This code is too"pseudo". Maybe post the script steps involved.
Probably have an = sign or two that shouldn't be there. The precise syntax for this would be:
Set variable [$$a ; value: 1]
Set Variable [$$a ; value: $$a + 1 ]
Show custom dialog [$$a]
and this will display 2 in the custom dialog. Note that there are no = signs anywhere in this script.
wrong: Set Variable [$$a ; value: $$a=$$a + 1 ]
right: Set Variable [$$a ; value: $$a + 1 ]
thanks a big lot guys
Unlike a few programming languages, = is only used to compare values, not to assign them. Thus $$a = $$a + 1 will return false (no number equals a value one greater than itself) which in FileMaker means that a 0 would be entered into $$a by the set variable step.