Undefined function or variable but variable is defined

I am getting an error message saying u1 isn't defined but it clearly is right above. Why is this error message ocurring?

I left out some of the initial conditions but they are irrelevent for the question at hand.

N0=f(1,1);
u0=f(2,1);

GTR = 51;
deltax = L/GTR;
B = 0.5;
t = 0;

for k = 1:GTR
    t=t+deltax;
    n1 = N0 - deltax*(B*c*LoadFunctionCP(t-deltax,L,type)+(1-B)*c*LoadFunctionCP(t,L,type));
    ul = u0 +((deltax/EA)*(B*N0+(1-B)*n1))
    u0 = u1;
    N0 = n1;
  
    storage2(k,:) = [t-deltax N0 u0];
end 


Undefined function or variable 'u1'.

Error in file (line 120)
    u0 = u1;

[KALYAN ACHARJYA]

Still, f is undefined here

Answer 1

[KALYAN ACHARJYA]

one is ul and other is u1

ul = u0 +((deltax/EA)*(B*N0+(1-B)*n1)); % This is UL
u0 = u1;  % This is U1

Hehe.. :)