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# How to avoid stepping back on random 2D walker

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Ege Tunç on 28 Dec 2018
Closed: MATLAB Answer Bot on 20 Aug 2021
for n = 1:N .
A = randi(4);
rr=rand;
if rr < 1/4
A = 1+mod(A,4);
elseif rr < 1/2
A = 1+mod(A-2,4);
end
if A==1
y_t(n+1)=y_t(n)+1;%north
x_t(n+1)=x_t(n);
elseif A==2
y_t(n+1)=y_t(n)-1;
x_t(n+1)=x_t(n); %south
elseif A==3
x_t(n+1)=x_t(n)+1;%east
y_t(n+1)=y_t(n);
elseif A==4
x_t(n+1)=x_t(n)-1;%west
y_t(n+1)=y_t(n);
else
end
end
Image Analyst on 28 Dec 2018
Please type control-a then control-i in MATLAB before pasting here to make sure your indenting is corrected.

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.
Ege Tunç on 28 Dec 2018
oh wait now i understood north,east,south,west order is important okay step back issue is done. Ty mate!
Walter Roberson on 28 Dec 2018
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.

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.
end
end