Example of DO with WHILE, UNTIL

The WHILE and UNTIL options make successive executions of the do-group dependent upon a specified condition.

Consider the following example:

  do while (A=B);
  .
  .
  .
  end;

This example is equivalent to the following statement:

  S:  if A=B then;
      else goto R;
           .
           .
           .
          goto S;
  R:  next statement

Consider the following example:

  do until (A=B);
  .
  .
  .
  end;

This example is equivalent to the following statement:

  S:
    .
    .
    .
    if (A=B) then goto R;
       goto S;
  R:  next statement

In the absence of other options, a do-group headed by a DO UNTIL statement is executed at least once, but a do-group headed by a DO WHILE statement might not be executed at all. That is, the statements DO WHILE (A=B) and DO UNTIL (A¬=B) are not equivalent.

In the following example, if A¬=B, when the DO statement is first encountered, the do-group is not executed at all.

  do while(A=B) until(X=10);

However, if A=B, the do-group is executed. If X=10 after an execution of the do-group, no further executions are performed. Otherwise, a further execution is performed provided that A is still equal to B.

In the following example, the do-group is executed at least once, with I equal to 1:

  do I=1 to 10 until(Y=1);

If Y=1 after an execution of the do-group, no further executions are performed. Otherwise, the default increment (BY 1) is added to I, and the new value of I is compared with 10. If I is greater than 10, no further executions are performed. Otherwise, a new execution commences.

The following statement specifies that the do-group executes ten times while C(I) is less than zero, and then (provided that A is equal to B) once more:

  do I = 1 to 10 while (C(I)<0),
         11 while (A = B);