Not able to access the INIT variable in a secondary FOR loop - for-loop

Question

I have the following code snippet:
TYPES: BEGIN OF ty_table_line,
idx TYPE i,
posnr TYPE i,
quan TYPE i,
END OF ty_table_line,
ty_internal_table TYPE SORTED TABLE OF ty_table_line WITH UNIQUE KEY idx.
DATA(lt_set) = VALUE ty_internal_table( ( idx = 1 posnr = 1 quan = 2 )
( idx = 2 posnr = 1 quan = 40 )
( idx = 3 posnr = 1 quan = 10 )
( idx = 4 posnr = 1 quan = 88 ) ).
DATA(lt_set_2) = VALUE ty_internal_table(
FOR i = 0 WHILE i <= 4
( LINES OF VALUE #( FOR <line> IN lt_set[]
WHERE ( idx = i + 1 )
( <line> ) ) ) ).
Here, my loop starts with i=0 and I am trying to use the value in i to filter out some values from an internal table LT_SET. What is the right approach for this error?
I tried with WHERE ( idx = i + 1 ) and WHERE ( idx <> i ), but I get this error:
The variable "I" cannot be used here.

Answer 1

I can't explain the reason (nothing found in the ABAP documentation) but I could reproduce and find a workaround with an additional auxiliary variable declared with LET. Tested with ABAP 7.52.
TYPES: BEGIN OF ty_table_line,
idx TYPE i,
posnr TYPE i,
quan TYPE i,
END OF ty_table_line,
ty_internal_table TYPE SORTED TABLE OF ty_table_line WITH UNIQUE KEY idx.
DATA(lt_set) = VALUE ty_internal_table( ( idx = 1 posnr = 1 quan = 2 )
( idx = 2 posnr = 1 quan = 40 )
( idx = 3 posnr = 1 quan = 10 )
( idx = 4 posnr = 1 quan = 88 ) ).
DATA(lt_set_2) = VALUE ty_internal_table(
FOR i = 0 WHILE i <= 4
LET j = i IN
( LINES OF VALUE #( FOR <line> IN lt_set[]
WHERE ( idx = j + 1 )
( <line> ) ) ) ).
ASSERT lt_set_2 = lt_set.