ABAP Programs:Using Field Symbols for components of a structure

REPORT ZSOURCE2403.
* Table work area for later use
TABLES CUSTOMERS.
* Defining a Field Symbol
FIELD-SYMBOLS

.

* Displaying all fields of all table entries
SELECT * FROM CUSTOMERS.

NEW-LINE.
DO.
ASSIGN COMPONENT SY-INDEX OF STRUCTURE CUSTOMERS TO <>.
IF SY-SUBRC <> 0.
EXIT.
ENDIF.
WRITE <>.
ENDDO.
ENDSELECT.