Running code and getting error about vector must be the same length

I'm running my code and I am getting an error about how the vector must be the same length. Does anyone have advice because I thought the vectors were fillled to the same length?

ntrails = 20;
N_run = zeros(ntrails,1);
BC = [5,-5,-5,5];
for trail = 1:1:ntrails
    xA = -5; xAK = xA;
    yA= 0; yAK = yA;
    xB = 5; xBK = xB;
    yB = 0; yBK = yB;
    nsteps = 1000;
    collision_flag = 0; k = 0;
 while collision_flag == 0 && k < nsteps
     [xAKp1, yAKP1] = RandWalk_2D(xAK,yAK,BC);
     [xBKp1, yBKP1] = RandWalk_2D(xBK,yBK,BC);
     
     xAK_val = [xAK-.5, xAK + .5, xAK + .5, xAK -.5];
     yAK_val = [yAK-.5, yAK + .5, yAK + .5, yAK - .5];
     
     xAKp1_val = [xAKp1-.5, xAKp1 + .5, xAKp1 + .5, xAKp1 -.5];
     yAKp1_val = [yAKp1-.5, yAKp1 + .5, yAKp1 + .5, yAKp1 - .5];
          
     xBK_val = [xBK-.5, xBK + .5, xBK + .5, xBK -.5];
     yBK_val = [yBK-.5, yBK + .5, yBK + .5, yBK - .5];
     
     xBKp1_val = [xBKp1-.5, xBKp1 + .5, xBKp1 + .5, xBKp1 -.5];
     yBKp1_val = [yBKp1-.5, yBKp1 + .5, yBKp1 + .5, yBKp1 - .5];
     
     figure(1)
     hold on
     xlim([-5.5,5.5,])
     ylim([-5.5,5.5,])
     fill(xAK_val,yAK_val,'r') %current step A
     fill(xAKp1_val, yAKp1_val, 'b')%next step for A
     fill(xBK_val,yBK_val,'r') %current step B
     fill(xBKp1_val, yBKp1_val, 'b')%next step for B
     hold off
     
     xAK = xAKp1; yAK = yAKp1; 
     xBK = xBKP1; yBK = yBKp1;
     k = k+1;
     
    if xAK == xBK && yAK == yBK
        collision_flag = 1;
        N_run(trail) = k;
    end
 end
end
 medval = median(N_run)
    
function [x,y] = RandWalk_2D(x0,y0,BC)
   r = rand;
   if r<0.2
        x = x0;
        y = y0+1;
        if y >= BC(1); y = BC(1);
        end
   else if 0.2 < r && r <= 0.4;
           x = x0;
           y = y0-1;
           if y <= BC(2);
               y = BC(2);
           end
       elseif 0.4< r && r<=0.6
           x = x0 - 1;
           y = y0;
           if x<= BC(3); x = BC(3)
           end
       elseif 0.6 < r && r <= 0.8;
           x = x0+1; y = y0;
           if x >= BC(4); x = BC(4);
           end
       elseif 0.8<r 
           x = x0; y = y0;
       end
   end
end

[Mario Malic]

The code is not working. Meanwhile, you can call the variables in the command window that report the error and check what's the issue. Also, post the error in the future.

Undefined function or variable 'yAKp1'.
Error in Marios (line 19)
     yAKp1_val = [yAKp1-.5, yAKp1 + .5, yAKp1 + .5, yAKp1 - .5]; 

Answer 1

[the cyclist]

It's the subtle difference between these two lines of code:

xAK_val = [xAK-.5, xAK + .5, xAK + .5, xAK -.5];
yAK_val = [yAK-.5, yAK + .5, yAK + .5, yAK - .5];

Specifically, the last element of each of these vectors.

yAK - .5

is a single element, equal to yAK minus 0.5. But

xAK -.5

is two elements, xAK and -0.5.

(Note that you have that same difference in several places.)

[Mario Malic]

Wow, nice catch!

[the cyclist]

In case you are unaware, the MATLAB debugger is a very useful tool for tracking down things like this.

Specifically, I stopped your code when the error occurred, by using

dbstop if error

then traced back why those two lines of code were different lengths.

[Marios Christofides]

Thank you!!! I would have not caught that.