老是错误subplot那里哪错了呀

clear all;clc;
a=2;
N=100000;r=rand();
for i=1:N
f=a*sqrt(1-r^2);
theta(i)=2*pi*r;
x(i)=r*cos(theta(i));
y(i)=r*sin(theta(i));
end
figure,subplot(x,y,'b');
figure,hist(r,50);
figure,hist(theta,50);