'How to show "if" condition on a sequence diagram?

I was wondering, how can one represent "if" statement on a sequence diagram?

if (somethingShouldBeDone) {
       // Do it
} else {
       // Do something else
}

Can it be represented at all? The thing is ... in my code, fair amount of conditions are checked to determine a variety of actions. If i am going to show the actions, I'd like to explicitly state that actions are caused by particular events.

If possible create an image representation of a solution.

umlsequence-diagram


Solution 1:[1]

If you paste

A.do() {
  if (condition1) {
   X.doSomething
  } else if (condition2) {
   Y.doSomethingElse
  } else {
   donotDoAnything
  }
}

onto https://app.zenuml.com. It will generate a diagram for you.If/else sequence diagram

Solution 2:[2]

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

ATM machine sequence diagram

Hope this helps!

Solution 3:[3]

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

Sources

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Source: Stack Overflow

Solution Source
Solution 1
Solution 2 vishwampandya
Solution 3 David Burg

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