[plt-scheme] ProfessorJ, ==, and double

From: Don Blaheta (dblaheta at knox.edu)
Date: Tue Jan 25 14:02:34 EST 2005

Quoth Carl Eastlund:
> The == operator doesn't operate on type double in ProfessorJ: Beginner
> in DrScheme v209.  It works fine in ProfessorJ: Advanced, but we've
> needed it in our earliest examples in the HtDCH class.  We can work
> around it with new Double(x).equals(new Double(y)), but yuck.  Is
> there any reason for this being left out of the Beginner level?

Because floating point numbers shouldn't be tested for equality, due to
numeric error.  Consider:

  > double x = (1.0 / 6) * 10000;
  > double y = 10000.0 / 6;
  > x
  > y

Now, there are situations once you know what you're doing, when you know
that numeric error can't arise, that you can make use of ==, but Java
beginners really shouldn't use those.  The correct way to test equality
on doubles is

  > double epsilon = 0.0001;
  > Math.abs (x - y) < epsilon

It's really the same reason that you can't use == on objects in Beginner
ProfJ: it doesn't mean what they might intuitively expect it to mean, so
you don't want them using it until they understand its problems.

What "earliest examples" in HtDCH are you referring to?

-=-Don Blaheta-=-dblaheta at knox.edu-=-=-<http://faculty.knox.edu/dblaheta/>-=-
"I can remember when a good politician had to be 75 percent ability and
25 percent actor, but I can well see the day when the reverse could be
true."						--Harry Truman

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