Search found 3806 matches

by tomfid
Thu Mar 28, 2024 6:17 pm
Forum: Vensim
Topic: double-click selection
Replies: 5
Views: 110

Re: double-click selection

Some of this will change with the new equation editor soon.
by tomfid
Thu Mar 28, 2024 6:16 pm
Forum: Vensim
Topic: How to calculate the difference in value for a variable in vensim over time?
Replies: 1
Views: 11

Re: How to calculate the difference in value for a variable in vensim over time?

You can use DELAY FIXED for this: Previous GDP = DELAY FIXED( Current GDP, one year, Current GDP ) ~ $/year one year == 1 ~ year In continuous time, you can use the SMOOTH function, i.e. Previous GDP = SMOOTH( Current GDP, one year ) ~ $/year You can also use the TREND function to compute a fraction...
by tomfid
Thu Mar 21, 2024 1:11 am
Forum: Vensim
Topic: it is frustrating!
Replies: 25
Views: 563

Re: it is frustrating!

Is the run "current" the first loaded dataset?
by tomfid
Thu Mar 21, 2024 1:10 am
Forum: Vensim
Topic: Continuos or discrete variables?
Replies: 4
Views: 90

Re: Continuos or discrete variables?

On the other hand, if you're interested in stochastic extinction of a disease in a small population, integer values may matter.

It's impossible to answer these questions in general; some specifics of the question to be answered must be known.
by tomfid
Thu Mar 21, 2024 1:08 am
Forum: Vensim
Topic: Continuos or discrete variables?
Replies: 4
Views: 90

Re: Continuos or discrete variables?

Like Forrester, I think that discrete quantities are rarely a good place to start. Things that are theoretically only available in integer quantities either don't matter, or have qualifications. For example, people are certainly available in integer quantities. But what about the work-equivalent of ...
by tomfid
Fri Mar 08, 2024 3:57 pm
Forum: Vensim
Topic: Help with PULSE TRAIN and random variable
Replies: 6
Views: 134

Re: Help with PULSE TRAIN and random variable

Try this instead ...
PULSE_test 2.mdl
(2.16 KiB) Downloaded 10 times
You can also use SAMPLE IF TRUE for this (though I think it's not available in all versions).
by tomfid
Tue Mar 05, 2024 2:06 pm
Forum: Vensim
Topic: it is frustrating!
Replies: 25
Views: 563

Re: it is frustrating!

2024-03-05_07-05-12.png
2024-03-05_07-05-12.png (8.56 KiB) Viewed 462 times
by tomfid
Mon Mar 04, 2024 4:36 pm
Forum: Vensim
Topic: where is the sim set up from the old GUI in the new one?
Replies: 9
Views: 10845

Re: where is the sim set up from the old GUI in the new one?

Not sure I'm tracking the issue here. In the old gui, PLE sim setup doesn't have a dialog. Everything is in the toolbar. 2024-03-04_09-33-54.png In the new gui, PLE does have a dialog (albeit without much content). 2024-03-04_09-32-44.png In both cases, constants and lookups are highlighted, and you...
by tomfid
Mon Feb 05, 2024 11:10 pm
Forum: Vensim
Topic: Reinstall Vensim (error 977).
Replies: 5
Views: 562

Re: Reinstall Vensim (error 977).

Typically this indicates a path problem. Have you moved the dll?
by tomfid
Fri Feb 02, 2024 4:00 pm
Forum: Vensim
Topic: RC Fonksiyon
Replies: 1
Views: 393

Re: RC Fonksiyon

I think you'll have to elaborate a bit, because there are a lot of different possible population structures, which might call for different approaches.

One option is to use the Little's Law equilibrium estimate, i.e. residence time = stock/flow_throughput. https://en.wikipedia.org/wiki/Little%27s_law
by tomfid
Fri Feb 02, 2024 2:00 pm
Forum: System Dynamics Discussion
Topic: Utilizing Ventana Systems Tools for enhancing data breach protection strategies ?
Replies: 3
Views: 938

Re: Utilizing Ventana Systems Tools for enhancing data breach protection strategies ?

For starters, google "sander zeijlemaker security system dynamics" - he's done quite a bit of interesting work. You should be able to find much of it in the ISDC conference proceedings.
by tomfid
Fri Feb 02, 2024 1:52 pm
Forum: Vensim
Topic: Can the Vensim PLE version achieve seasonal forecasting?
Replies: 2
Views: 531

Re: Can the Vensim PLE version achieve seasonal forecasting?

I think the month variable is intentional, for creating a seasonal input.

Since the seasonal behavior has 3 episodes, switched in an IF THEN ELSE statement, I think you could create the same thing for seasonal smoothing - have 3 smoothing structures, one active during each episode.
by tomfid
Thu Feb 01, 2024 3:45 pm
Forum: Vensim
Topic: Units tips or laws
Replies: 7
Views: 727

Re: Units tips or laws

Your flows are dimensioned dmnl/hour, but your time unit in Model>Settings is Mes.
by tomfid
Wed Jan 31, 2024 3:57 pm
Forum: Vensim
Topic: Units tips or laws
Replies: 7
Views: 727

Re: Units tips or laws

There are at least two rules: - a flow has the same units as the stock it connects to, divided by the time unit - EXP, LN, and a number of other functions require dimensionless units beyond that, if you have y = a / b the units follow y units = a units/b units One consequence of this is that you can...
by tomfid
Mon Jan 29, 2024 6:30 pm
Forum: Vensim
Topic: Copy & Paste between models
Replies: 4
Views: 496

Re: Copy & Paste between models

If you open them in the same application instance, this works. (If you open the files by double-clicking from the finder/explorer, you have two application instances, and it won't work.)

In pro/DSS, general settings, you can enable opening multiple models simultaneously.
by tomfid
Thu Jan 25, 2024 5:35 pm
Forum: Vensim
Topic: Max file size
Replies: 5
Views: 1067

Re: Max file size

I think the 3221226505 return code may be a variable type mismatch - this has been reported in other contexts.
by tomfid
Sun Jan 14, 2024 11:19 pm
Forum: Vensim
Topic: Finding MAX of simulation variable
Replies: 2
Views: 937

Re: Finding MAX of simulation variable

If you can make the calculation at FINAL TIME, you can use SAMPLE IF TRUE to hold the max value over the simulation - there's an example in the help system.
by tomfid
Wed Jan 10, 2024 5:44 pm
Forum: Vensim
Topic: Get data xls with multiple subscripts
Replies: 4
Views: 3571

Re: Get data xls with multiple subscripts

In a case like this, importing the data to vdf would be less error-prone.
by tomfid
Wed Jan 10, 2024 5:42 pm
Forum: Vensim
Topic: Get data xls with multiple subscripts
Replies: 4
Views: 3571

Re: Get data xls with multiple subscripts

The GET DATA calls are basically 1-dimensional because the spreadsheet is 2D, and one is taken by time.

One possible way to semi-automate a GET call with more dimensions is to put the sheet names or first-cell references in a string variable with the appropriate subscripting.
by tomfid
Sat Jan 06, 2024 5:33 pm
Forum: Vensim
Topic: Vensim not correctly calculating stock from flows
Replies: 4
Views: 6552

Re: Vensim not correctly calculating stock from flows

Right. Personally I find it hard to remember the beginning/ending balance sequencing. It would be nice if we could update the table tool to make it easy to toggle display of stocks as start/end of period, though that might be tricky for some variables using functions like SAMPLE IF TRUE. It would al...
by tomfid
Sat Jan 06, 2024 3:23 pm
Forum: Vensim
Topic: Vensim not correctly calculating stock from flows
Replies: 4
Views: 6552

Re: Vensim not correctly calculating stock from flows

I think that the computation is actually correct. The Euler update in your spreadsheet is: stock(t + dt) = stock(t) + flows This is fine if the TIME STEP (dt) = 1, but in this model it's 0.25. The correct Euler update is: stock(t + dt) = stock(t) + flows*dt In this case, the value 1618 in cell F37 i...
by tomfid
Tue Jan 02, 2024 10:46 pm
Forum: Vensim
Topic: Solving for implicit functions before starting time
Replies: 7
Views: 9540

Re: Solving for implicit functions before starting time

I agree. The drawback of a lookup is that it might not be totally general (i.e. you'd have to recompute the values if some assumption changed). My guess is that the speed penalty for FIND ZERO is small. Unfortunately, you can't wrap FIND ZERO in an INITIAL statement, but you could set the error vect...
by tomfid
Fri Dec 29, 2023 4:02 pm
Forum: Vensim
Topic: Solving for implicit functions before starting time
Replies: 7
Views: 9540

Re: Solving for implicit functions before starting time

Another option would be to put a table of solutions (Moody diagram) into a lookup or fitted function.
by tomfid
Fri Dec 22, 2023 3:14 pm
Forum: Vensim
Topic: Can't view model as sketch
Replies: 12
Views: 11878

Re: Can't view model as sketch

This means there are syntax errors in the equations. Use Check Model <ctl><T> to see the error window.