[Rd] unique.matrix issue [Was: Anomaly with unique and match]

[Henrik Bengtsson]

It should be possible to run unique()/duplicated() column by column
and incrementally update the set of unique/duplicated rows.  This
would avoid any coercing.  The benefit should be even greater for
data.frame():s.

My $.02

/Henrik

On Thu, Mar 10, 2011 at 12:29 AM, Petr Savicky <savicky at cs.cas.cz> wrote:
> On Wed, Mar 09, 2011 at 02:11:49PM -0500, Simon Urbanek wrote:
>> match() is a red herring here -- it is really a very specific thing that has to do with the fact that you're running unique() on a matrix. Also it's much easier to reproduce:
>>
>> > x=c(1,1+0.2e-15)
>> > x
>> [1] 1 1
>> > sprintf("%a",x)
>> [1] "0x1p+0"               "0x1.0000000000001p+0"
>> > unique(x)
>> [1] 1 1
>> > sprintf("%a",unique(x))
>> [1] "0x1p+0"               "0x1.0000000000001p+0"
>> > unique(matrix(x,2))
>>      [,1]
>> [1,]    1
>>
>> and this comes from the fact that unique.matrix uses string representation since it has to take into account all values of a row/column so it pastes all values into one string, but for the two numbers that is the same:
>> > as.character(x)
>> [1] "1" "1"
>
> I understand the use of match() in the original message by Terry Therneau
> as an example of a situation, where the behavior of unique.matrix() becomes
> visible even without looking at the last bits of the numbers.
>
> Let me suggest to consider the following example.
>
>  x <- 1 + c(1.1, 1.3, 1.7, 1.9)*1e-14
>  a <- cbind(rep(x, each=2), 2)
>  rownames(a) <- 1:nrow(a)
>
> The correct set of rows may be obtained using
>
>  unique(a - 1)
>
>            [,1] [,2]
>  1 1.110223e-14    1
>  3 1.310063e-14    1
>  5 1.709743e-14    1
>  7 1.909584e-14    1
>
> However, due to the use of paste(), which uses as.character(), in
> unique.matrix(), we also have
>
>  unique(a)
>
>    [,1] [,2]
>  1    1    2
>  5    1    2
>
> Let me suggest to consider a transformation of the numeric columns
> by rank() before the use of paste(). For example
>
>  unique.mat <- function(a)
>  {
>      temp <- apply(a, 2, rank, ties.method="max")
>      temp <- apply(temp, 1, function(x) paste(x, collapse = "\r"))
>      a[!duplicated(temp), , drop=FALSE]
>  }
>
>  unique.mat(a)
>
>    [,1] [,2]
>  1    1    2
>  3    1    2
>  5    1    2
>  7    1    2
>
> Petr Savicky.
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>