DO and WHILE — counting loops

Level: 101 · newcomer

Status: stub — the shape is here; the worked program is not yet.

One line: DO n TIMES repeats a fixed number of times, WHILE cond repeats as long as a condition holds, both set sy-index to the current pass — and sy-index belongs to the innermost loop, which is what nested loops trip over.

What it does

DO 5 TIMES.
  WRITE: / sy-index.
ENDDO.

WHILE lv_remaining > 0.
  lv_remaining = lv_remaining - 1.
ENDWHILE.

DO without TIMES loops forever until an EXIT — a legitimate shape for "read until there is nothing left", and the reason an accidental missing EXIT hangs a work process.

sy-index counts passes of the current loop, starting at 1. In a nested DO, the inner loop overwrites it, so a value needed after the inner loop must be saved before it. Note also that LOOP AT sets sy-tabix, not sy-index — two fields, two loops, and mixing them up is a classic.

What this page still needs

• [ ] a snippets/ program with nested loops showing sy-index being overwritten
• [ ] DO … VARYING, and why it is on the obsolete list
• [ ] the modern replacements: FOR … UNTIL in an expression, and when a loop is still clearer

See also

• LOOP AT — the loop over a table, and sy-tabix
• FOR — counting inside an expression instead
• CHECK, CONTINUE, EXIT, RETURN — getting out of the loop
• System fields — sy-subrc, sy-tabix, sy-index and the rest