Variable 'KD' is not fully defined on some execution paths

function KD = fcn(delta_KD)

persistent KD_actual; 
persistent KD_anterior;

if isempty(KD_actual)
    K=2;
    t0=8;
    tao=4;
    KD=(0.51/abs(K))*(tao/t0)^0.76;
    KD_actual=0
    KD_anterior=0;
end

KD_anterior=KD;
KD_actual=delta_KD;

KD=KD_anterior+KD_actual;

Answer 1

[Stephen23]

Look at this line of code

KD_anterior=KD;

and now consider what is the value of KD the second time the function is called. The first time the function is called the variable KD_actual is empty and so the code inside the if block is evaluated and so KD is defined. But the second time you call the function KD_actual will not be empty (you just defined it with the first call and persistent keeps whatever value it had) and so the if block does not run and so KD is not defined (which is why MATLAB warns you about this).