Sequence Diagram to Communication Diagram

Question

I have created a Sequence Diagram that I wish to convert to a Communication/Collaboration Diagram. I understand the conversion and the numbering process but I am wondering since my Sequence diagram have loops that are under the alt fragment, how is their numbering going to be in Communication Diagram?

Here a sample of my Sequence Diagram that have one of those loops:

Edit: What I want to know here is that since the loop is an IF situation, is it still OK to number those sequences? I don't think it would make logic if user's communication is, 1.0 Enter registered Username and Password, 2.0 Re-enter registered Username and Password...2.0 here is the IF loop situation

Answer 1

Communication diagram and sequence diagram represent the same interactions from a different angle. But the simplified graphical representation of the messages implies restrictions in the communication diagram when it comes to combined fragments.

In the communication diagram you can represent:

• simple sequence of messages and nested messages, via the chronological numbering, e.g.: 1,2,3 and decimal numbering, e.g. 2.1, 2.2, 2.2.1
• loops (loop operator in sequence diagram), using a * following the number. More specific looping specifications could follow the start between brackets e.g. *[1..n] or *[element in collection]. Your loop conditions are unspecified in the sequence diagram, so a * would do.
• simple conditions (opt operator in sequence diagram), using a condition between brackets after the number e.g.[t>10]

In your example, we could therefore imagine: `

1: Enters username and passord
2: Check validity
2.1*: Mismatch of name or password    // return message, not usual     
2.2:  Prompt user to re-enter credentials
2.3:  Re-enter username and password
2.4:  Re-check validity               // missing in the SD in the loop
3: Verify user
4: USer verified
5: Display profile  

Now there are two issues here: first, in your sequence diagram, there is a missing check validity in the loop. Second, I showed for simplicity all the messages, but in a communication diagram you would usually not sho return messages, to keep things simpler. So we'd have a slightly simpler model:

1: Enters username and passord
2: Check validity   //message, return is implicit
3.1*[credentials invalid]: Prompt user to re-enter credentials      
3.2: Re-enter username and password
3.3: Check validity 
4: Read user
5: Display profile  

Advice for the choice between SD and CD:

• if you have many involved lifelines, either make several smaller SDs or opt for the CD.
• if you have many message exchanges, especially between the same lifelines, prefer SD, because following the numbers on a CD is exponentially unintuitive with the increase of messages, and long lists of messages above the arrows look ugly.
• if you have many combined fragments, or if you have more than loops and optionals, stay with SD.