Gri: `while' command

Chapters: 1: Introduction 2: Simple example 3: Invocation 4: Finer Control 5: X-Y Plots 6: Contour Plots 7: Image Plots 8: Examples 9: Gri Commands 10: Programming 11: Environment 12: Emacs Mode 13: History 14: Installation 15: Gri Bugs 16: Test Suite 17: Gri in Press 18: Acknowledgments 19: License Indices: Concepts Commands Variables | ### 9.3.52: ``while`'
    `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
- Read number pairs from a file, plotting bullets at the indicated locations. Note the use of an infinite loop, with a break condition following an end-of-file test. (Do not be tempted to write such loops as ``while !..eof..`' because that would not catch the end of file until the next time through the loop. The result would be to draw the last bullet twice, since the ``read`' will not update the variables when the end of file is encountered.)

      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