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From: l107353@cliffy.lfwc.lockheed.com (Garlington KE)
Subject: Fixed-point question
Date: 1995/04/01
Message-ID: <3li7ec$k37@butch.lmsc.lockheed.com>#1/1
X-Deja-AN: 100649478
organization: Lockheed Missiles and Space Co.
newsgroups: comp.lang.ada
Date: 1995-04-01T00:00:00+00:00
List-Id: <comp.lang.ada>

There's probably a simple answer to this question, and I'm just too tired
to see it, but here goes anyway:

We have a user who has defined a fixed-point of the general form

   type A_Type is delta 0.1 range ...;

and some objects like the following:

   A: constant A_Type := 2.2;
   B: constant A_Type := 2.3;

and an abject C of A_Type which gets set to the expression 9.0 * 0.25.

He then runs the following code:

  if C = A then
     Text_IO.Put_Line("C equals A");
  elsif C = B then 
     Text_IO.Put_Line("C equals B"); 
  else
     Text_IO.Put_Line("C equals neither!");
  end if;

and of course it says that C equals neither, since C is stored as the
model number 2.25, which is neither 2.1875 nor 2.3125. I explain this
to the user, and all is well until he asks: "How should I do this
comparison, then?" Thinking back to my old FORTRAN days with floating-point,
I say, "Well, you compute the absolute difference between the two
numbers, and then see if the difference is less than some epsilon."
However, I can't for the life of me decide how to write the
equality function such that (1) C will be "equal" to B and (2) C will not
be "equal" to A, nor what epsilon is right (A_Type'Small?)

If someone could post the "roughly equal" function I'm describing, I would
appreciate it. (Note: this would need to be a pre-Ada 95 solution).

--------------------------------------------------------------------
Ken Garlington                  GarlingtonKE@lfwc.lockheed.com
F-22 Computer Resources         Lockheed Fort Worth Co.

If LFWC or the F-22 program has any opinions, they aren't telling me.