12.49: The `while' Command

`while .test.|{rpn ...}'

Perform statements in loop while the value of `.test.' or the RPN expression is nonzero. The end of the loop designated by a line containing the words `end while'. The value `.test.' may be an rpn expression. To leave the loop prematurely, use a `break' statement. Upon encountering a `break' statement, Gri jumps to the line immediately following the loop. If the `-chatty' option is nonzero, a notification is printed every 1000 passes through the loop, as a debugging measure to catch unintended infinite loops.

Examples:

• Loop forever, printing a message over and over.

  while 1 show "This loops forever. Need to 'break'" end while

• Repeatedly read two numbers, and plot a bullet at the indicated location. If (or, hopefully, ``when'') the end of the file is encountered, break out of the loop; otherwise continue plotting forever.

  while 1 read .x. .y. if ..eof.. break end if draw symbol bullet at .x. .y. end while

• Loop 10 times, printing the values of `.i.' as they range 0, 1, ..., 9. After exiting from the loop, `.i.' will equal 10. Be careful to use the correct rpn greater-than test to avoid an infinite loop.

  .i. = 0 while {rpn .i. 10 >} show .i. .i. += 1 end while