Initialising variables in an embedded function

Hello everyone,

I'm trying to get an embedded function within Matlab Simulink to perform a finite difference calculation for me.

The idea is that the output argument will be fed back into itself after a unit delay in order for the next set of values to be computed by the function.

The problem is that I can't seem to initialise output variables.

I'm unable to declare the output variable as persistent as Matlab won't allow it... Here is my code:

function Temps = fcn(Ti,aw,aa,L,Tstep,Tamb,Tcline,Tprev)
%#eml
Nodes = 100;
persistent Temps;
  if isempty ( Temps ); 
    Temps = [1:1:Nodes]./[1:1:Nodes]; 
  end

for k = 1:Nodes;
    if k>2;
        Tnew(k) = aw.*Temps(k-1)+0.2*k;
    else
        Tnew(k) = 1;
    end
end
Temps = Temps.*Tnew
end

And here is the error:

The PERSISTENT declaration cannot be used on function outputs.

Function 'Finite Difference Conduction Calculator' (#31.76.86), line 4, column 1:
"persistent"

Thanks in advance

Pete

Answer 1

[Kaustubha Govind]

Why not use a local variable as the persistent variable instead?

function output = fcn(Ti,aw,aa,L,Tstep,Tamb,Tcline,Tprev)
%#eml
Nodes = 100;
persistent Temps;
  if isempty ( Temps ); 
    Temps = [1:1:Nodes]./[1:1:Nodes]; 
  end

for k = 1:Nodes;
    if k>2;
        Tnew(k) = aw.*Temps(k-1)+0.2*k;
    else
        Tnew(k) = 1;
    end
end
Temps = Temps.*Tnew;
output = Temps;
end

Answer 2

[Mike Hosea]

Right. The output variable cannot be a persistent variable. There's more than one way to go about this, but all involve using two variables where now you're trying to use one. Perhaps the simplest solution is to rename your output variable and make the last line of your code a copy from the persistent variable to the output variable. Something like

function yout = foo(x)
persistent yp
if isempty(yp)
    yp = x;
end
<Some calculations involving only x and yp go here.>
yout = p;

Answer 3

[Peter]

Dear All,

Thanks very much for the rapid response, it turns out that simply renaming the variable works perfectly.

Azzi, I like your suggestion of initialising the unit delay I hadn't thought of it and I guess that would have worked too.

Best Regards

Pete Armstrong