Here you go. Thanks for interest 
DATA: l_string TYPE string.
DATA: l_index TYPE n.
TYPES: BEGIN OF t_tab,
ind TYPE n,
END OF t_tab.
DATA: lt_ind TYPE TABLE OF t_tab.
DEFINE m_add.
if not &1 is initial.
if &2 is initial.
&2 = &1.
else.
&2 = &2 && '/' && &1.
endif.
endif.
END-OF-DEFINITION.
PERFORM test1.
CLEAR: l_index, l_string.
PERFORM test2.
CLEAR: l_index, l_string.
*&---------------------------------------------------------------------*
*& Form TEST1
*&---------------------------------------------------------------------*
FORM test1.
DO 100000 TIMES.
l_index = l_index + 1.
m_add l_index l_string.
ENDDO.
ENDFORM. " TEST1
*&---------------------------------------------------------------------*
*& Form TEST2
*&---------------------------------------------------------------------*
FORM test2.
DO 100000 TIMES.
l_index = l_index + 1.
APPEND l_index TO lt_ind.
ENDDO.
DELETE lt_ind WHERE ind IS INITIAL.
CONCATENATE LINES OF lt_ind INTO l_string SEPARATED BY '/'.
ENDFORM. " TEST2