Read internal table - field symbol has not been assigned yet

I have wrote this small piece of code which updates the data in it_alv based on values in another internal table it_alv_to_check.

At some point while executing the code in the ELSE statement block, I got the error:
> field symbol has not been assigned yet

But I had it at first assigned dynamically using READ TABLE statement.

How to make sure that even if sy-subrc <> 0, the field symbol will be assigned?

 FIELD-SYMBOLS: <to_check> LIKE LINE OF it_alv.

   LOOP AT it_alv ASSIGNING FIELD-SYMBOL(<fs_alv_2>).
      READ TABLE it_alv_to_check ASSIGNING <to_check>  WITH KEY belnr = <fs_alv_2>-belnr.
      IF sy-subrc = 0.
        <fs_alv_2>-statement_year = <to_check>-statement_year.
        <fs_alv_2>-laufi = <to_check>-laufi.
        <fs_alv_2>-user_field_1 = <to_check>-user_field_1.
        <fs_alv_2>-stceg = <to_check>-stceg.
        <fs_alv_2>-name1 = <to_check>-name1.
        <fs_alv_2>-monat_from = <to_check>-monat_from.
      ELSE.
        <fs_alv_2>-statement_year = <to_check>-statement_year.
        <fs_alv_2>-laufi = <to_check>-laufi.
        <fs_alv_2>-user_field_1 = <to_check>-user_field_1.
        <fs_alv_2>-stceg = <to_check>-stceg.
        <fs_alv_2>-name1 = <to_check>-name1.
        <fs_alv_2>-monat_from = <to_check>-monat_from.
        <fs_alv_2>-color = VALUE #( ( color-col = 6
                        color-int = 0
                        color-inv = 1
                        )
                         ).
      ENDIF.
    ENDLOOP.

You cannot have your field-symbol assigned in this scenario. You read an internal table using READ TABLE statement and in case the entry was found with the given key (i.e. sy-subrc = 0), the found table row is assigned to a field symbol and the field symbol points to the table row in the memory.

Otherwise, if no table row is found, you have no entry to assign field-symbol to, and the field symbol remains unchanged or initial. In this case you cannot rely on it being assigned (it can be assigned but point to the wrong entry) or assume the assignment itself (remains initial because it was not previously assigned).

More about it: READ TABLE, result ... ASSIGNING

Depending on what you want to achieve you can assign some default values in the else branch or implement some other logic to handle this scenario.

What your code appears to be doing is:

1. If a corresponding row is found it_alv_to_check, take the values of that row to overwrite various values in it_alv.
2. If no corresponding row is found in it_alv_to_check, then also take those values (from the row that does not exist!) and write them into it_alv. And also set the color to a fixed value.

The first part of the second point is obviously impossible. You can not do something with data which does not exist. This just does not make sense on a basic logical level.

So what you need to do here first is some conceptual work. What exactly do you expect to happen with the values statement_year, laufi, user_field_1, stceg, name1 and monat_from in the row of it_alv when it_alv_to_check does not contain a row for that belnr? Leave them as they are? Fill them with fixed placeholder data? Fill them with values from a different row, and if so which one?

Answer this question (probably with the help of the person who gave you this requirement) and the answer to your actual question will probably appear on its own. If not, feel free to ask a new question where you describe what exactly you want to happen and what you need to know in order to do that.

I've changed declaring of the field symbol to:

READ TABLE it_alv_to_check ASSIGNING FIELD SYMBOL(<to_check>)  WITH KEY belnr = <fs_alv_2>-belnr.

and now it works fine