# [R] Odp: strange issue with "which" on "seq"

Petr PIKAL petr.pikal at precheza.cz
Wed Jun 9 14:28:37 CEST 2010

```Hi

Well, then please explain what you really want to do. I use such
fractional sequences only for evaluation of some models and in that case
this finite precision issue is not important.

Regards
Petr

r-help-bounces at r-project.org napsal dne 09.06.2010 14:20:16:

> which(abs(v - .1) <= .Machine\$double.eps)
>
> seems to me too cumbersome to write. Any other easier way? all.equal
> does not quite work
>
> Nikhil
>
> On Jun 9, 2010, at 7:54 AM, Petr PIKAL wrote:
>
> > Hi
> >
> > r-help-bounces at r-project.org napsal dne 09.06.2010 13:16:40:
> >
> >> Dear R community,
> >> I am puzzled by the following lines:
> >>
> >>> v <- seq(-0.5,0.5,by=0.1)
> >>> v
> >>  [1] -0.5 -0.4 -0.3 -0.2 -0.1  0.0  0.1  0.2  0.3  0.4  0.5
> >>> which(v == -0.4)
> >>  [1] 2
> >>> which(v == 0)
> >>  [1] 6
> >>> which(v == 0.1)
> >> integer(0)
> >>> which(v == 0.2)
> >> integer(0)
> >>> which(v == 0.3)
> >> integer(0)
> >>> which(v == 0.4)
> >>  [1] 10
> >>> which(v == 0.5)
> >>  [1] 11
> >>
> >> Why "which" can only match some of the values in "v"? Are the numbers
> >> generated by "seq" not exact fractional numbers?
> >> Please, help me to understand this.
> >
> > Well FAQ 7.31 was not here some time. Computing in binary results in
> > finite precision of fractional numbers.
> >
> > v <- seq(-0.5,0.5,by=0.1)
> > v[7]-0.1
> > [1] 8.326673e-17
> >
> > Regards
> > Petr
> >
> >
> >
> >>
> >> J
> >>
> >> Dr James Foadi PhD
> >> Membrane Protein Laboratory (MPL)
> >> Diamond Light Source Ltd
> >> Diamond House
> >> Harewell Science and Innovation Campus
> >> Chilton, Didcot
> >> Oxfordshire OX11 0DE
> >>
> >> Email    :  james.foadi at diamond.ac.uk
> >> Alt Email:  j.foadi at imperial.ac.uk
> >>
> >> --
> >> This e-mail and any attachments may contain confidential...
> >> {{dropped:8}}
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

```

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