DO.HELPREXX

DO instruction

+-----------------------------------------------------------------------------+
| DO [repetitor] [conditional];                                               |
| [instruction-list]                                                          |
| END [symbol];                                                               |
|                                                                             |
| repetitor is { exprr | FOREVER |                                            |
|                name = expri [TO exprt] [BY exprb] [FOR exprf]               |
|                                                                             |
| conditional is { WHILE exprw | UNTIL expru }                                |
+-----------------------------------------------------------------------------+

The DO instruction groups instructions together. Depending upon the optional
parts specified, the group may be repeated. A DO instruction may include both a
repetitor and a conditional.

Repetitors
    If the FOREVER keyword is specified, the instructions are repeated until a
    LEAVE or SIGNAL instruction terminates the repetitions.

    If the exprr expression is specified, which must evaluate to a non-negative
    whole number, the group is repeated that many times.

    If "name = expri" is specified, the specified variable is initialized to
    the value of the expri expression before the first repetition. When the
    variable exceeds the expression value, the repetitions stop.

    If the BY keyword is specified, the variable specified by name is
    incremented by the value of the exprb expression at the start of each
    repetition, before the variable is compated to exprt. If the BY keyword is
    omitted, it defaults to incrementing the variable specified by name by 1
    at the start of each repetition.

    If the FOR keyword is specified, the group is repeated exprf times. exprf
    must evaluate to a non-negative whole number.

    If the TO keyword is specified, the variable specified by name is compared
    to the value of the exprt expression at the start of each repetition. If
    exprt is negative, and the variable is less than exprt, the repetitions
    are terminated. If exprt is non-negative, and the variable exceeds exprt,
    the repetitions are terminated.

    All the repetitor expressions are evaluated just once, before the first
    instruction in the instruction-list, must evaluate to a number, and their
    values are rounded according to the current NUMERIC DIGITS settings.

Conditionals
    If the UNTIL keyword is specified, the expru expression,
    which must evaluate to either 0 or 1, is evaluated at the end of each
    repetition, and the repetitions are terminated if it is 1. If the WHILE
    keyword is specified, the exprw expression, which must evaluate to either
    0 or 1, is evaluated at the start of each repetition, and the repetitions
    are terminated if it is 1.

Instruction list
    Any instruction is permitted in the instruction-list.

    If a symbol is specified on the END instruction that closes the group, it
    must name the name specified in the repetitor. This can be helpful when
    closing nested DO loops, to ensure each is properly closed.

NOTES:
    The keywords UNTIL and WHILE are reserved in a DO instruction, and may not
    be used as symbols in any of the expressions.

    The keywords BY, FOR, and TO are reserved in a DO repetitor, and may not
    be used as symbols in any of the repetitor's expressions.

    The BY, FOR, and TO phrases may be specified in any order within the
    repetitor, and are evaluated in the order they are written.

Examples:

    /* performs two instructions where only one would otherwise be allowed */
    if a=3 then do
      a=a+2
      say 'Smile!'
      end

    /* displays 'Go caving!' at least once, and possibly more */
    do forever
      say 'Go caving!'
      if random(5)=1 then leave
      end

    /* displays 'Hello' five times */
    do 5
      say 'Hello'
      end

    /* displayss 3 , 2, 1, 0, -1, -2 */
    do i = 3 to -2 by -1
      say i
      end

    /* displays 0.3, 1.0, 1.7, 2.4, 3.1, 3.8 */
    x=0.3
    do y=x to x+4 by 0.7
      say y
      end

    /* displays 0.3, 1.0, 1.7 */
    do y=0.3 to 4.3 by 0.7 for 3
      say y
      end

    /* displays 1, 3, 5, 7 */
    do i=l to 10 by 2 until i>6
      say i
      end