[R] seq

[Prof Brian Ripley]

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