File APIs for Java Developers
Manipulate DOC, XLS, PPT, PDF and many others from your application.
http://aspose.com/file-tools
The moose likes Beginning Java and the fly likes The workings of  floats Big Moose Saloon
  Search | Java FAQ | Recent Topics | Flagged Topics | Hot Topics | Zero Replies
Register / Login


Win a copy of EJB 3 in Action this week in the EJB and other Java EE Technologies forum!
JavaRanch » Java Forums » Java » Beginning Java
Bookmark "The workings of  floats" Watch "The workings of  floats" New topic
Author

The workings of floats

Fitz Herbert
Greenhorn

Joined: Mar 22, 2003
Posts: 21
SCJ2 question

This prints true or false?
I would say false as:
4.0/3.0 is a reoccuring number 1.333333333.........
the varialbe f of float type holds a specific number of decimal places,
and so when f is then multiplyed by 3.0F the value will be 3.9999999999999.......... reoccuring and not 4.0F
But when compiled prints true???
Thomas Paul
mister krabs
Ranch Hand

Joined: May 05, 2000
Posts: 13974
A computer can not store a recurring number so f actually contains 1.3333334. Multiply that by 3 and you get 4.0000002 which rounds to 4.0 because 4.0000002 is beyond the precision of a float.


Associate Instructor - Hofstra University
Amazon Top 750 reviewer - Blog - Unresolved References - Book Review Blog
Orin Malcolm
Greenhorn

Joined: May 20, 2003
Posts: 5
Well said thomas


Knowledge is a disease..<br />Be sure to get infected.<p>Soon to be SCJP
Layne Lund
Ranch Hand

Joined: Dec 06, 2001
Posts: 3061
I don't know whether the exact details given by either answer are the way it actually happens. In fact, I'd bet it depends on the environment you run this in. Two different machines may give two different answers.
The moral of the story is that you should NEVER compare floating point numbers with the == operator. As the above discussions illustrate, round off error makes such a comparison unreliable.


Java API Documentation
The Java Tutorial
Thomas Paul
mister krabs
Ranch Hand

Joined: May 05, 2000
Posts: 13974
Originally posted by Layne Lund:
I don't know whether the exact details given by either answer are the way it actually happens. In fact, I'd bet it depends on the environment you run this in. Two different machines may give two different answers.

They should't be different. Java religiously follows the IEE 754 floating point standard for the behaviour of floats and doubles.
This is a pretty good write-up on the standard even if it is by Microsoft:
http://research.microsoft.com/~hollasch/cgindex/coding/ieeefloat.html
Jim Yingst
Wanderer
Sheriff

Joined: Jan 30, 2000
Posts: 18671
It turns out that yes, 4.0F/3.0F should be exactly the same in any Java implementation - but there are other cases where similar expressions are not so bound. It depends on whether a given expression is considered FP-strict or not. (Any compile-time constant expression is FP-strict). From the JLS:
Within an FP-strict expression, all intermediate values must be elements of the float value set or the double value set, implying that the results of all FP-strict expressions must be those predicted by IEEE 754 arithmetic on operands represented using single and double formats. Within an expression that is not FP-strict, some leeway is granted for an implementation to use an extended exponent range to represent intermediate results; the net effect, roughly speaking, is that a calculation might produce "the correct answer" in situations where exclusive use of the float value set or double value set might result in overflow or underflow.

I'm not sure what would be a good example of an expression that can evaluate differently in a non-FP-strict context. But it seems there's at least a possibility of different results...


"I'm not back." - Bill Harding, Twister
 
I agree. Here's the link: http://aspose.com/file-tools
 
subject: The workings of floats
 
Similar Threads
Question from JQ+
Error with JQPlus Question
Floating Confusion
Doubt
Precision of float and double