循环和迭代

COBOL 中的循环和迭代构造不是由 for 或 while 等关键字定义的，而是使用相同 PERFORM 语句的语法变体。

固定数字循环（执行次数）

如果我们需要重复执行一个段落固定次数，我们可以在 PERFORM 命令中添加 TIMES 修饰符。

COBOL

PERFORM PROCESS-LINE 10 TIMES.

在这种情况下，编译器将恰好执行 PROCESS-LINE 段落 10 次，然后恢复代码的线性执行。

条件循环（PERFORM UNTIL）

为了迭代直到某个条件成立（相当于反向 while 循环），我们使用 PERFORM ... UNTIL 语句。

COBOL

PERFORM INCR-COUNTER UNTIL WS-COUNTER > 5.

COBOL

PROCEDURE DIVISION.
MAIN-PROCEDURE.
    PERFORM PROCESS-ITEM UNTIL WS-COUNTER > 3.
    STOP RUN.

PROCESS-ITEM.
    DISPLAY "Counter: " WS-COUNTER.
    ADD 1 TO WS-COUNTER.

---

## Try it yourself

<CobolExercise
  id="cobol.m3.l2.e1"
  prompt="Write a statement inside MAIN-PROCEDURE to execute the PROCESS-ITEM paragraph exactly 10 times."
  starter={`       PROCEDURE DIVISION.\n       MAIN-PROCEDURE.\n           * WRITE CODE HERE\n           STOP RUN.\n`}
  solution={`       PROCEDURE DIVISION.\n       MAIN-PROCEDURE.\n           PERFORM PROCESS-ITEM 10 TIMES.\n           STOP RUN.\n`}
  rules={{
    requiredKeywords: ['PERFORM PROCESS-ITEM', '10', 'TIMES.', 'STOP RUN.'],
  }}
  hint="Use the syntax: PERFORM PROCESS-ITEM 10 TIMES."
/>

<CobolExercise
  id="cobol.m3.l2.e2"
  prompt="Repeatedly execute the PROCESS-STEP paragraph until the WS-COUNTER variable becomes greater than 5."
  starter={`       PROCEDURE DIVISION.\n       MAIN-PROCEDURE.\n           * WRITE CODE HERE\n           STOP RUN.\n`}
  solution={`       PROCEDURE DIVISION.\n       MAIN-PROCEDURE.\n           PERFORM PROCESS-STEP UNTIL WS-COUNTER > 5.\n           STOP RUN.\n`}
  rules={{
    requiredKeywords: ['PERFORM PROCESS-STEP', 'UNTIL', 'WS-COUNTER', '>', '5', 'STOP RUN.'],
  }}
  hint="Use the syntax: PERFORM PROCESS-STEP UNTIL WS-COUNTER > 5."
/>