[#CLJ-1006] Quotient on bigdec may produce wrong result

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[CLJ-1006] Quotient on bigdec may produce wrong result Created: 01/Jun/12 Updated: 01/Mar/13 Resolved: 09/Nov/12
Status:	Closed
Project:	Clojure
Component/s:	None
Affects Version/s:	Release 1.4
Fix Version/s:	None

Type:

Defect

Priority:

Major

Reporter:

laurent joigny

Assignee:

Unassigned

Resolution:

Declined

Votes:

Labels:

bug

Environment:

Linux 3.2.0-24-generic #39-Ubuntu SMP i686 GNU/Linux

Attachments:

TestBigDecimalQuotient.java

Description

Hi,

As discussed on the mailing list in the message "When arithmetic on a computer bite back" (01/jun)

There may be bug in the way quotient is implemented for bigdec.

user> (quot 1.4411518807585587E17 2) ;; correct with doubles
7.2057594037927936E16
user> (quot 1.4411518807585587E+17M 2) ;; wrong with BigDecs
72057594037927935M

–
Laurent

Comments

Comment by laurent joigny [ 01/Jun/12 5:48 PM ]

I can reproduce the bug when using BigDecimal constructor on String.
See attached file for a test class.

More infos :
java version "1.6.0_24"
OpenJDK Runtime Environment (IcedTea6 1.11.1) (6b24-1.11.1-4ubuntu3)
OpenJDK Client VM (build 20.0-b12, mixed mode, sharing)

Comment by laurent joigny [ 01/Jun/12 5:49 PM ]

A simple test file, that you can drop in Clojure sources and execute to reproduce the bug on BigDecimal constructor using String as argument.

Comment by Tassilo Horn [ 03/Jun/12 4:30 AM ]

Seems to be a general precision problem. Note that in

user> (quot 1.4411518807585587E17 2) ;; correct with doubles
7.2057594037927936E16
user> (quot 1.4411518807585587E+17M 2) ;; wrong with BigDecs
72057594037927935M

the double result is actually wrong and the bigdec one is correct. The problem which lead to the wrong conclusion is that in your calculation the input number is already wrong.

So the moral is: don't use any floating points (neither doubles nor bigdecs) for computations involving divisibility tests.

For bigdecs, you can set the math context for making computations throw exceptions if they lose precision, though:

user> (binding [*math-context* (java.math.MathContext. 1 java.math.RoundingMode/UNNECESSARY)]
	       (quot (bigdec (Math/pow 2 58)) 2))
;Division impossible
;  [Thrown class java.lang.ArithmeticException]

Comment by Stuart Sierra [ 09/Nov/12 8:49 AM ]

Not a bug. Just floating-point arithmetic.

Generated at Sun May 19 12:01:15 CDT 2013 using JIRA 4.4#649-r158309.

[CLJ-1006] Quotient on bigdec may produce wrong result Created: 01/Jun/12 Updated: 01/Mar/13 Resolved: 09/Nov/12