Problem populating an internal table

Question

Hello,
I am having a problem with filling an internal table. I cannot seem to grasp this and I struggle with this in every program. I am attaching pieces of my program so that maybe someone can help me. I have a selection screen that I populate with dates. if the dates are populated, I want to use these dates for the selection. If the dates are initial, I want to read an entry from the TVARV table and populate the dates that will used in the selection. I tried various code statements and I cannot get it to work. i get messages like "is a table without a header line and therefore has no component "LOW" or other messages similar to this. I would like that if the s_udate is populated, I could fill the tvarv table and not have to call the form.
also if anyone has some documentation on working with internal tables without header lines (I think I read in here that internal tables without header lines is the way to go ), could you please include that as well so that I could try to understand how this suppose to work 
thanks in advance for the help
TYPES: BEGIN OF ty_tvarv_date,
        low     TYPE   tvarv_val,          "start date
        high    TYPE   tvarv_val,          "end date
       END OF ty_tvarv_date.
TYPES: ty_t_tvarv_date TYPE STANDARD TABLE OF ty_tvarv_date INITIAL SIZE 0,
DATA   it_tvarv_date TYPE STANDARD TABLE OF ty_tvarv_date INITIAL SIZE 0.
SELECTION-SCREEN BEGIN OF BLOCK a WITH FRAME TITLE text-001.
SELECTION-SCREEN BEGIN OF LINE.
SELECTION-SCREEN COMMENT 2(22) text-002.
SELECT-OPTIONS s_udate FOR sy-datlo.
SELECTION-SCREEN END OF LINE.
SELECTION-SCREEN END OF BLOCK a.
PERFORM select_date_tvarv CHANGING it_tvarv_date.
PERFORM select_data643_mseg USING    it_tvarv_date
                                     it_zfi_gl_subcontrk[]
                              CHANGING it_mseg643.
<b>*******************************************************************</b>
FORM select_date_tvarv CHANGING p1_it_tvarv_date TYPE ty_t_tvarv_date.
IF s_udate IS INITIAL.
  SELECT low high
    FROM tvarv
    INTO  TABLE p1_it_tvarv_date
    WHERE name EQ  'Z_SUBCONTRK_DATES'.
  IF sy-subrc NE 0.
    IF sy-batch IS INITIAL.
      MESSAGE i899 WITH text-027.
      LEAVE LIST-PROCESSING.
    ELSE.
      MESSAGE e899 WITH text-027.
    ENDIF.
  ENDIF.
ELSE.
     MOVE s_udate to p1_it_tvarv_date.
     MOVE s_udate-low(8)  to p1_it_tvarv_date-low.
     MOVE s_udate-high(8) to p1_it_tvarv_date-high.
ENDIF.
ENDFORM. " FORM SELECT_DATE_TVARV
<b>*******************************************************************</b>
FORM select_data643_mseg USING p1_tvarv_date       TYPE ty_t_tvarv_date
                               p1_zfi_gl_subcontrk TYPE ty_t_zfi_gl_subcontrk
                      CHANGING p1_it_mseg          TYPE ty_t_mseg643.
  DATA: wa_tvarv_date type ty_tvarv_date.
  READ TABLE p1_tvarv_date INTO wa_tvarv_date INDEX 1.
  SELECT abukrs  awerks amatnr aerfmg a~erfme
         akostl abwart amblnr awempf acharg akzbew algort bbudat
    FROM mseg AS a INNER JOIN mkpf AS b ON amblnr EQ bmblnr
      AND b~budat <= wa_tvarv_date-high
      AND b~budat >= wa_tvarv_date-low
        INTO TABLE p1_it_mseg
        FOR ALL ENTRIES IN p1_zfi_gl_subcontrk
        WHERE a~bukrs   EQ p1_zfi_gl_subcontrk-bukrs_from
        AND   a~werks   EQ p1_zfi_gl_subcontrk-werks_from
        AND   a~wempf   EQ p1_zfi_gl_subcontrk-werks_to643
        AND   a~matnr   EQ p1_zfi_gl_subcontrk-matnr
        AND   a~bwart   EQ'643'.
  IF sy-subrc NE 0.
    IF sy-batch IS INITIAL.
      MESSAGE i899 WITH text-027.
      LEAVE LIST-PROCESSING.
    ELSE.
      MESSAGE e899 WITH text-027.
    ENDIF.
  ENDIF.
ENDFORM. " FORM SELECT_DATA643_MSEG

Answer 1

In tables with header line, you need not define an explicit work area.
System will create a so-called work area, which is called header line.
This header line is like any other work area. the difference is that it can be accessed using internal table name. A record will be by default read into header line in a READ statement.
For ex,
READ TABLE p1_tvarv_date INTO wa_tvarv_date INDEX 1.
Here you used to read the 1st record into wa_tvarv_date.
But after making it table with header line you can use
READ TABLE p1_tvarv_date INDEX 1.
This will move the data to header line and it can be accessed by name p1_tvarv_date.
See the code below.
TYPES: BEGIN OF ty_tvarv_date,
low TYPE tvarv_val, "start date
high TYPE tvarv_val, "end date
END OF ty_tvarv_date.
TYPES: ty_t_tvarv_date TYPE STANDARD TABLE OF ty_tvarv_date INITIAL SIZE 0,
DATA it_tvarv_date TYPE STANDARD TABLE OF ty_tvarv_date <b>WITH HEADER LINE INITIAL</b> SIZE 0.
SELECTION-SCREEN BEGIN OF BLOCK a WITH FRAME TITLE text-001.
SELECTION-SCREEN BEGIN OF LINE.
SELECTION-SCREEN COMMENT 2(22) text-002.
SELECT-OPTIONS s_udate FOR sy-datlo.
SELECTION-SCREEN END OF LINE.
SELECTION-SCREEN END OF BLOCK a.
PERFORM select_date_tvarv CHANGING it_tvarv_date.
PERFORM select_data643_mseg USING it_tvarv_date
it_zfi_gl_subcontrk[]
CHANGING it_mseg643.
FORM select_date_tvarv CHANGING p1_it_tvarv_date TYPE ty_t_tvarv_date.
IF s_udate IS INITIAL.
SELECT low high
FROM tvarv
INTO TABLE p1_it_tvarv_date
WHERE name EQ 'Z_SUBCONTRK_DATES'.
IF sy-subrc NE 0.
IF sy-batch IS INITIAL.
MESSAGE i899 WITH text-027.
LEAVE LIST-PROCESSING.
ELSE.
MESSAGE e899 WITH text-027.
ENDIF.
ENDIF.
ELSE.
MOVE s_udate to p1_it_tvarv_date.
MOVE s_udate-low(8) to p1_it_tvarv_date-low.
MOVE s_udate-high(8) to p1_it_tvarv_date-high.
ENDIF.
ENDFORM. " FORM SELECT_DATE_TVARV
FORM select_data643_mseg USING p1_tvarv_date TYPE ty_t_tvarv_date
p1_zfi_gl_subcontrk TYPE ty_t_zfi_gl_subcontrk
CHANGING p1_it_mseg TYPE ty_t_mseg643.
<b>READ TABLE p1_tvarv_date INDEX 1.</b>
SELECT abukrs awerks amatnr aerfmg a~erfme
akostl abwart amblnr awempf acharg akzbew algort bbudat
FROM mseg AS a INNER JOIN mkpf AS b ON amblnr EQ bmblnr
<b>AND b~budat <= p1_tvarv_date-high
AND b~budat >= p1_tvarv_date-low</b>
INTO TABLE p1_it_mseg
FOR ALL ENTRIES IN p1_zfi_gl_subcontrk
WHERE a~bukrs EQ p1_zfi_gl_subcontrk-bukrs_from
AND a~werks EQ p1_zfi_gl_subcontrk-werks_from
AND a~wempf EQ p1_zfi_gl_subcontrk-werks_to643
AND a~matnr EQ p1_zfi_gl_subcontrk-matnr
AND a~bwart EQ'643'.
IF sy-subrc NE 0.
IF sy-batch IS INITIAL.
MESSAGE i899 WITH text-027.
LEAVE LIST-PROCESSING.
ELSE.
MESSAGE e899 WITH text-027.
ENDIF.
ENDIF.
ENDFORM. " FORM SELECT_DATA643_MSEG