Ventana software support forum

JJ, I do see how your model works, but I fail to see why you seem to be quite found of using TIME STEP in equations (cf. your post re the World 3 in the SD section). Whatever x = Time/Time Step means in a simulation (once again: TIME STEP is nothing to be " seen " anywhere in the reality t...

Hi Irene, what JJ has suggested will only work, if you make the two different DELAY FIXED functions separate variables and then use them in the equation for the delayed flow. Vensim gives an error if you mix IF THEN ELSE with the DELAY FIXED in one equation -- at least my instance of it ;-) I would ...

In the SD-section of this forum Jean-Jacques Laublé has posted questions regarding robustness of equation formulation in the World 3 model shipping with Vensim. To test this model JJ uses Reality Checks - a distinguished feature of Vensim that sets it quite nively apart from other tools. To answer J...

Hi JJ, sorry to bother you with this - but as your thread here was still fresh I have quite naturally assumed differently. Could you please give me the link to the previous thread on the forum that you are refering to? From the formula: maturing = population x (1 - mortality x time step) x 1/15 I wo...

Regarding the issue of two outflows with one being dominant, Jean-Jacques Laublé has just pointed to an issue in the World 3 model that ships with Vensim. In the population sub model there the outflows to the stock of adolescents (0 to 14 year-old persons) is modeled as a proportional split as Tony ...

Hi JJ, ok, I am starting to see what you mean -- the formulation that I have provided will correct the flows by mathematical force using MIN and MAX functions -- in accordance to stock control as proposed by Sterman and many others. Your formulation is more to the point as one indeed has to look ahe...

Hello JJ, what a coincidence: your post came almost at the same time Irene had posted a fundamental question on how to deal with two outflows from a stock: http://www.ventanasystems.co.uk/forum/viewtopic.php?f=2&t=4715 It seems that the answer to your question is twofold: There is first the fund...

Hi Irene and Tony, regarding Tonys suggestion to use a proportional split to distribute the total outflow onto the two (or however many) outflows one might want to make sure that this adds up to the maximum possible outflow. The reason being rounding errors. So maybe it is a good idea to have all bu...

Thank you, Tony. That helps for a more focused seach. Scanning some of the documentation provided by Mathematica I find a lot of reference to the ubiquitous .NET-framework. Would you have some comments on how .NET is different from DLL (I believe there is a difference)? As far calling Vensim from Ma...

Hello, I am wandering about how to best link Mathematica -- of which I have continuously grown rather found of albeit with a steep learning curve -- and Vensim DSS: Can Mathematica's MathLink be applied? (structured communication with external programs) Is it possible to write external functions in ...

Hello Irene, you should think very systematically through your model and ask yourself "what is having influence on this variable?" or "what information is needed to determine this variable?" - with variable being stock or flow/auxiliary. If you do this you will end up having to a...

I am certainly not enough of an expert with regard to programming, but the necessity of 'NaN' (which is not the same as a NULL-pointer as far as I know?) slowing down things beyond being reasonable seems to be worth reconsidering and checking. As far as I had understood it NaN is a 'special represen...

I can't help but to notice that the fact that 'something silly happens' if :NA: enters calculations does not really approach very robust program design. It looks to me that mapping a 'special value' like :NA: onto a numeric range should not be done if it can be avoided. Wouldn't C/C++ know things li...

Dear all: since I have gained some experience with Vector Functions and their use in hybrid models where microsimulation or ABM - appraoches couple with SD (cf. my thread in this forum) I have stumbled upon some rather tricky issue and its provisional fix. Vensim's use of subscripts and their mappin...

Having had some trouble with a modell lately a suggestion came up to introduce a variable Not a Number to be -9999 for testing as I had made intense use of :NA: in my model to label missing values. This caused floating point errors somewhere and the suggestion should help to bypass this. This had me...

That is a pity as these commentaries certainly help in making the code much more readable especially if you are using Vensim in a team for prototyping. Woul you suggest to completely remove curly brackets or will Vensim either run or not run - but not say do something unexpected because of their use...

Thank you, Tom, ACCEPTING ENTER within the equation editor certainly helps foster readability as does a non-proportional font. I have tested the commentaries now with VECTOR LOOKUP and VECTOR ELM MAP - being vector functions like the VECTOR SELECT. The problem does not arise there as far as I can te...

Dear all: I have to admit that while I tried hard to keep my model structure (the sketch view) nice and readable I have not done quite enough to keep this readability down to the equations level. Lately though I (re-)discovered that using CTRL-TAB (for indentions), CTRL-RETURN (for additional line s...

Hi, since I used to work with i-Think a bit I noted that the concept of modules there is a neat one. I am not a computer expert but I would assume that this comes a bit closer to object oriented approaches to code/models. To me the reuse of Vensim models (especially if large and subscripted) tends t...

Hi JJ, I quite agree now. The way numbers are treated without 'strict units checking' there should not be a 'UNITS ARE A.OK." message without a statement that strict unit checking is not enforced as that is a bit like fooling oneself. Unless there is an additional statement it is better not to ...

Tom, that is indeed a most valuable feature - for both the purist and the rigorist :wink: - and it is easily missed. The feature is as far as I can see not really treated in the reference manual, so maybe this could be added and maybe it might also be included as an option (right click) for the unit...

Hi JJ, while I can see the problem I would tend to disagree with your postulation of automatically assigning implicit constants in an equation a dimension - or to deny it a dimension by having it be dimensionless by default. As far as I would see it we are treating physical entities that is you have...

Hi Tom, that is exactly what I ment. As hinted at in the thread regarding 'bigger models' the bottleneck is runtime and not diskspace for that matter. I may be mathematically wrong but in principle one should be able to bring most SD model down to the constants, the stocks and the functions that giv...

Tom, what a great support! And you bet that it does what it is supposed to do: It will nicely count the agents going from cell to cell without blowing up the subscript ranges. Now if you could really make Vensim forget everything during runtime besides what is needed at any instance in simulation ti...

Hello, one of my last threads here in the forum touched the question of how to deal with bigger models using Vensim (DSS for that). The answers were enlightening albeit a bit sobering. One of the possibilities is to use an agent-based approach if the model is very disaggregated in attributes as the ...