MATLAB problem

clear
clc
clf


% Create a 2D array of size 8x8 containing zeros
a = zeros(8, 8);

% Set the pixel at position (3,3) to 1
a(3, 3) = 1;

% Determine the size of the matrix a
[y_len, x_len] = size(a);

% Nested for loop to iterate through all y and x coordinates
for y = 1:y_len
    for x = 1:x_len
        % Check if the value at a(y,x) is equal to 1 (gas particle exists)
        if a(y, x) == 1
            % Choose a random number between 0 and 3
           rtest = floor(rand*3.9);
           r = rtest;                                            % <<<<<<<<<<<<<< problem here. When r = rtest then the only values generated are 0 and 2 (so not behaving as it should) however if you set r to equal 1 for example and not rtest then rtest then starts behaving as it should. so somehow by setting r = rtest its changing rtests behaviour.

            % Move the gas particle based on the random number
            if r == 0 && y > 1
                % Move up
                a(y-1, x) = 1;
                a(y, x) = 0;
            elseif r == 1 && y < y_len
                % Move down
                a(y+1, x) = 1;
                a(y, x) = 0;
            elseif r == 2 && x > 1
                % Move left
                a(y, x-1) = 1;
                a(y, x) = 0;
            elseif r == 3 && x < x_len
                % Move right
                a(y, x+1) = 1;
                a(y, x) = 0;
            end
        end
    end
end

% g)
if r == 0
        % Move up
       disp('Moving particle up')
    elseif r == 1
        % Move down
        disp('Moving particle down')
    elseif r == 2
        % Move left
        disp('Moving particle left')
    elseif r == 3
        % Move right
        disp('Moving particle right')
end