Adding a Decision

Activity diagrams can also implement logic that is based on conditions. You can branch a control flow using the Decision element.

The following example shows how to use decisions in activity diagrams. The user can enter a positive number. The process will then return a message depending on if the number is smaller or greater than 10 and divisible by 4.

The branching conditions of the decision must evaluate to a Boolean value (boolean expression) and be defined on the outgoing control flows of the decision node as a Guard Expression. Page Logical Operators provides a list of all possible logical operators that can be used within a guard expression. You can also use Boolean operators (and, or) as described in Boolean Operators.

In the example above, there are three outgoing control flows:

The one or more of the control flows contain the actual condition(s) ((1) and (2)). Exactly one control flow must contain an else expression (3): This control flow is followed if all other expressions evaluate to false.

Adding a Guard Expression

To add a guard expression to a control flow proceed as follows. Select the control flow you want to add a guard expression to from the activity diagram and open the Attributes panel:

Enter a boolean expression into the field Guard Expression:

The entered expression is also displayed in a box next to the corresponding control flow in the diagram:

When the guard expression box is selected, you can move it any time. Click the box and drag it to the desired position:

You can resize the guard expression box by clicking the square in the bottom right corner of the element and dragging it:

Specifying an Evaluation Order

The order attribute on the decision flow defines the order in which the guards should be evaluated. To add an order to a control flow proceed as follows. Select the control flow you want to add an order to from the activity diagram and open the Attributes panel:

Enter the order number you want to assign to the selected control flow:

Examples

The following tables shows some examples regarding the activity diagram above:

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