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on the usage of pre()

on the usage of pre()

Mar-13-14 19:32:59

Hello,

I don't understand why it is possible to call pre() during integration time. With omc 1.9.1+dev (r18345) I can simulate equation

Code:


  Real x (start=0);

  x = pre(x) + 1;

where I observe that the growth of x depends on the numberOfIntervals parameter, which seems odd to me, I would expect the compiler to reject this program.

My other question is on the use of pre() in a discrete context. Does anyone know why equation

Code:


  Integer cpt (start=0);

  cpt = if time <= 10 then pre(cpt) + 1 else pre(cpt);

returns

Code:

ERROR: Too many event iterations.

?

Thank you,
Antoine

Edited by: antoinem - Mar-14-14 13:24:31

antoinem: 7 Posts; Thank you [ 0 ]

#4295 Permalink

Re: on the usage of pre()

Mar-14-14 09:51:14

I've just tried both models in Wolfram System Modeler. Both simulations "work", but in both cases x respectively cpt jump instantly on 11 and don't change after that.
I've also tried in in an older Version of OM but both models didn't compile.
I often have trouble using the pre()-function as well, so I am looking forward to hearing an answer on this current/wink

SKD: 42 Posts; Thank you [ 4 ]

#4296 Permalink

Re: on the usage of pre()

Mar-14-14 13:15:31

The following model is not valid according to the specification:

Code:


model M

  Real x

equation

  x = pre(x) +1;

end M;

x is a continuos variable, so it's not allowed to use pre of x outside when.

The model:

Code:


model M

 Integer cpt (start=0);

equation

  cpt = if time <= 10 then pre(cpt) + 1 else pre(cpt);

end M;

fails, since every time the tool execute the code pre(cpt) != cpt, so we need to perform an further event iteration.

Please, see Modelica Specification page 27 ff:
3.7.3
Event-Related Operators with Function Syntax

so long.
Willi