For loop and its reverse based on if-condition/ternary operation in Oracle - oracle

Question

In Oracle, I have the following almost identical SQL in an if-else block of a stored procedure:
if v_endsid = '15' then
FOR i IN 1..v_num LOOP --loop around the number from the beginning
v_item := TRIM(SUBSTR(v_str, (i - 1) * 12 + 1, 12)); --now this is the evaluated item
if v_item = TRIM(cursorElement.ITEM) then --this is the time to break
break;
end if;
END LOOP;
else
FOR i IN REVERSE 1..v_num LOOP --loop around the number from the last
v_item := TRIM(SUBSTR(v_str, (i - 1) * 12 + 1, 12)); --now this is the evaluated item
if v_item = TRIM(cursorElement.ITEM) then --this is the time to break
break;
end if;
END LOOP;
end if;
As you can see, the only difference between the SQL in the if and else blocks is the FOR loop. One is a forward for loop, another is a reversed (backward) for loop.
Is there any way to combine the block? I am trying this:
FOR i IN (case when v_endsid = '15' then 1..v_num else REVERSE 1..v_num end) LOOP
v_item := TRIM(SUBSTR(v_str, (i - 1) * 12 + 1, 12)); --now this is the evaluated item
if v_item = TRIM(cursorElement.ITEM) then --this is the time to break
break;
end if;
END LOOP;
But it gives me compilation error in the 1..v_num:
Found: '..' Expecting: END -or- ELSE -or- WHEN -or- OR -or- AND -or-
BETWEEN IN LIKE LIKE2 LIKE4 LIKEC MEMBER SUBMULTISET -or- ! != < <= <>
= > >= ^ ^= IS NOT ~

Answer 1

There is no way of dynamically changing the direction of the for loop. The only thing you can do here if you want to combine the two blocks is to use a basic loop
if v_endsid = '15' then
i := 1;
reverse := false;
else
i := v_num;
reverse := true;
end if;
LOOP --loop around the number from the beginning
v_item := TRIM(SUBSTR(v_str, (v_num - 1) * 12 + 1, 12)); --now this is the evaluated item
if v_item = TRIM(cursorElement.ITEM) then --this is the time to break
break;
end if;
if reverse = true then
if i = 1 then
exit;
else
i := i - 1;
else
if i = v_num then
exit;
else
i := i + 1;
end if;
end if;
END LOOP;

Answer 2

The final solution I adapt is by using basic LOOP and some ternary operations:
i := case when v_endsid = '15' then 1 else v_num end; --evaluates from front or back depending on the case
Loop
v_item := TRIM(SUBSTR(v_str, (i - 1) * 12 + 1, 12)); --now this is the evaluated item
--other queries
i := i + (case when v_endsid = '15' then 1 else -1 end);
exit when i = 0 or i = v_num + 1; --exceeds the elements
end loop;
This, I think, is a fairly neat working replacement for the original SQL

Answer 3

How about the another way round ? Instead of manipulating the loop condition encapsulate the code evaluating the break condition into a function that can be called from the different loops.
declare
v_reverse constant boolean := true;
-- your parameters and break rule can be arbitrary complex, mine is simple
-- as this is just a demonstration
function break(i in pls_integer) return boolean is
begin
return 13 = i;
end;
begin
if v_reverse
then
for i in reverse 1 .. 15
loop
dbms_output.put_line(i);
exit when break(i);
end loop;
else
for i in 1 .. 15
loop
dbms_output.put_line(i);
exit when break(i);
end loop;
end if;
end;
/