MATLAB Answers


How to avoid stepping back on random 2D walker

Asked by Ege Tunç on 28 Dec 2018
Latest activity Edited by Ege Tunç on 6 Jan 2019
for n = 1:N .
A = randi(4);
if rr < 1/4
A = 1+mod(A,4);
elseif rr < 1/2
A = 1+mod(A-2,4);
if A==1
elseif A==2
x_t(n+1)=x_t(n); %south
elseif A==3
elseif A==4

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Please type control-a then control-i in MATLAB before pasting here to make sure your indenting is corrected.

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2 Answers

Answer by Walter Roberson
on 28 Dec 2018

you obtained the main code from me in another question . The code I provided cannot step backwards . The code keeps the current direction number 50 percent of the time and alters the direction number by 1 otherwise . A direction number change of 2 would be needed to go backwards and the code II provided cannot change by 2.


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changes on y direction are north and south, changes on x direction east and west. Am I wrong?
Also if neutrons leaves shield any side they must continues as a straigth line.The only problem is checking last step's direction to avoid step back
oh wait now i understood north,east,south,west order is important okay step back issue is done. Ty mate!
Yes, the order is important. The code maintains a current direction, X, which is maintained (50%) or turn left relative to the current direction (X-1, wrapped if necessary), or turn right (X+1, wrapped if necessary) relative to the current direction.

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Answer by Image Analyst
on 28 Dec 2018

What I'd probably do is to compute all your directions in advance, and then remove disallowed ones. That way you don't have to check each one in the loop.
maxNumExpectedSteps = 100; % A million, or whatever - some big number, bigger than you expect to ever need.
directions = randi(4, 1, maxNumExpectedSteps)
% Remove elements where the next direction
% would have been in a disallowed direction.
% If it's going in direction 1, don't let it go in direction 3
% So replace the pattern [1, 3] with 1
directions = strrep(directions, [1, 3], 1);
% If it's going in direction 2, don't let it go in direction 4
% So replace the pattern [2, 4] with 2
directions = strrep(directions, [2, 4], 2);
% If it's going in direction 3, don't let it go in direction 1
% So replace the pattern [3, 1] with 3
directions = strrep(directions, [3, 1], 3);
% If it's going in direction 4, don't let it go in direction 2
% So replace the pattern [4, 2] with 4
directions = strrep(directions, [4, 2], 4);
% Now you're guaranteed to never go in a disallowed direction.
Now, start your loop getting the direction (1 through 4) from the "directions" array.
for k = 1 : numSteps
thisDirection = directions(k);
if thisDirection == 1
% Code to move in direction 1.
elseif thisDirection == 2
% Code to move in direction 2.
elseif thisDirection == 3
% Code to move in direction 3.
elseif thisDirection == 4
% Code to move in direction 4.


yes, i know but, after his warning i tried to find non modified version of the code where i used after some modification. However, i cannot find that version so i added codes which belongs to you.
So if the TA's don't let you use code by us, what can you do? I suppose renaming the variables is also not enough. So I guess if you don't want any code, you should state that in advance. I guess we could just alter your existing code written by you rather than giving improved algorithms.
well, taking some part of codes or getting some modifications are not strictly forbidden. since, my code needed some modifications and these modifications were included in your code. I have take some part of them with some modifications. however, one of the TA’s is so sensitive, I just realized this(idk why). when she sees any little similarity, she prefers to assume the whole paper might be stolen and seeks for . When I realize this, i paniced. however, codes from this topic used for modifications of some part of mine. this is my bad, sorry for that. tried to fix/readd codes that i deleted

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