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Date: 19 Nov 91 00:23:10 GMT
From: cochiti.lanl.gov!jlg@lanl.gov  (Jim Giles)
Subject: Re: Numeric problem with Ada
Message-ID: <1991Nov19.002310.23171@beta.lanl.gov>
List-Id: <comp.lang.ada>

At least on the Sun/Sparc, C is compiled so that it _always_ uses
double precision for intermediate calculations - no matter how
explicitly you ask for just (float).  

   main()
   { 
      float z;
      int mz;
      mz = 7;
      z = 2.6 * mz - 0.2;
      if ( z == 18.0 ) printf ("%2.6f\n", z);
   }

This program always prints `18.000000' even if mz is set in a separately
compiled procedure.  The reason is that the calculation in the assignment
statement to `z' is always done in double and then rounded to float when
the assignment is done.  The same continues to be true even if the
expression is altered to:
      z = (float) 2.6 * (float) mz - (float) 0.2;

So, that's why C always gives 18.000000 on Sun/Sparc.  I suspect the
same may be true of other C implementations.

J. Giles