[R] strange (?) behavoir of expand.grid()

[David Winsemius]

http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

--  
David Winsemius

On Apr 7, 2009, at 10:35 AM, stefan.duke at gmail.com wrote:

> Hello,
>
> I came across a strange behavior of expand.grid (or at least strange  
> to me).
>
> For certain values of one of my input variables - created by seq() - I
> have to use strings (e.g ==".6") to select a row of the object
> created by expand.grid(), for other values numerical (e.g. ==.8) and
> for some both work. Please find an example below.
>
>
> #Example
> x<- seq(0,1,1/10)
> y  <- seq(0,1,1/10)
>
> gridd <- expand.grid(x,y )
>
>
> gridd[which(  gridd[,2]=="0.6" ),] #gives me the right data
> gridd[which(  gridd[,2]==0.6 ) ,]  #gives error message
>
> gridd[which(  gridd[,2]=="0.4" ),] #gives right data
> gridd[which(  gridd[,2]==0.4 ),]  #gives right data
>
> gridd[which(  gridd[,2]=="0.0" ),] #gives error message
> gridd[which(  gridd[,2]==0.0 ),]  #gives me the right data
> #End of Example
>
>
> I also encountered this phenomenon on another version of R and on
> another computer.
>
> Has anybody an idea what this is and how to overcome it?
>
> Best,
> Stefan
>
>
>
> Sys.getlocale()
> [1] "LC_COLLATE=English_United Kingdom.1252;LC_CTYPE=English_United
> Kingdom.1252;LC_MONETARY=English_United
> Kingdom.1252;LC_NUMERIC=C;LC_TIME=English_United Kingdom.1252"
>
> version
>               _
> platform       i386-pc-mingw32
> arch           i386
> os             mingw32
> system         i386, mingw32
> status
> major          2
> minor          8.0
> year           2008
> month          10
> day            20
> svn rev        46754
> language       R
> version.string R version 2.8.0 (2008-10-20)