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:
Nr. | Control Flow | Condition | Guard Expression |
|---|---|---|---|
(1) | Decision → smallerTen | entered number smaller than ten |
|
(2) | Decision → divisibleByFour | entered number divisible by four |
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(3) | Decision → greaterEqualTen | all other cases |
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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.
The boolean operators of the xUML Runtime support short-circuiting . This means that the second operand is evaluated only when the result is not fully determined by the first operand.
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:
Example | Entered Number | Guard Expression | Condition | Result |
|---|---|---|---|---|
1 | 7 |
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| Branch 1 is followed without evaluating the other two branches |
2 | 12 |
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| Branch 2 is followed without evaluating the third branch |
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| |||
3 | 14 |
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| Branch 3 is followed |
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| |||
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entered number smaller than ten
entered number smaller than tenActivity_Decision_Example
Click here to download a simple example model that shows how to use decisions in activity diagrams with Scheer PAS Designer.