simile... modeling software that is pretty cool!

I'm taking my first "big girl" engineering class-- NOTE THAT I AM NOT AN ENGINEER-- and we got this really neat software to use to make conceptual models. What's great is that

1. it's visual

2. it runs C ++

3. put the two above together and you can see that essentially I consider it a GUI for C++... I think C++ looks very hard but I admire how efficiently well-written scripts in it run, and people who can figure out all that stuff with the parenthesis. However I don't know how to do that, and I haven't had the chance to learn yet. OKAY!

Okay, so I built a basic (bad) model for stand density to show you how it works:

So the basic rules of the road are in this toolbar here. Look to the lower left, and you've got boxes for "steady state variables"-- the things you want to "track" with your differential equations. In this case, it is "stand density" that I want to track. The circle with the X in it is "variables"-- these are parameters and they can be functions, constant, boolean, arrays, logical... you name it. More in a second. Then there is hourglass-with-a-line-through-it, this is a flow to move something (like trees) from one state to another. The units of these must agree with the boxes. You can also use them to move things into the system or out of it. Finally there is little-curvy arrow, which is used to make influence. For example, if you are writing a differential equation where the CHANGE IN X OVER TIME = RATE OF SOMETHING * X, then you would want rate of something and X to influence change in X over time. I've found you can't really have too many of these, and simile will delete the ones that you don't put into the model math.

There's some other neat stuff up there too that I won't go into, including creation (sun), migration (bird), and submodel (the cartoonish talking cloud). Oh! Most importantly, to click on something, use the little arrow. I know it sounds dumb but I took a bit to understand that.

So you have this model now that you built for stand density. As you can see, the above variables are described and labelled, and lines of flow and influence are drawn. This is just point and click.

 Now you may want to fill it in. Well, okay, you do want to fill it in. So click on the variables, boxes, and flows, and you'll get a screen like this (you don't fill in the influences, they are more like operators, I guess). Anyway, here I called rate of seeding 0.5 (fertile stand, huh?)

And here you can see me filling in the flow for birth. Notice in the background it's green while you are filling it in. Also notice that because of the "influences" I have the choice to fill it in using the variables that influence it. I've just made it rate*stand density. This is a per-capita stand. :D

Ok! Time to plot! And there's a lot of ways to do it. You'll see now that my model is black because I've fillled everything in. I plotted stand density versus time (I had a low death rate, something like 0.4 per capita). Oh NO! OVERCOMPETITION OCCURED! TIME FOR THINNING (see below!)

Well, that's your brief guide to simile. If you like it you should play the tutorials here