@@CloneMe    SG
From: tedb@hpcvra.cv.hp.com. (Ted Beers)
Newsgroups: comp.sys.hp48
Subject: HP 48 Program Procreation
Date: 23 Aug 91 22:06:57 GMT
Organization: Hewlett-Packard Co., Corvallis, OR, USA


    I was intrigued by a challenge made by Ken Thompson
    (coauthor of Unix) many years ago and mentioned again
    in the September, 1991, issue of Dr. Dobb's Journal.
    To paraphrase, as a test of a computer language's
    power, Thompson suggested that one write a program
    that, when compiled and executed, will produce an
    exact copy of itself. Taking up the gauntlet, here's
    what I came up with for the HP 48:

      Warning!  Examining this program too closely can
      prove hazardous to your sanity.

@ CloneMe:  A program that creates itself.
@ Reference: Dr. Dobb's Journal #180, September, 1991,
@ pg. 18.
@ Problem first proposed by Ken Thompson.
@
\<<
  C$ 31 "\<< C$ 31 " OVER + SWAP + STR\-> \>>
  "\<< C$ 31 " OVER + SWAP + STR\->
\>>

    (Note: C$ <n> is a little-known built-in object
    delimiter for an <n>-character string.)

    When this program is executed, it will output itself!

    At first this exercise might seem either trivial or
    impossible, but it's neither. The trick at work here
    is to make the first half of the program create a
    string that represents the second half of the
    program, and to make the second half of the program
    put together the first half of the program with the
    second half and change the result into a program
    (whew!).

    Variations are possible. Here's a program that
    creates TWO copies of itself, changing the motive
    from procreation to proliferation:

\<<
  C$ 35 "\<< C$ 35 " OVER + SWAP + STR\-> DUP \>>
  "\<< C$ 35 " OVER + SWAP + STR\-> DUP
\>>

    And then of course there's the suicidal variation
    who's sole purpose is to disappear:

\<< \>>

    Can anyone create a shorter version of the program
    that creates itself?  (Removing unnecessary but
    friendly formatting doesn't count!)

                                Ted W. Beers
                                Hewlett-Packard