Issue #5321 has been updated by Charles Nutter.


Why is BigDecimal considered inexact? Unlike a float, a value in a BigDrcimal is always represented exactly. The value itself may be and estimate for various reasons (irrational, repeating, ...), but that is a property of the value, not a property of BigDecimal. Or put another way, BigDecimal is not inexact since if it is possible to exactly represent the number in base 10, BigDecimal can represent it exactly.

Perhaps the idea of inexact is too fuzzy here...even Float can be exact if the value can be exactly represented in base 2 decimal form with Float's precision.

It seems to me that the interesting characteristic of these classes is not whether the value they represent is exact, but whether arithmetic operations using them can be done exactly. By that definition, only Float is clearly inexact; even if the two operands of a floating-point arithmetic operation are exact, the result may not be. The other types do not have this property.
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Feature #5321: Introducing Numeric#exact? and Numeric#inexact?
http://redmine.ruby-lang.org/issues/5321

Author: Kenta Murata
Status: Open
Priority: Normal
Assignee: Yukihiro Matsumoto
Category: 
Target version: 


Ruby has several numeric classes such as Integer.
These classes are classified whether their instances represent numbers exactly.
According to this, Integer and Rational are exact, and Float and BigDecimal are inexact because they're always including error.
The exactness of a Complex depends on its real and imaginary parts.

Now, Ruby's numeric classes doesn't provide predicators to investigate the exactness of their instances.
So, if we want to examine whether a number is exactly zero, we must investigate the class of the number.
I want simple way to examine the number exactness.

I propose to introduce Numeric#exact? and/or Numeric#inexact? for resolving this inconvenience.



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