Im having trouble with unit step functions

I need to create a program that evaluates 4 signals correlation coefficients. The signals are

a. X1(t)=u(t+0.5)-u(t)-u(t)+u(t-0.5)

b. X2(t)=(t+0.5)*[u(t+0.5)-u(t)] + (0.5-t)*[u(t)-u(t-0.5)]

c. X3(t)=sin(2*pi*t)*[ u(t+0.5)-u(t-0.5)]

d. X4(t)=cos(4*pi*t)*[ u(t+0.5)-u(t-0.5)]

I keep getting this error "Output argument "u" (and maybe others) not assigned during call to "u".

Error in sign_cor (line 5)

x1=u(t+0.5)-u(t)-u(t)+u(t-0.5);

This is my code so far:

Dt=0.01;

T=6.0;

t=[-1:Dt:T];

x1=u(t+0.5)-u(t)-u(t)+u(t-0.5);

x2=(t+0.5)*[u(t+0.5)-u(t)]+(0.5-t)*[u(t)-u(t-0.5)];

x3=sin(2*pi*t)*[u(t+0.5)-u(t-0.5)];

x4=cos(4*pi*t)*[u(t+0.5)-u(t-0.5)];

E0=sum(x1.*conj(x1))*Dt;

E1=sum(x2.*conj(x2))*Dt;

E2=sum(x3.*conj(x3))*Dt;

E3=sum(x4.*conj(x4))*Dt;

ab=(x1.*conj(x2))*Dt/(sqrt(E0*E1))

ac=(x1.*conj(x3))*Dt/(sqrt(E0*E2))

ad=(x1.*conj(x4))*Dt/(sqrt(E0*E3))

bc=(x2.*conj(x3))*Dt/(sqrt(E1*E2))

bd=(x2.*conj(x4))*Dt/(sqrt(E1*E3))

cd=(x3.*conj(x4))*Dt/(sqrt(E2*E3))

Answer 1

[Adam Danz]

As the error indicates 'u' is not defined. Assuming it's a variable, it has no value and is not defined. If it's a function, Matlab doesn't know it exists.