[R] round() seems inconsistent when rounding 5s

[Rashid Nassar]

Many thanks to all for the very quick replies.  I should have stated the
apparent problem more clearly: round() does not seem to always round 5 to
the nearest even value as I expected, as my examples (repeated below)
showed:

> round(2.45, 1)
[1] 2.5               # shouldn't this be 2.4?

> round(1.05, 1)
[1] 1.1               #  1.0 ?

> signif(3.445, 3)
[1] 3.45             # 3.44 ?

Apologies for not stating this more clearly in my first message, and again
thanks for the replies.

Rashid Nassar



On Sat, 15 Mar 2003, Warnes, Gregory R wrote:

> Actually, under the default IEEE rounding mode, decimal values ending in 5
> are always rounded to the nearest even value.   This intended to ensure that
> rounding does not systematically bias computations in one direction, as
> would happen if 5's were always rounded either up or down.
>
> A very useful review of the issues in floating point computations on
> computer and under IEEE standard arithmetic is the paper "What Every
> Computer Scientist Should Know About Floating-Point Arithmetic", by David
> Goldberg, published in the March, 1991 issue of Computing Surveys.  This
> paper is reprinted on the Sun website at
> http://docs.sun.com/source/806-3568/ncg_goldberg.html.
>
> -Greg
>
> > -----Original Message-----
> > From: Marc R. Feldesman [mailto:feldesmanm at pdx.edu]
> > Sent: Saturday, March 15, 2003 5:49 PM
> > To: Rashid Nassar; r-help
> > Subject: Re: [R] round() seems inconsistent when rounding 5s
> >
> >
> > At 01:53 PM 3/15/2003, Rashid Nassar wrote:
> >  >It may be my lack of unerstanding, but round() seems to me to give
> >  >inconsistent results when rounding 5s as in the following examples?
> >
> > Not really.  Floating point numbers can't be precisely represented in
> > binary and so the internal representation of 1.5 might be
> > very different
> > from what you think it is.  If you've done any programming at
> > all, this is
> > one of the first lessons you learn about "real" numbers and computers.
> >
> >
> > Dr. Marc R. Feldesman
> > Professor and Chairman Emeritus
> > Anthropology Department - Portland State University
> > email:  feldesmanm at pdx.edu
> > email:  feldesman at attglobal.net
> > fax:    503-725-3905
> >
> >
> > "Sometimes the lights are all shining on me, other times I
> > can barely see,
> > lately it's occurred to me, what a long strange trip it's
> > been..."  Jerry &
> > the boys
> >
> > ______________________________________________
> > R-help at stat.math.ethz.ch mailing list
> > https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> >
>
>
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