Handling the x = INTEG(_,_) equation

Post date: Feb 28, 2012 11:57:49 PM

For all the models in Hartmut Bossel's System Zoo, the equation listing includes equations of the form

x = INTEG(a,b).

If you look carefully at the model diagram, you will see that a is always a stock (compartment).

There are 3 ways of handling this.

1. If b is a numeric value...

... then use this as the initial value for the stock called x.

2. If b is a variable (usually called INITIAL x)...

... then ignore this equation altogether, and use the value of the variable called INITIAL x for the stock x.

3. If b is some expression, e.g. c*d....

... then make sure that your diagram has the specified variables, give each the value specified in the equation listing, draw influences from them to the stock x, and enter the expression (e.g. c*d) into the stock x.

Note that you never need to use the first argument (in the example above, a). This is simply some expression which adds up all the (positive) inflows and the (negative) outflows, to give the net rate of change for the stock x. Simile or Similette know this already, from the model diagram.