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Guards are conditions that need to be used throughout UML diagrams to control flow. Remember that a guard could only be assigned to a single message. To draw a guard on a sequence diagram, you placed the guard element above the message line being guarded and in front of the message name, as shown below.
What is the symbol for a true/false condition on a sequence diagram? Brackets [ ] indicate a true/false condition. Explain what parameters of a message are. Shows the data that is passed with the message.
If else condition, also called alternatives in UML terms can indeed be represented in sequence diagrams. Here is a link where you can find some nice resources on the subject http://www.ibm.com/developerworks/rational/library/3101.html
If you paste
A.do() {
if (condition1) {
X.doSomething
} else if (condition2) {
Y.doSomethingElse
} else {
donotDoAnything
}
}
onto https://www.zenuml.com. It will generate a diagram for you.
Very simple , using Alt fragment
Lets take an example of sequence diagram for an ATM machine.Let's say here you want
IF card inserted is valid then prompt "Enter Pin"....ELSE prompt "Invalid Pin"
Then here is the sequence diagram for the same
Hope this helps!
In Visual Studio UML sequence this can also be described as fragments which is nicely documented here: https://msdn.microsoft.com/en-us/library/dd465153.aspx
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