[R] seq
Prof Brian Ripley
ripley at stats.ox.ac.uk
Fri Sep 3 14:17:01 CEST 2004
On Fri, 3 Sep 2004, Roger Bivand wrote:
> On Fri, 3 Sep 2004, Henric Nilsson wrote:
>
> > Hi everyone,
> >
> > I've tried the below on R 1.9.1 and the 2004-08-30 builds of R 1.9.1
> > Patched and R 2.0.0 on Windows 2000, and the results are consistent.
> >
> > > seq(0.5, 0, by = -0.1)
> > [1] 0.5 0.4 0.3 0.2 0.1 0.0
> >
> > > seq(0.7, 0, by = -0.1)
> > [1] 7.000000e-01 6.000000e-01 5.000000e-01 4.000000e-01 3.000000e-01
> > 2.000000e-01 1.000000e-01 -1.110223e-16
> >
> > Is this really the intended behaviour? I ended up using
>
> Well, you are using a floating point representation in a digital computer,
> so I don't think you should be surprised. Note that the internal
> representation is also modified by the print() functions, so what you see
> when an object is printed is not always exactly what is inside the object.
>
> >
> > > seq(0.7, 0, length = 8)
> > [1] 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
> >
> > which does what I want.
To expand a little, the code is in seq.default.
seq(0.7, 0, by = -0.1) is done by from + (0:n) * by for n=8
seq(0.7, 0, length = 8) is done by
c(from, from + (1:(length.out - 2)) * by, to))
so the last value is handled differently. Since 0.1 cannot be represented
exactly on a binary computer,
> print(7*0.1, digits=16)
[1] 0.7000000000000001
and you are seeing the result of 0.7 - 7*0.1.
--
Brian D. Ripley, ripley at 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 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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