[R] which() function not finding certain numbers

[Petr Savicky]

On Wed, Aug 15, 2012 at 10:37:17PM +1000, Tom Bird wrote:
> Hi,
> 
> I am using 32-bit R v 2.15.1, on Mac OsX v 10.6.8 with R GUI 1.52 Leopard
> build 32-bit (6188).  I have also replicated this error on R 2.13.2 on a
> windows 7 machine.
> 
> In some sequences of numbers,  I am unable to use the which() function to
> index certain values.    It seems to be primarily numbers with a 3 in the
> decimal.
> For example:
> 
> > S=seq(0,4,0.01)
> 
> > which(S==2.02)
> 
> [1] 203
> 
> > which(S==2.03)
> 
> integer(0)
> 
> > which(S==2.04)
> 
> [1] 205
> 
> > which(S==2.04)
> 
> [1] 205
> 
> > which(S==3.03)
> 
> integer(0)
> 
> > which(S==3.3)
> 
> integer(0)
> 
> > which(S==3.)
> 
> [1] 301
> 
> > which(S==3.5)
> 
> [1] 351

Hi.

This is a rounding problem.

  print(S[204], digits=20)
  [1] 2.0300000000000002487

  print(2.03, digits=20)
  [1] 2.0299999999999998046

Use function round(, digits), for example

  S <- round(seq(0,4,0.01), digits=7)

  which(S==2.03)

  [1] 204

  which(S==3.03)

  [1] 304

Hope this helps.

Petr Savicky.