From ripley@stats.ox.ac.uk  Thu Sep 13 09:53:56 2001
From: ripley@stats.ox.ac.uk (Prof Brian Ripley)
Date: Thu, 13 Sep 2001 09:53:56 +0100 (BST)
Subject: [Rd] sum overflow (PR#1091)
In-Reply-To: <200109130843.KAA28348@pubhealth.ku.dk>
Message-ID: <Pine.LNX.4.31.0109130948310.14470-100000@gannet.stats>

It's not a problem with sum:

> sum(a*a)
[1] 333833500
> sum(b*b)
[1] 333833500

are accurate.

The overflow is in the integer arithmetic for *.  That's a question for
your C run-time system.  On a 64-bit machine you might get different
results (although most use 32-bit ints, including mine).

If you use integers you need to be aware of the consequences.  It's a
feature not a bug.


On Thu, 13 Sep 2001 wsi@gcal.ac.uk wrote:

> Full_Name: Bill Simpson
> Version: 1.2.3
> OS: linux
> Submission from: (NULL) (193.62.250.209)
>
>
> I think I have found an accuracy problem with sum()
>
> > a<-1:1000
> > b<-1:1000
> >
> > crossprod(a,a)*crossprod(b,b)
>              [,1]
> [1,] 1.114448e+17
> > drop(.Last.value)
> [1] 1.114448e+17
> >
> #this should give same answer as above
> > sum(a*a)*sum(b*b)
> [1] -652010736
> #seems to be due to sum using integer representation which overflows instead of
> using
> #double representation
>
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-- 
Brian D. Ripley,                  ripley@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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[Rd] sum overflow (PR#1091)
Prof Brian Ripley