
                   end 
              end 
          end 
       end 
    end 
end

hist(X)

You are getting your error because your code never reaches the line where X is defined. The heart of the problem seems to be where you have a second for loop over the same variable:

    for i= i:1:128

This is a very confusing line of code, and certainly doesn't do what you intend. I'm guessing you probably want a second looping variable j, that loops from the current value of i to 128.

But I don't understand the indexing of that section of code, and am not sure which index values should be i, and which should be j.

Furthermore, you have the statement

if i==0

and MATLAB is never going to enter that section, because neither looping variable is ever equal to zero.

Answer 1

[ad lyn]

actually this the ziggurat method programm for generating gaussians randoms numbers.and what's i'm trying to do is to draw the histogramm of X but i'm reveing this message error .

this is the "nrmlpdf" function

function [ y ] = nrmlpdf( x )
y=exp(-x^2/2);

end

Answer 2

[Steven Lord]

Instead of calling hist (whose use is discouraged) use histogram.

And I second the cyclist's question for more information about the difficulty you're experiencing when you try to create the histogram.