ASSIGN COMPONENT <comp> OF STRUCTURE <s> TO <FS>.

REPORT Y_TEST.

type-pools ABAP .

DATA: BEGIN OF LINE,
         COL1 TYPE I VALUE '11',
         COL2 TYPE I VALUE '22',
         COL3 TYPE I VALUE '33',
      END OF LINE.

DATA COMP(5) VALUE 'COL3'.

FIELD-SYMBOLS: <F1>, <F2>, <F3>.

ASSIGN LINE TO <F1>.
ASSIGN COMP TO <F2>.

DO 3 TIMES.
   ASSIGN COMPONENT SY-INDEX OF STRUCTURE <F1> TO <F3>.
   WRITE <F3>.
ENDDO.

ASSIGN COMPONENT <F2> OF STRUCTURE <F1> TO <F3>.
WRITE / <F3>.