Random Walk boundaries not working

The particles end up outside the barrier

M = 100;            % the number of particles
N = 80;           % the number of jumps to take
Deltax = 1;        % the size of the jumps in x
Deltay = 1;        % the size of the jumps in y
[row,col] = find(mask); %'mask' is the barrier in which 0 are the barrier and 1 is where particles are free to roam
Position=randi(length(row),M,1);
x=col(Position); % set x position for the particle
y=row(Position); % set y position for the particle
hold on
plot(x,y, '.', 'MarkerSize', 20)

for n = 1:N        % for each of the N jumps

    

    r = rand(1,M); % generate M random numbers between 0 and 1

   

    left_mask  = find(r < 0.25);                            % mask identifying the left-moving particles
    
    for idx = 1:length(left_mask)
        xLoc = x(left_mask);
        yLoc = y(left_mask);
        if mask(xLoc - Deltax, yLoc) == 0
            x(left_mask(idx)) = x(left_mask(idx))  - Deltax;
        end
    end
   % move those particles left

    right_mask = find((0.25 <= r) & (r < 0.5)); 
  for idx = 1:length(right_mask)
        xLoc = x(right_mask);
        yLoc = y(right_mask);
        if mask(xLoc + Deltax, yLoc) == 0
            x(right_mask(idx)) = x(right_mask(idx))  + Deltax;
        end
  end
    
   

    down_mask = find((0.5<= r) & (0.75>r));   % mask identifying the down-moving particles
 for idx = 1:length(down_mask)
        xLoc = x(down_mask);
        yLoc = y(down_mask);
        if mask(xLoc - Deltay, yLoc) == 0
            x(down_mask(idx)) = x(down_mask(idx))  - Deltay;
        end
 end
    
          % move those particles down
           
    up_mask = find(r >= 0.75);                              % mask identifying the up-moving particles
 for idx = 1:length(up_mask)
        xLoc = x(up_mask);
        yLoc = y(up_mask);
        if mask(xLoc + Deltay, yLoc) == 0
            x( up_mask(idx)) = x( up_mask(idx))  + Deltay;
        end
  end