A task can be either (1) a task alone, or (2) a prerequisite task for another task. To be able to describe in one equation the concept of both tasks and prerequisites (which are also tasks), we define another subscript range — prereqtask — which is exactly the same as the other subscript range (task). This process is called mapping. For this project model, each task is a potential prerequisite for some other task, so we define another subscript range as equivalent to the first, using the mapping symbol <->.
| Ø | Click on the Subscript Control button on the Toolbar. Click on the New… button, in the dialog box type the name prereqtask then press Enter. |
| Ø | Click on the variable Type lower drop down box that currently says Normal and choose Equivalence. |
The Equation Editor displays the Equivalence symbol <-> left of the editing box.
| Ø | Type in the name task in the equation field. It should appear without any brackets. |
| Ø | Click OK to close the Equation Editor. |
You will notice that Vensim did not add a new tab to the Subscript Control. An Equivalence is treated as a Subrange. Thus prereqtask is a Subrange of task and does not get a separate tab. If you want to edit the equation for prereqtask you need to use the Choose button in the Equation Editor.
