The expression block can be used to specify an arbitrary function of the model parameters and /or the quantity in other blocks (such as compartments and lookup tables. The value of this block is available for use during the simulations.

From the Projects window

• Expand the Project to which you wish to add the expression by clicking the + symbol next to it. Right-click on ‘Blocks’, or a sub-system in ‘Blocks’, and select Expression from the pop-up menu.

From the Matrix window

• Right click an empty diagonal cell of the matrix and select Expression from the the pop-up menu.

From the Blocks window

• Place the mouse cursor over the table and right-click on it. Select Create | Expression from the pop-up menu, or
• In the title bar, select Expression from the Type drop-down list and click on the button.

The block editor is shown by right-clicking the block in a window and selecting Edit. In the Blocks window, you can also show the block editor by selecting the block and clicking the button, or by double-clicking the right-most “validation” column of the block.

The block editor has three tabs:

This page lets you edit basic properties such as name, sub-system and base unit. See editing properties for more information.

By clicking Settings button Evaluation mode is available. Evaluation mode allows the user to specify when, during simulation, the expression is evaluated.

This page lets you edit the visual properties of the block in the matrix window and graph window. See editing appearance for more information.

This page lets you specify the algebraich expression and other index specific properties. See Editing values for more information.

Dimension - You can specify the dimension of the block by using the Type drop down list. Depending on the dimension the index drop down lists are activated.

Data

• Indices - For blocks that are non-scalar, the first columns will specify indices from the index lists. This allows you to specify different expressions for different indices. The top row is named “default” and can be used when the same value should be set for all indices.
• Expression - The expression can contain any number of parameters, blocks and functions. The Expression editing tool can be used to quickly find available functions and objects to use in the expression. Also refer to expression syntax for details on mathematical format and how to specify conditions.
• Information - By using the Table control button you can display more columns that will help you document your model.
• Arguments - Used to define arguments that should be sent to the expression (see below).

In some situations it is useful to require arguments to be sent to the expression. These arguments then be used in the equation, together with parameters and other blocks. The expressions then becomes a function, and will only be used as an “abstract”.

• Arguments allow you to define an equation in one place, to be used in many places using different parameters.
• An expression for which arguments have been defined will never be available as a simulation result.

Use the right-hand side table to add (or remove arguments. Click in the table fields to change argument names or to write descriptions for the arguments.

Once an argument has been added, you can use it in the equations of the expressions.

You reference an expression by writing the name (or ID) of the expression. If the expression requires arguments, you also add parenthesis and a comma-separated list of argument values - like with any function.

Examples:

• For an expression named MyExpression, which takes two arguments: MyExpression( 134.2, time/2 )
• Calling the equation for Lead defined in a material dependent expression which requires one argument: AnotherExpression[Lead]( 432.2 )

• Blocks
• Blocks window
• Matrix window
• Graph window