Variable ... is undefined on some execution paths

I'm doing a project in Simulink/Simscape, and it contains a Matlab Function block, which contains the code that must be executed each cycle by the simulator. In this block i need to calculate the discrete derivative of temperature doing (T1-T1_prev)/dt, where T1 is one of the inputs of the block.

In theory, the problem of this methode is the definition of T1_prev at the time 0 of the simulation, so i did an

function y = TCU(u)
%% input
...
time = u(6);

if time <= 0
    
    ...
        
    T1_prev = 0; 
    
end

temperature_tank1 = temperatureCheck (T1, T1_prev, Tmax, Tsig, dT_dt_max, deltat);

T1_prev = T1;

y
     
end

I reported just the parts which i have problems with. In this way matlab says T1_prev is not fully defined due to the lack of an else case, but actually i don't want it, and T1_prev results fully defined, because it's updated in the end of the cycle for each cycle that is not the first. (T1 is updated each cycle too)

How can i solve the problem?

Thank you guys

[John D'Errico]

Learn to use the debugger? Clearly, even though you think it is always defined, there are some paths where it is not.

[Federico Avino]

Thank you, I know technically it's not fully defined for the compiler (absence of T1_prev in else case), but for the simulation it is defined because the structure of the execution, so the real case, is cyclical. How can I manage to overcome this false error?

Answer 1

[Walter Roberson]

persistent T1_prev
if isempty(T1_prev); T1_prev = 0; end