Random Walk on 2D Modification

I have 2D random walk code part for neutrons. However, the code includes some conditions, 2 of them are:

• on the next step the neutron cannot step back, but only forward, left or right ,
• the probability to go forward is twice more than changing a direction

I could not modified the code for "cannot step back" part. Also, i have no idea how to make the probability is doubled for not changing direction. Since, if it's once changed direction it has the change to go straight forward is still doubled.

Here's the code:

clc;
clearvars;
N = 100; % Length of the x-axis, also known as the length of the random walks.
M = 100; % The amount of random walks.
x_t(1) = 0;
y_t(1) = 0;
for m=1:M
  for n = 1:N 
    A = sign(randn); 
    x_t(n+1) = x_t(n) + A;
    A = sign(randn); 
    y_t(n+1) = y_t(n) + A;
  end
  plot(x_t, y_t);
  hold on
end
grid on;

set(gcf, 'Units', 'Normalized', 'Outerposition', [0, 0.05, 1, 0.95]);
axis square;

[Walter Roberson]

Please do not close questions that have an answer. The volunteers spend their time researching and testing solutions for you, and it is not fair to the volunteers to have their contributions suddenly disappear.

[Ege Tunç]

I though "close" means, the question is answered, no more answers needed. Am i wrong?

[Walter Roberson]

"close" means make it invisible to the public, such as for a question that needs clarification from the author.

[Ege Tunç]

okay, i opened it. sorry for my misunderstanding

Answer 1

[Walter Roberson]

dd = randi(4)
  for ss = 1:500
    rr = rand;
   if rr < 1/4
    dd = 1+mod(dd,4)   %next higher direction
  elseif rr < 1/2
     dd = 1+mod(dd-2,4)     %next lower direction
  end    % 50 percent stays same direction 
  if dd==1
   yy=yy+1;     %north 
  elseif dd==2
    xx=xx+1;    %east
  elseif dd==3
    yy=yy-1;     %south
   else
     xx=xx-1;    %west
   end
   plot here probably 
  end