The following Modelica code resets variable 'bb' to 0 after it sets to 123. Can anyone explain why? I am using OpenModelica v1.13.2.
model test2
import Modelica.Utilities.Streams.print;
Real b(start=0, fixed=true);
Real bb(start=0, fixed=true);
Integer c(start=0,fixed=true);
algorithm
when sample(0,0.1) then
c := pre(c) + 1;
if c == 1 then
b := 12.3;
elseif c == 2 then
bb := 123;
end if;
print(String(time)+", "+String(b)+", "+String(bb));
end when;
end test2;
Simulation prints:
0, 12.3, 0
0, 12.3, 123
0, 12.3, 0
...(repeats)
Also screenshot of the plot:
The result is incorrect according to the Modelica 3.4 specification, and bb should not be set to zero:
At the start of the algorithm bb
should be set to pre(bb)
which is 123; according to "11.1.2 Execution of an algorithm in a model" https://specification.modelica.org/master/Ch11.html#execution-of-an-algorithm-in-a-model
Note that the semantics for when in equations give similar results but in a different way, in that case the specific when-clause is instead implicitly mapped to an if-then-else where the else-branch would set bb=pre(bb)
according to section "8.3.5.1" - https://specification.modelica.org/master/Ch8.html#defining-when-equations-by-if-expressions-in-equality-equations