Variables provide a source of numeric information in the model.  Variables are utilized by model components that perform a calculation. 


Four variable types are defined for the Model Builder process:

  1. •external variable is a value, record, array or file that is stored on a locally or available from a remote data source such as a USB drive, another connected computer or a website on the internet

  2. •internal variable is a value from another model component such as a stock level or flow rate or a converter result

  3. •user input variable is a value that is input directly by the user

  4. •adjusted variable is defined by the user with a movable control in the user interface


The variable symbol is a line with an arrow pointing to the component performing the calculation.  The variable symbol may optionally be drawn with enforced 90° transitions or as a semicircle.  The following are variable symbols:


from External Variable source


from Internal Variable source


from User Input Variable source


from Adjusted Variable source


Attributes configured by the Model Builder user for an external variable:

  1. •EXTERNAL REF - the file location or URL for an external variable

  2. •QUERY RULE - the rule to submit a query to the remote data source

  3. •PARSE RULE - the rule to interpret the response to the query


Attributes configured by the Model Builder user for an adjusted variable:

  1. •ADJUST RANGE - slider adjustment range from MIN to MAX

  2. •ADJUST INCREMENT - the increment of the slide adjustment


Component Interfaces on an internal variable allowed by the Model Builder process:

  1. •from or to a modeled stock, a modeled flow, a transformer, a factory of a converter


Component Interfaces on an external, user input or adjusted variable allowed by the Model Builder process:

  1. •to a modeled stock, a modeled flow, a transformer, a factory of a converter


Error Conditions detected and prohibited by the Model Builder process:

  1. •ILLEGAL VARIABLE CONNECTION
        – an attempt to connect a variable from or to an unbounded stock
        – an attempt to connect a variable from or to an unrestricted flow


Other Behaviors and Characteristics:

  1. •none


System Dynamics Calculator  ⇑

The SD Model in Depth  ↑

Factories →

 

        System Dynamics Calculator

                The Calculator

                The SD Model in Depth

                        Stocks

                        Flows

                        Transformers

                        Converters

                        Variables

                        Factories

                        Customized Factory Packages

                        Abstraction Layers, Fragments and Scripts

                        Functions

                The Model Builder

                The Model Simulator

                Appendix: The Modeling Process

                System Dynamics CalculatorR1.pdf

                SD2ProjectProposalSDS.pdf