DO : ABAP Keyword a day

DO
Variants

1. DO.
2. DO n TIMES.
Variant 1
DO.
Addition

… VARYING f FROM f1 NEXT f2
Effect
Repeats the processing enclosed by the DO and ENDDO statements until the loop is terminated by EXIT , STOP or REJECT .

You can use the CONTINUE statement to end the current loop pass prematurely and continue with the next loop pass.

The system field SY-INDEX counts the number of loop passes, starting from 1. You can nest DO loops. When the processing leaves a DO loop, the value of SY-INDEX belonging to the outer DO loop is restored.
Example

DO.
WRITE: / 'SY-INDEX - Begin:', (3) SY-INDEX.
IF SY-INDEX = 10.
EXIT.
ENDIF.
WRITE: 'End:', (3) SY-INDEX.
ENDDO.

This DO loop outputs 9 lines of the form

” SY-INDEX – Begin: n End: n “.

Here, n stands for the numbers 1 to 9.

The last line displayed is

” SY-INDEX – Begin: 10 “.

On the 10th pass, the loop is terminated.
Note
The danger with this statement is programming endless loops. Therefore, you must ensure that the loop contains at least one EXIT , STOP or REJECT statement which is executed at least once.
Addition 1
… VARYING f FROM f1 NEXT f2
Effect
This addition is useful if you have a series of fields of the same type and the same distance from each other.
f is a variable which you define in a DATA statement. On each loop pass, f contains a new value. The field f1 after ” FROM ” specifies the first value of the variable f , while the field f2 after ” NEXT ” specifies the value to be assigned to the variable f in the second pass. For each subsequent pass, the variable f contains the next value in the sequence determined by the distance between the fields f1 and f2 in memory.
The fields f1 and f2 should be type-compatible and convertible to f .
If the value of f changes during the loop pass, the new value is then placed in the appropriate field fn assigned to f (transfer type: pass by value and result). If the loop pass terminates because of a dialog message, the new value is not passed back if f changes.
The addition … VARYING f FROM f1 NEXT f2 can be used several times in a DO statement.
Example

DATA: BEGIN OF WORD,
ONE VALUE 'E',
TWO VALUE 'x',
THREE VALUE 'a',
FOUR VALUE 'm',
FIVE VALUE 'p',
SIX VALUE 'l',
SEVEN VALUE 'e',
EIGHT VALUE '!',
END OF WORD,
LETTER1, LETTER2.
DO VARYING LETTER1 FROM WORD-ONE THEN WORD-THREE
VARYING LETTER2 FROM WORD-TWO THEN WORD-FOUR.
WRITE: LETTER1, LETTER2.
IF LETTER2 = '!'.
EXIT.
ENDIF.
ENDDO.

The resulting output is the character string

“E x a m p l e !”.
Note
When using this addition, ensure that the DO loop terminates at the “right” time, in order to avoid assigning meaningless values that happen to be in memory after this sequence of fields. This could result in a runtime error.
Variant 2
DO n TIMES.
Addition

… VARYING f FROM f1 NEXT f2 (similar to variant 1)
Effect
Repeats the processing enclosed by the DO and ENDDO statements n times. If n changes within the loop, this has no effect on loop passes.
Example

DATA COUNT TYPE I.
DO 10 TIMES.
ADD SY-INDEX TO COUNT.
ENDDO.

The field COUNT now contains 55 (1+2+…+10).
Related WHILE , LOOP
Note
Performance
The runtime required to pass once through an empty DO loop is about 11 msn (standardized microseconds). For 100 loop passes, about 230 msn would be needed.
If possible, use a WHILE loop instead of a DO / EXIT construction because this improves the performance slightly and is clearer.

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