Create a Header belong to a field.

Hello all, i am new of Crystal Report. I am designing a report, and i want create a Header belong to a field.

Example: List fields of Group Header can contain: field_a, field_b, field_c, field_d, field_e, ... But field_b in some cases can disappear.

So i want when have field_b, the tittle of report is "This is report for field_b", else is "This is report for others" My idea is get a string (Str_allfields) contain all fields then use InStr to find, then create 2 headers, and the appear of what header belong to InStr(Str_allfields,field_b).

But the How to creat the Str_allfields.

Please help me find the best way to resolve my trouble. Thank all very much.