how to express alternative flows in activity diagram

Question

I want to express the below events in an activity diagram:

• An alternative flow, such as "at step x of basic flow, user clicks cancel instead of ok".
• An alternative entry to the use case, such as "instead of click the bold button, use can press Ctrl-B".
• An error, such as "when user clicks save, the system is unable to save the file to disk."

How can I do these?

Thank you.

Answer 1

you need something called constraints or guardian conditions.

see where it says [loopguard] in that diagram? you can put [if user does x] implying the next state won't occur until the condition is true.

for the second question, I'd just write "user presses ctrl+B or selects Bold" in the same state circle or as a note to the side.

for the third question, you can use exceptions. It looks like this:

Answer 2

I'm not an expert on diagramming, but I like using shapes with multiple points to specify an alternative path, such as Diamonds or Triangles