The previous tip on this subject can be superseded by the simple use of field-symbols in connection with the 'Assign component idx' statement in SAP.
Gives the same result , but also has the benefit of making use of existing conversion exit's.
DATA: BEGIN OF REC, A VALUE 'a', B VALUE 'b', C VALUE 'c', D VALUE 'd', END OF REC, CN(5) VALUE 'D'. FIELD-SYMBOLS
. DO 3 TIMES. ASSIGN COMPONENT SY-INDEX OF STRUCTURE REC TO . IF SY-SUBRC <> 0. EXIT. ENDIF. WRITE . ENDDO. ASSIGN COMPONENT CN OF STRUCTURE REC TO . WRITE .