MATLAB中while循环的离散值画图，下角标该怎么用呀

求求大佬看看这个代码哪里错误了

VA=0.15;

VB=0.1;

A=[0,6];

B=[2*pi,0];

d=norm(B-A);

AX(1)=0;

AY(1)=6;

BX(1)=2*pi;

BY(1)=0;

t=0.01;

k=1;

while d>10^(-4)

    k=k+1;

    AX(k)=AX(k-1)+VA*t*(BX(k-1)-AX(k-1))/d;

    AY(k)=AY(k-1)+VA*t*(BY(k-1)-AY(k-1))/d;

    BX(k)=BX(k-1)-VB*t*abs(cos(atan(cos(BX(k-1)))));

    BY(k)=sin(BX(k));

    A=[AX(k),AY(k)];

    B=[BX(k),BY(k)];

    d=norm(B-A)

    D(k)=d;

end

plot(AX,AX,'ro',BX,BY,'b*')

hold on 

x=linspace(0,0.1,50);

plot(x,D)

VA = 0.15;
VB = 0.1;
A = [0, 6];
B = [2*pi, 0];
d = norm(B-A);
AX(1) = 0;
AY(1) = 6;
BX(1) = 2*pi;
BY(1) = 0;
t = 0.01;
k = 1;

while d > 10^(-4)
    k = k + 1;
    AX(k) = AX(k-1) + VA*t*(BX(k-1)-AX(k-1))/d;
    AY(k) = AY(k-1) + VA*t*(BY(k-1)-AY(k-1))/d;
    BX(k) = BX(k-1) - VB*t*abs(cos(atan(cos(BX(k-1)*pi/180))));
    BY(k) = sin(BX(k));
    A = [AX(k), AY(k)];
    B = [BX(k), BY(k)];
    d = norm(B-A);
    D(k) = d;
end

plot(AX, AY, 'ro', BX, BY, 'b*')
hold on 
x = linspace(0, 0.1, 50);
plot(x, D)