zhang

my code is like this:

clear all
t0=0;
dt=0.05;
t1=1;
tf=5;
t=[t0:dt:tf];
st=length(t);

n1=floor((t1-t0)/dt)
x1=zeros(1,st);
x(n1)=1/dt;
subplot(2,2,1),stairs(t,x1),grid on

x2=[zeros(1,n1-1),ones(1,st-n1+1);
subplot(2,2,3);
stairs(t,x2);
grid on
w=10;;
u=-0.5;
x3=exp((u+j*w)*t);
subplot(2,2,2),plot(t,real(x3)),grid on;
subplot(2,2,4),plot(t,imag(x3)),grid on;

but when I run it ,an error returned. The error is:Error: File: lab3.m Line: 19 Column: 2
The expression to the left of the equals sign is not a valid target for an assignment.
Can anyone help me help me
Professional Interests: singal processing