Time updated plot has a triangle instead of square

I wrote a program to produce a time updated plot of two particles. The next position for B is a triangle instead of a square. Does anyone have a fix?

ntrail = 5000;

%tic used to time the code

for trail = 1:1:1000

nsteps = 5000;collision_flag = 0; k = 0;

%Initial Position of A and B

xa = -5; xak = xa; xb = 5; xbk = xb; %For Part D, replace xa and xb with (-4,0) or (-3,0) to simulate them closer together.

ya = 0; yak = ya; yb = 0; ybk = yb;

BC = [5,-5,-5,5];%Set a boundary position

while collision_flag == 0 && k < 1000

[xakp1, yakp1] = RandWalk_2D(xak,yak,BC);

[xbkp1, ybkp1] = RandWalk_2D(xbk,ybk,BC);

%Create Particle A on Grid for Step(k)

xakval = [xak-.5, xak+.5, xak+.5, xak-.5];

yakval = [yak-.5, yak-.5, yak+.5, yak+.5];

%Create Particle A on Grid for Step(k+1)

xakvalp1 = [xakp1-.5, xakp1+.5, xakp1+.5, xakp1-.5];

yakvalp1 = [yakp1-.5, yakp1-.5, yakp1+.5, yakp1+.5];

%Create Particle B on Grid for Step(k)

xbkval = [xbk-.5, xbk+.5, xbk+.5, xbk-.5];

ybkval = [ybk-.5, ybk-.5, ybk+.5, ybk+.5];

%Create Particle B on Grid for Step(k+1)

xbkvalp1 = [xbkp1-.5, xbkp1+.5, xbkp1+.5, xbkp1-.5];

ybkvalp1 = [ybkp1-.5, ybkp1-.5, xbkp1+.5, ybkp1+.5];

figure(1)

hold on

xlim([-5.5,5.5,])

ylim([-5.5,5.5,])

fill(xa,ya,'r')

fill(xakval,yakval,'r') %current step A

fill(xakvalp1, yakvalp1, 'b')%next step for A

fill(xbkval,ybkval,'y') %current step B

fill(xbkvalp1, ybkvalp1, 'p')%next step for B %For problem C comment

%out the next step so B stays in one place.

title('Initial Position of A and B')

title('Final Position of A and B')

hold off

%Update the New Position for Step(k+1)

xak = xakp1; yak = yakp1;

xbk = xbkp1; ybk = ybkp1; %For problem C comment out the update value for B.

k = k+1;

if xak == xbk && yak == ybk

collision_flag = 1;

niter = k;

end

end

end

medval = median(niter);

fprintf("Median = %d\n", medval);

%Time = toc

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

Answer 1

[Marios Christofides]