Declare and append values to local variable

No. Variables in Appian are immutable, following the functional paradigm of the SAIL expression language. But you can just create a new variable holding the values. And, you do not have to declare a local variable, just to define the output of your expression.

As Stefan mentioned already, no.  You're thinking about it backwards.  There's no need at all to "declare" an empty value for "a" if you're just going to go on and perform some calculations:  perform your calculations first, then declare your local variable based on that result.

Thank you [mention:a11f77a729fb4db2af76b09948583328:e9ed411860ed4f2ba0265705b8793d05]  and [mention:b45a4f6a20b14a008e4e0ec732a8bf98:e9ed411860ed4f2ba0265705b8793d05] , That was helpful. The above example is just a protype that I had created. The expression is quiet complex in the original code.