Can not assign values to an array when using sample() function - modelica

Question

I'm trying to implement a DFT on my own because the FFT example in OpenModelica is not working for me and I can't figure out why.
But I'm already stuck sampling a sine function and assigning the sampled values to a buffer array.
This is why I tried to make it even simpler and just assign a counter variable "iTick" to the array which still doesn't work.
See the basic example.
Can anyone tell me why this is not working and how I can actually assign a value to the array when using the sample() function ??
block DFT
import Modelica.Constants.pi;
parameter Integer N = 360 "Total number of samples";
Integer iTick;
Real y_buf[N];
algorithm
when sample(0, 0.1) then
iTick :=iTick + 1;
if iTick >= 1 and iTick <= N then
y_buf[iTick] := iTick;
end if;
end when;
end DFT;
[358] 14:56:15 Symbolisch Warnung
The linear system:
1 : $PRE.y_buf[2] = y_buf[2]
2 : y_buf[2] = $PRE.y_buf[2]
[
-1.0 , 1.0 ;
1.0 , -1.0
]
*
[
y_buf[2] ;
$PRE.y_buf[2]
]
=
[
0.0 ;
0.0
]
might be structurally or numerically singular for variable $PRE.y_buf[2] since U(2,2) = 0.0. It might be hard to solve. Compilation continues anyway.
[359] 14:56:15 Symbolisch Warnung
The linear system:
1 : $PRE.y_buf[1] = y_buf[1]
2 : y_buf[1] = $PRE.y_buf[1]
[
-1.0 , 1.0 ;
1.0 , -1.0
]
*
[
y_buf[1] ;
$PRE.y_buf[1]
]
=
[
0.0 ;
0.0
]
might be structurally or numerically singular for variable $PRE.y_buf[1] since U(2,2) = 0.0. It might be hard to solve. Compilation continues anyway.
[360] 14:56:15 Übersetzung Warnung
Assuming fixed start value for the following 2 variables:
y_buf[360]:DISCRETE(fixed = false ) type: Real [360]
iTick:DISCRETE(fixed = false ) type: Integer

Answer 1

After a long search and try and error I found out that the magic word "discrete" solves my problem! I can't yet explain why, but see below the working example:
model Test
import Modelica.Constants.pi;
parameter Integer N = 360 "Total number of samples";
Integer iTick(start=0, fixed=true);
discrete Real y_buf[N](start=fill(0,N), fixed=fill(true, N));
algorithm
when sample(0, 0.1) then
iTick :=iTick + 1;
if iTick >= 1 and iTick <= N then
y_buf[iTick] := iTick;
end if;
end when;
end Test;
Hopefully this is a help to someone!

Answer 2

Your "Symbolisch Warnung" disappears once you initialize iTick and y_buf. However, the code still does not work. OpenModelica simulates it, but the items of y_buf are never updated.
This issue might be related to this question where the delay operator is not working in algorithm sections. Therefore, I suggest a similar workaround: Try to avoid the algorithm section. With an equation section and proper initialization, your minimal example could look as follows:
block DFT
import Modelica.Constants.pi;
parameter Integer N = 360 "Total number of samples";
Integer iTick(start=0, fixed=true);
Real y_buf[N](start=fill(0, N), fixed=fill(true, N));
equation
when sample(0, 0.1) then
iTick = pre(iTick) + 1;
end when;
for i in 1:N loop
when iTick >= i then
y_buf[i] = iTick;
end when;
end for;
end DFT;