From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 28 Jul 2007 08:54:26 -0700

Message-ID:

<f8forn$17dv$1@ihnp4.ucsd.edu>

On Jul 28, 7:38 am, Michael Jung <m...@phantasia.org> wrote:

That wont really help, because you will lose certain features of

comparing.

Explain please.

No.

Do not bluntly contradict me in a public forum.

* First, sorting a NaN in some arbitrary but consistent way, e.g. NaN

< -inf < -10 < -1 < 0 < 1 < 10 < inf, allows well-defined behavior and

makes collections of floats that may include NaNs safe to put into

e.g. a TreeSet. So I'm not sure I disagree with Float's implementation

of this behavior.

< -inf < -10 < -1 < 0 < 1 < 10 < inf, allows well-defined behavior and

makes collections of floats that may include NaNs safe to put into

e.g. a TreeSet. So I'm not sure I disagree with Float's implementation

of this behavior.

That wont really help, because you will lose certain features of

comparing.

Explain please.

- You can usually determine algebraically whether they are equal or

not.

not.

No.

Do not bluntly contradict me in a public forum.

Shrug. Different people have different preferences, so it is hard for

writers to keep track. I'm here to learn, as well as to pass on any

relevant insights I have, so if someone disagrees with me I *want* them

to say so, simply and directly. I won't be offended as long as they

explain their reasoning, so that I can either be convinced and learn, or

be able to explain why I disagree with their reasoning.

Depending on what you do, solutions to these problems are unknown.

There's an equation that can in principle be solved to see if they are

equal. In practise it will be difficult with big enough (degree 5 or

more) polynomials and various transcendental functions. In many cases,

you know one is algebraic and the other transcendental and therefore

they must be unequal.

I think you may be underestimating the difficulties of dealing with

general real numbers. In many cases we can calculate the value to any

required finite number of decimal, or binary, places. That is enough to

give the order for two unequal values, but not enough to ever be able to

declare equality.

Even if a class represents values of some transcendental function there

can be some points at which the function has an algebraic solution. For

example, consider the sines of angles that can be expressed as rational

numbers of degrees. sin(30), sin(45), and sin(60) are exact solutions to

quadratic equations.

One could write a comparator between sines and quadratic equation

solutions by special casing angles that are exact multiples of three

degrees, and then calculating the binary expansions of the other values

until there is a difference.

However, to make Number implement Comparable we would need a general

approach to comparing that does not depend on writing special code for

every pair of Number subclasses.

Here's a challenge for those who want to make Number implement

Comparable<Number>. Write the API documentation for the compareTo

method. Tell me what I have to do if I am writing a new Number subclass

in order to conform to the contract.

Patricia

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