Auwvax.343
net.lang.apl
utzoo!decvax!harpo!uwvax!greg
Mon Apr 26 11:04:12 1982
example of self-replicating expression
Jeff Shallit sent me several back issues of APL News, a publication of
APL Press, Box 27, Swarthmore, PA 19081,  (215) 328-9771 in response to
my request for an article on self-replicating APL expressions.
This is a fascinating, insightful newsletter on APL, and I highly
recommend it.
   A self-replicating expression is a character vector which
includes no alphabetics and which when typed in returns itself.
Formally, a vector C is self-replicating if

   ^/ ( C = execute C ),
      ~ C epsilon 'ABC...XYZ_delta_underscore A...underscore delta'

yields one.
(I use underbar to indicate that one symbol is ending and another is starting.)
The shortest example they give is the following:

     22_rho_1_transpose_11_rho_'_'_'22_rho_1_transpose_11_rho_'_'_'

If you write the above expression on your blackboard using apl characters,
you will notice that it is 22 characters long, and can be viewed either as a
character vector with some manipulations prepended or as a vector of 11
characters that is followed by a repitition of itself.
				- Greg Johnson
				  UW - Madison

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