跪求大神进来解决关于MATLAB的问题~~求进

A=rand(4,3);
idx=find(A>0.5);
num=length(idx);
A(idx)=0;

确定么，大神

为什么会没有结果啊

x=0:0.1:7;
y1=sin(2*x);
subplot(1,3,1);plot(x,y1);xlabel('x');ylabel('y1');title('y1=sin(2*x)')
y2=sin(x);
subplot(1,3,2);plot(x,y2);xlabel('x');ylabel('y2');title('y2=sin(x)')
y3=sin(x);
subplot(1,3,3);plot(x,y3);xlabel('x');ylabel('y3');title('y3=sin(x)')

x=0:0.1:7;
y1=sin(2*x);
y2=sin(x.^2);
y3=(sin(x)).^2;
figure
plot(x,y1,'r',x,y2,'g',x,y3,'b')
legend('sin(2x)','sin(x^2)','(sin(x))^2')
第五题

编写一个函数f(t)=sin2t t小于等于0
=sint t>0
并给f（t-1) t=[-2π +2π]范围的曲线