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Path: utzoo!watmath!watnot!watdaisy!mpdevine
From: mpdevine@watdaisy.UUCP (Michel P. Devine)
Newsgroups: net.lang
Subject: Cute little liar program (for hackers only)
Message-ID: <7772@watdaisy.UUCP>
Date: Thu, 12-Jun-86 21:16:54 EDT
Article-I.D.: watdaisy.7772
Posted: Thu Jun 12 21:16:54 1986
Date-Received: Sun, 15-Jun-86 03:49:59 EDT
Distribution: net
Organization: U of Waterloo, Ontario
Lines: 84


  During my CS340 lectures (a second course in Data Structures), I alluded to 
some of the unusual features of some programming languages, in particular Pascal
and Fortran.  On the spur of the moment (as an (spectacularly unsuccessful) 
attempt at stimulating class participation), I suggested an exercise: write a 
program which prints the identity of the compiler used to compile it.  In the 
following script session, both liar.f and liar.p are links to the data file liar
(which is the program in question). 

Script started on Mon Jun  9 11:52:23 1986
%
% ls -l liar*
-rw-r--r--  1 mpdevine           440 Jun  9 11:50 liar
lrwxr-xr-x  1 mpdevine             4 Jun  9 11:45 liar.f@ -> liar
lrwxr-xr-x  1 mpdevine             4 Jun  9 11:45 liar.p@ -> liar
%
% pc liar.p
% a.out
  Compiled by a Pascal  compiler
%
% f77 liar.f
liar.f:
   MAIN liar:
% a.out
  Compiled by a FORTRAN compiler
%

If you like challenges, you can mark this article for later reference,
otherwise skip quickly to the supplied solution (hold on to your hat!).

% cat liar
      program liar                                                          (*
c *)               (output);					            (*
c *)  begin
                                                                   writeln  (*
     1  ( *,* )    'Compiled by a FORTRAN compiler'
c *)            ('  Compiled by a Pascal  compiler');
      end                                                               .   (*
c				       			    column 72  ^    *) 
% 
script done on Mon Jun  9 11:53:42 1986


Nice, no?  The main tricks to notice are:

	a) FORTRAN ignores all characters beyond the 72nd column (these were 
	   originally used for sequence numbers).  
	
	b) '(*' and '*)' are Pascal's comment delimiters.  In FORTRAN, the 
	   character 'c' in the first column comments out the whole line,
	   while a '1' in column 6 indicates a continuation of the previous
	   statement.

	c) 'write' is a valid FORTRAN statement, while 'writeln' is correct 
	   Pascal.

	d) Pascal requires a period after the 'end' keyword (which is found
	   at column 73!!!)

Also, notice the blanks around the logical unit and format specification:
there are necessary to avoid nested comments (for Pascal, of course).  For 
some  FORTRAN compilers, *any* character in column 1 as the effect of 
commenting out the line and therefore a simpler (ah!) solution would be:

      program liar
(output);  begin	
                                                                   writeln (*  
     1  ( *,* )    'Compiled by a FORTRAN compiler'                        *)
('  Compiled by a Pascal  compiler');
      end                                                               .  


A further challenge: write a similar program for C and Pascal.

ps.

Before any comments are made about the good government money I wasted doing
this silliness, I should point out that only 10 minutes were required to
write and debug the program.
-- 
--------------------------------------------------------------------------------
mpdevine%watdaisy@waterloo.csnet            (519) 884-7123 Michel P. Devine
mpdevine%watdaisy%waterloo.csnet@csnet-relay.arpa          CS Dept., U. Waterloo
{allegra,decvax,inhp4,utzoo}!watmath!watdaisy!mpdevine     Waterloo, Ont. N2L3G1