Friday, May 16, 2008

When does x/2 != x>>1 for integer x?

As I discovered, when porting the Ruby big-integer unit tests to C# to test a BigInteger class --

Answer: Sometimes but not always, when x is negative and odd.

Python and Ruby -- native and DLR -- do maintain the identity:

IronPython 1.1.1 (1.1.1) on .NET 2.0.50727.1433
Copyright (c) Microsoft Corporation. All rights reserved.
>>> i = -57
>>> a = i/2
>>> b = i>>1
>>> print a,b
-29 -29

version 2.0β2, IronRuby pre-alpha, Python 2.5.2, Ruby 1.8.6 equivalents all omitted.

Indeed the Ruby big-integer unit tests contain a couple of tests explicitly to assert this identity. But doing

gives
/2 -> -28; >>1 -> -29

following the 'C' family "round towards zero" approach.

This is a potential peril in polyglot programming within an application -- the "same" arithmetic expression can yield different results depending on the language being used locally. Just being on the CLR, for example, does not mandate your arithmetic -- the language definition does. This is not something that one would at first consider might happen.

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