I need help for a very simple function in Matlab

In the picture a part of my simulink model is shown. It works like this:

"Data" is what is sent from serialport. "Status" is 1 when there is data on serialport and 0 when there is not any data sent.

When I start simulation, if there is data sent "Data" has the value of it. When there is not any data sent "Data" sends 0 as output.

What I want it to do is "If there is any data, give "y" the value of sent data. If there is no data sent keep "y" as the previous value"

So I added my own User Defined Function

function y = fcn(u,x)
if (x == 0)
    y = y;
else
    y = u;
end 
end

But this gives me the error says "y" is not defined. How can I achieve this simple solution wiht or without any user defined function? Can somebody, please, figure it out? Thanks in advance

Answer 1

[the cyclist]

You have not input the previous value of y to fcn(), so at the line

y = y;

the value is unknown. You need something like

function y = fcn(u,x,y0)
if (x == 0)
    y = y0;
else
    y = u;
end 
end

where y0 is the prior value of y.