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From: dhinds@portia.Stanford.EDU (David Hinds)
Newsgroups: comp.lang.pascal
Subject: Re: Absolute troubles
Message-ID: <8470@portia.Stanford.EDU>
Date: 22 Jan 90 21:51:12 GMT
References: <7559@chaph.usc.edu>
Sender: David Hinds <dhinds@Portia.stanford.edu>
Organization: Stanford University
Lines: 44

In article <7559@chaph.usc.edu>, ajayshah@aludra.usc.edu (Ajay Shah) writes:
> type
> 	VectorType = array[1..10] of real;
> 
> procedure xxx(var theta:VectorType);
> var
> 	sigma : real absolute theta;
> 	beta : VectorType absolute theta[2];

    The 'sigma' declaration apparantly works, but the 'beta' declaration
fails.  Actually, I'm surprised either of them works.  You are asking
the compiler to give a variable the same address as a parameter passed
by reference, which inherantly means that the address is not defined at
compile time.  The compiler might not catch it as an error, but if I
were you I would try running some kind of test program with just a
'sigma'-like declaration, to see if it ACTUALLY works.  You definitely
can't get away with the theta[2] declaration.
    If a 'sigma'-like declaration really can be handled properly by the
compiler, you could instead declare a record with two fields, called
'sigma' and 'beta', of the appropriate types, 'absolute theta'.  The
subfields of the record would then correctly refer to the parts of the
passed vector. i.e.,

   procedure xxx(var theta: VectorType);
   type 
     thetatype = record
       sigma: real;
       beta: VectorType;
     end;
   var
     y: thetatype absolute theta;

and use 'y.sigma' and 'y.beta' as the parts of 'theta'.  I have a bad
feeling about the use of 'absolute' with parameters, though.  At worst,
you could declare:

     y: ^thetatype;
  begin
     y := @theta;

and refer to 'y^.sigma' and 'y^.beta' as your vector parts.

 - David Hinds
   dhinds@portia.stanford.edu