The differences between FIELD-SYMBOLS and Internal

I will show you a way to write the lines of a table without knowing the different components of its line type. First of all you can reuse these lines of code to output any result of a SELECT that is stored in an internal table. And secondly, you encounter two important means to realize dynamic programming in ABAP: the field-symbol and the dynamic assign.

First we have to declare such a field-symbol, this is a particular field type in ABAP. Without going into the details of it, you should know that a field symbol works like a pointer without pointer arithmetics, but has a value semantics. That is you can assign it to different data fields and when accessing their content there is no need for dereferentiation.

FIELD-SYMBOLS: TYPE ANY .

The typing ANY is necessary because the field symbol should be able to refer to data of any type. And this is what our loop looks like now using a dynamic assignment to the different components of the work area:

The most complex activity is contained in the third line. The rest is simple and straight forward: The system variable sy-index contains the loop counter that is counted up with every processing of the loop. The n-th component of the structure is assigned to the field symbol. If this is successful (if sy-subrc =0 ) the content of the field symbol is output to the list using the write-statement. If there is no more component to be assigned to the field symbol, the assign is not successful (sy-subrc <>0 ) and the we leave the DO-loop (the inner loop) and go on with the outer loop.

To check if our new lines yield the same result we outcomment the original loop. You mark the relevant lines and press CRT and "<" simultaneously. We run the program and the result is almost the same. There is one difference: This table has some more columns because this time we output all columns, while in the first alternative we have not shown the columns SEATSMAX_B, SEATSOCC_B, SEATSMAX_F, SEATSOCC_F. If you doubt this just double click on sflight in the program and look in the declaration of the table in the ABAP Dictionary

Here is the related codes:

FIELD-SYMBOLS: TYPE ANY .

LOOP AT itab_flight INTO wa_flight.

  DO.

    ASSIGN COMPONENT sy-index OF STRUCTURE wa_flight TO .

    IF sy-subrc <> 0.

      SKIP.

      EXIT.

    ENDIF.

    WRITE .

  ENDDO.

ENDLOOP.

Section 2:

Do you want to squeeze more performance out of your Internal Table processing?The new ABAP object extensions in SAP require internal table definitions to look a little different. With this new style of definition comes many options for better performance like hash tables and sorted tables. The use of field symbols takes this performance concept one step further. 

The new style of definition of internal tables usually doesn't include a header area for the internal table. Furthermore, ABAP Objects will not allow internal tables with header lines. For added performance, instead of declaring a work area--use a field symbol. The work area concept and header line format both require data to be moved from the internal table to the work area or header area. A field symbol is just a pointer that will point to the proper line of the itab. With field symbols, no data needs to be moved. The field symbol just stores the proper memory address to point at the right line. The field symbol can have structure and be made up of fields similar to a work area or header line. Because no data needs to be copied, processing is much faster.