Associative array EXISTS() DML

Question

Is there a way to include the EXISTS() method of an associative array in a DML statement? For example:
This is fine...
SQL> declare
2 type distTblTyp is table of number index by varchar2(8);
3 testTbl distTblTyp; testVar varchar2(30);
4 begin
5 testTbl('a') := 1;
6 if (testTbl.exists('a')=true) then
7 null;
8 end if;
9* end;
SQL> /
PL/SQL procedure successfully completed.
But this fails...
SQL> declare
2 type distTblTyp is table of number index by varchar2(8);
3 testTbl distTblTyp; testVar varchar2(30);
4 begin
5 testTbl('a') := 1;
6 select decode(testTbl.exists('a'),true,'a','b')
7 into testVar from dual;
8* end;
SQL> /
select decode(testTbl.exists('a'),true,'a','b')
ERROR at line 6:
ORA-06550: line 6, column 24:
PL/SQL: ORA-01747: invalid user.table.column, table.column, or column
specification
ORA-06550: line 6, column 2:
PL/SQL: SQL Statement ignored
PL/SQL: SQL Statement ignored
Since I won't know which index elements exist until execution time, I want to have an insert statement (ran within a loop) with ~100 field values like:
insert into table [tablename]
values (
decode(testTbl.exists('a'), true, testTbl('a'), null),
decode(testTbl.exists('b'), true, testTbl('b'), null)
Any suggestions appreciated.

Answer 1

As Gerd suggested...
SQL> create or replace package my_test_pk as
  2   type my_type is table of number index by varchar2(2);
  3   v_my_type my_type;
  4   function ifExists(v_i varchar2) return number;
  5   procedure runTest;
  6  end;
  7  /
Package created.
SQL> create or replace package body my_test_pk as
  2  function ifExists(v_i varchar2) return number is
  3     v_r number;
  4     begin
  5     v_r := 0;
  6     if (v_my_type.exists(v_i)) then v_r := 1; end if;
  7     return v_r;
  8     end;
  9  
10    procedure runTest is
11    v_check varchar2(2);
12    begin
13     v_my_type('a') := 1;
14    select decode(ifExists('a'),1,'a',null) into v_check from dual;
15   dbms_output.put_line(v_check);
16     select decode(ifExists('b'),1,'b','N') into v_check from dual;
17    dbms_output.put_line(v_check);
18    end;
19     end;
20  /
Package body created.
SQL> exec my_test_pk.runtest;
a
N
PL/SQL procedure successfully completed.
SQL>