elseif t(K)>10 && t(K)<20
Notice that if you are going to use if statements and t is a vector, then you need to loop over all of the entries in the vector.
However, in many cases, it is better to use logical indexing instead of looping.
No need for an else because all elements were initialized to 0 anyhow.
Caution: when t = 10 exactly your code falls through to the else and assigns 0 there.
Caution: your code does not implement the same as the function definition. The function definition does not define any value for complex-valued t or for t >= 20, but your code would assign 0 to those locations. When a function definition does not define a result for a case, the code should either error or generate NaN if that situation is encountered.